Re: RCU nocb list not reclaiming causing OOM
On Mon, Jul 30, 2018 at 07:08:55PM +, David Chen wrote:
> Hi Paul,
>
> Just want to know what's your plan on stable branches regarding this issue.
> Do you intend to backport the ->nocb_lock? Or are you going with just the
> memory barrier change?
That depends on you. I am not going to submit to -stable unless someone
(in this case you) have tested it and pronounced it good. So if you
backport and test the ->nocb_lock changes and test them, I will submit
the patch for -stable. (Or you can submit to -stable, it does not have
to be me. Either way works.)
I should have asked this earlier, and perhaps could have saved you quite
a bit of work had I thought to do so (please accept my apologies if so).
Have you filed a bug report with your Linux distribution?
Thanx, Paul
> Thanks,
> David
>
>
> From: Paul E. McKenney
> Sent: Saturday, July 28, 2018 1:29 AM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 27, 2018 at 06:47:00PM -0700, Paul E. McKenney wrote:
> > On Sat, Jul 28, 2018 at 12:07:19AM +, David Chen wrote:
> > > Hi Paul,
> > >
> > > I wasn't talking about the xchg() though.
> > >
> > > The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
> > > rdp->nocb_gp_head;`
> > > it's stated so in the comment. And I do think we need ordering between
> > > `*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking
> > > on head not tail.
> > >
> > > swait_event_interruptible(rdp->nocb_wq,
> > >
> > >READ_ONCE(rdp->nocb_follower_head));
> > >
> > > So what I'm saying is that since we need to maintain ordering between
> > > `*tail = rdp->nocb_gp_head;`
> > > and wake up, we need to change the smp_mb__after_atomic() to smp_mb().
> > > Because
> > > smp_mb__after_atomic() wouldn't guarantee
> > >
> > > So this is what I'm proposing.
> >
> > Good eyes!
> >
> > Hmmm... What do I do about this in mainline? Ah, I introduced a
> > ->nocb_lock in mainline to prevent this from happening. In that same
> > commit that you didn't want to use because it is hard to backport. ;-)
> >
> > So yes, but there might well be other misorderings fixed by the
> > hard-to-backport commit that your change below does not cover. Still I
> > do agree that you need full ordering at that point.
>
> Another approach would be to backport only the ->nocb_lock portions
> of that patch. This would still potentially leave failure-to-wake
> (as opposed to misordering-on-wake) issues, but it should cover all
> of the misordering-on-wake issues.
>
> Thanx, Paul
>
> > > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > > linux-4.9.37/kernel/rcu/tree_plugin.h
> > > --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> > > 06:42:41.0 -0700
> > > +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
> > > -0700
> > > @@ -2076,7 +2076,7 @@
> > > /* Append callbacks to follower's "done" list. */
> > > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > > *tail = rdp->nocb_gp_head;
> > > - smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > > + smp_mb(); /* Store *tail before wakeup. */
> > > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > > /*
> > > * List was empty, wake up the follower.
> > >
> > > Thanks,
> > > David
> > >
> > > From: Paul E. McKenney
> > > Sent: Friday, July 27, 2018 4:47 PM
> > > To: David Chen
> > > Cc: linux-kernel@vger.kernel.org
> > > Subject: Re: RCU nocb list not reclaiming causing OOM
> > >
> > >
> > > On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> > > > Hi Paul,
> > > >
> > > > Thanks for the advice.
> > > > The bug is kind of hard to hit, so I can't say for certain yet.
> > >
> > > Well, you can always remove the "tail == >nocb_follower_head" as an
> > > extra belt-and-suspenders safety net. I am not putting that in mainline,
> > > but in the privacy of your own copy of the kernel, I d
Re: RCU nocb list not reclaiming causing OOM
On Mon, Jul 30, 2018 at 07:08:55PM +, David Chen wrote:
> Hi Paul,
>
> Just want to know what's your plan on stable branches regarding this issue.
> Do you intend to backport the ->nocb_lock? Or are you going with just the
> memory barrier change?
That depends on you. I am not going to submit to -stable unless someone
(in this case you) have tested it and pronounced it good. So if you
backport and test the ->nocb_lock changes and test them, I will submit
the patch for -stable. (Or you can submit to -stable, it does not have
to be me. Either way works.)
I should have asked this earlier, and perhaps could have saved you quite
a bit of work had I thought to do so (please accept my apologies if so).
Have you filed a bug report with your Linux distribution?
Thanx, Paul
> Thanks,
> David
>
>
> From: Paul E. McKenney
> Sent: Saturday, July 28, 2018 1:29 AM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 27, 2018 at 06:47:00PM -0700, Paul E. McKenney wrote:
> > On Sat, Jul 28, 2018 at 12:07:19AM +, David Chen wrote:
> > > Hi Paul,
> > >
> > > I wasn't talking about the xchg() though.
> > >
> > > The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
> > > rdp->nocb_gp_head;`
> > > it's stated so in the comment. And I do think we need ordering between
> > > `*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking
> > > on head not tail.
> > >
> > > swait_event_interruptible(rdp->nocb_wq,
> > >
> > >READ_ONCE(rdp->nocb_follower_head));
> > >
> > > So what I'm saying is that since we need to maintain ordering between
> > > `*tail = rdp->nocb_gp_head;`
> > > and wake up, we need to change the smp_mb__after_atomic() to smp_mb().
> > > Because
> > > smp_mb__after_atomic() wouldn't guarantee
> > >
> > > So this is what I'm proposing.
> >
> > Good eyes!
> >
> > Hmmm... What do I do about this in mainline? Ah, I introduced a
> > ->nocb_lock in mainline to prevent this from happening. In that same
> > commit that you didn't want to use because it is hard to backport. ;-)
> >
> > So yes, but there might well be other misorderings fixed by the
> > hard-to-backport commit that your change below does not cover. Still I
> > do agree that you need full ordering at that point.
>
> Another approach would be to backport only the ->nocb_lock portions
> of that patch. This would still potentially leave failure-to-wake
> (as opposed to misordering-on-wake) issues, but it should cover all
> of the misordering-on-wake issues.
>
> Thanx, Paul
>
> > > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > > linux-4.9.37/kernel/rcu/tree_plugin.h
> > > --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> > > 06:42:41.0 -0700
> > > +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
> > > -0700
> > > @@ -2076,7 +2076,7 @@
> > > /* Append callbacks to follower's "done" list. */
> > > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > > *tail = rdp->nocb_gp_head;
> > > - smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > > + smp_mb(); /* Store *tail before wakeup. */
> > > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > > /*
> > > * List was empty, wake up the follower.
> > >
> > > Thanks,
> > > David
> > >
> > > From: Paul E. McKenney
> > > Sent: Friday, July 27, 2018 4:47 PM
> > > To: David Chen
> > > Cc: linux-kernel@vger.kernel.org
> > > Subject: Re: RCU nocb list not reclaiming causing OOM
> > >
> > >
> > > On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> > > > Hi Paul,
> > > >
> > > > Thanks for the advice.
> > > > The bug is kind of hard to hit, so I can't say for certain yet.
> > >
> > > Well, you can always remove the "tail == >nocb_follower_head" as an
> > > extra belt-and-suspenders safety net. I am not putting that in mainline,
> > > but in the privacy of your own copy of the kernel, I d
Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
Just want to know what's your plan on stable branches regarding this issue.
Do you intend to backport the ->nocb_lock? Or are you going with just the
memory barrier change?
Thanks,
David
From: Paul E. McKenney
Sent: Saturday, July 28, 2018 1:29 AM
To: David Chen
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 06:47:00PM -0700, Paul E. McKenney wrote:
> On Sat, Jul 28, 2018 at 12:07:19AM +, David Chen wrote:
> > Hi Paul,
> >
> > I wasn't talking about the xchg() though.
> >
> > The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
> > rdp->nocb_gp_head;`
> > it's stated so in the comment. And I do think we need ordering between
> > `*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking on
> > head not tail.
> >
> > swait_event_interruptible(rdp->nocb_wq,
> >
> >READ_ONCE(rdp->nocb_follower_head));
> >
> > So what I'm saying is that since we need to maintain ordering between
> > `*tail = rdp->nocb_gp_head;`
> > and wake up, we need to change the smp_mb__after_atomic() to smp_mb().
> > Because
> > smp_mb__after_atomic() wouldn't guarantee
> >
> > So this is what I'm proposing.
>
> Good eyes!
>
> Hmmm... What do I do about this in mainline? Ah, I introduced a
> ->nocb_lock in mainline to prevent this from happening. In that same
> commit that you didn't want to use because it is hard to backport. ;-)
>
> So yes, but there might well be other misorderings fixed by the
> hard-to-backport commit that your change below does not cover. Still I
> do agree that you need full ordering at that point.
Another approach would be to backport only the ->nocb_lock portions
of that patch. This would still potentially leave failure-to-wake
(as opposed to misordering-on-wake) issues, but it should cover all
of the misordering-on-wake issues.
Thanx, Paul
> > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > linux-4.9.37/kernel/rcu/tree_plugin.h
> > --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> > 06:42:41.0 -0700
> > +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
> > -0700
> > @@ -2076,7 +2076,7 @@
> > /* Append callbacks to follower's "done" list. */
> > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > *tail = rdp->nocb_gp_head;
> > - smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > + smp_mb(); /* Store *tail before wakeup. */
> > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > /*
> > * List was empty, wake up the follower.
> >
> > Thanks,
> > David
> >
> > From: Paul E. McKenney
> > Sent: Friday, July 27, 2018 4:47 PM
> > To: David Chen
> > Cc: linux-kernel@vger.kernel.org
> > Subject: Re: RCU nocb list not reclaiming causing OOM
> >
> >
> > On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> > > Hi Paul,
> > >
> > > Thanks for the advice.
> > > The bug is kind of hard to hit, so I can't say for certain yet.
> >
> > Well, you can always remove the "tail == >nocb_follower_head" as an
> > extra belt-and-suspenders safety net. I am not putting that in mainline,
> > but in the privacy of your own copy of the kernel, I don't see any really
> > serious problem with it. (As long as you aren't going for absolute maximum
> > performance, but even then there are other more important tuning actions
> > and code changes you could make.)
> >
> > > Though after another look at the code, I found out the
> > > `smp_mb__after_atomic();`
> > > seems to be only a compiler barrier on x86.
> >
> > Yes, and that is because the locked xchg instruction used on x86 to
> > implement xchg() already provides full ordering. ;-)
> >
> > > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > > *tail = rdp->nocb_gp_head;
> > > smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > > swake_up(>nocb_wq);
> > >
> > > But that wouldn't be enough right? Because from 6b5fc3a13318, it stated
> > > that
> > > wakeu
Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
Just want to know what's your plan on stable branches regarding this issue.
Do you intend to backport the ->nocb_lock? Or are you going with just the
memory barrier change?
Thanks,
David
From: Paul E. McKenney
Sent: Saturday, July 28, 2018 1:29 AM
To: David Chen
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 06:47:00PM -0700, Paul E. McKenney wrote:
> On Sat, Jul 28, 2018 at 12:07:19AM +, David Chen wrote:
> > Hi Paul,
> >
> > I wasn't talking about the xchg() though.
> >
> > The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
> > rdp->nocb_gp_head;`
> > it's stated so in the comment. And I do think we need ordering between
> > `*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking on
> > head not tail.
> >
> > swait_event_interruptible(rdp->nocb_wq,
> >
> >READ_ONCE(rdp->nocb_follower_head));
> >
> > So what I'm saying is that since we need to maintain ordering between
> > `*tail = rdp->nocb_gp_head;`
> > and wake up, we need to change the smp_mb__after_atomic() to smp_mb().
> > Because
> > smp_mb__after_atomic() wouldn't guarantee
> >
> > So this is what I'm proposing.
>
> Good eyes!
>
> Hmmm... What do I do about this in mainline? Ah, I introduced a
> ->nocb_lock in mainline to prevent this from happening. In that same
> commit that you didn't want to use because it is hard to backport. ;-)
>
> So yes, but there might well be other misorderings fixed by the
> hard-to-backport commit that your change below does not cover. Still I
> do agree that you need full ordering at that point.
Another approach would be to backport only the ->nocb_lock portions
of that patch. This would still potentially leave failure-to-wake
(as opposed to misordering-on-wake) issues, but it should cover all
of the misordering-on-wake issues.
Thanx, Paul
> > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > linux-4.9.37/kernel/rcu/tree_plugin.h
> > --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> > 06:42:41.0 -0700
> > +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
> > -0700
> > @@ -2076,7 +2076,7 @@
> > /* Append callbacks to follower's "done" list. */
> > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > *tail = rdp->nocb_gp_head;
> > - smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > + smp_mb(); /* Store *tail before wakeup. */
> > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > /*
> > * List was empty, wake up the follower.
> >
> > Thanks,
> > David
> >
> > From: Paul E. McKenney
> > Sent: Friday, July 27, 2018 4:47 PM
> > To: David Chen
> > Cc: linux-kernel@vger.kernel.org
> > Subject: Re: RCU nocb list not reclaiming causing OOM
> >
> >
> > On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> > > Hi Paul,
> > >
> > > Thanks for the advice.
> > > The bug is kind of hard to hit, so I can't say for certain yet.
> >
> > Well, you can always remove the "tail == >nocb_follower_head" as an
> > extra belt-and-suspenders safety net. I am not putting that in mainline,
> > but in the privacy of your own copy of the kernel, I don't see any really
> > serious problem with it. (As long as you aren't going for absolute maximum
> > performance, but even then there are other more important tuning actions
> > and code changes you could make.)
> >
> > > Though after another look at the code, I found out the
> > > `smp_mb__after_atomic();`
> > > seems to be only a compiler barrier on x86.
> >
> > Yes, and that is because the locked xchg instruction used on x86 to
> > implement xchg() already provides full ordering. ;-)
> >
> > > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > > *tail = rdp->nocb_gp_head;
> > > smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > > swake_up(>nocb_wq);
> > >
> > > But that wouldn't be enough right? Because from 6b5fc3a13318, it stated
> > > that
> > > wakeu
Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 06:47:00PM -0700, Paul E. McKenney wrote:
> On Sat, Jul 28, 2018 at 12:07:19AM +, David Chen wrote:
> > Hi Paul,
> >
> > I wasn't talking about the xchg() though.
> >
> > The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
> > rdp->nocb_gp_head;`
> > it's stated so in the comment. And I do think we need ordering between
> > `*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking on
> > head not tail.
> >
> > swait_event_interruptible(rdp->nocb_wq,
> >
> > READ_ONCE(rdp->nocb_follower_head));
> >
> > So what I'm saying is that since we need to maintain ordering between
> > `*tail = rdp->nocb_gp_head;`
> > and wake up, we need to change the smp_mb__after_atomic() to smp_mb().
> > Because
> > smp_mb__after_atomic() wouldn't guarantee
> >
> > So this is what I'm proposing.
>
> Good eyes!
>
> Hmmm... What do I do about this in mainline? Ah, I introduced a
> ->nocb_lock in mainline to prevent this from happening. In that same
> commit that you didn't want to use because it is hard to backport. ;-)
>
> So yes, but there might well be other misorderings fixed by the
> hard-to-backport commit that your change below does not cover. Still I
> do agree that you need full ordering at that point.
Another approach would be to backport only the ->nocb_lock portions
of that patch. This would still potentially leave failure-to-wake
(as opposed to misordering-on-wake) issues, but it should cover all
of the misordering-on-wake issues.
Thanx, Paul
> > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > linux-4.9.37/kernel/rcu/tree_plugin.h
> > --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> > 06:42:41.0 -0700
> > +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
> > -0700
> > @@ -2076,7 +2076,7 @@
> > /* Append callbacks to follower's "done" list. */
> > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > *tail = rdp->nocb_gp_head;
> > - smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > + smp_mb(); /* Store *tail before wakeup. */
> > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > /*
> > * List was empty, wake up the follower.
> >
> > Thanks,
> > David
> >
> > From: Paul E. McKenney
> > Sent: Friday, July 27, 2018 4:47 PM
> > To: David Chen
> > Cc: linux-kernel@vger.kernel.org
> > Subject: Re: RCU nocb list not reclaiming causing OOM
> >
> >
> > On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> > > Hi Paul,
> > >
> > > Thanks for the advice.
> > > The bug is kind of hard to hit, so I can't say for certain yet.
> >
> > Well, you can always remove the "tail == >nocb_follower_head" as an
> > extra belt-and-suspenders safety net. I am not putting that in mainline,
> > but in the privacy of your own copy of the kernel, I don't see any really
> > serious problem with it. (As long as you aren't going for absolute maximum
> > performance, but even then there are other more important tuning actions
> > and code changes you could make.)
> >
> > > Though after another look at the code, I found out the
> > > `smp_mb__after_atomic();`
> > > seems to be only a compiler barrier on x86.
> >
> > Yes, and that is because the locked xchg instruction used on x86 to
> > implement xchg() already provides full ordering. ;-)
> >
> > > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > > *tail = rdp->nocb_gp_head;
> > > smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > > swake_up(>nocb_wq);
> > >
> > > But that wouldn't be enough right? Because from 6b5fc3a13318, it stated
> > > that
> > > wakeup operation don't guarantee ordering. And when the follower wakes
> > > up, it checks
> > > for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;`
> > > which doesn't
> > > have LOCK prefix. So it's possible for follower to wake up and see the
> > > list is empty and go
> > > back to sleep.
> >
Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 06:47:00PM -0700, Paul E. McKenney wrote:
> On Sat, Jul 28, 2018 at 12:07:19AM +, David Chen wrote:
> > Hi Paul,
> >
> > I wasn't talking about the xchg() though.
> >
> > The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
> > rdp->nocb_gp_head;`
> > it's stated so in the comment. And I do think we need ordering between
> > `*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking on
> > head not tail.
> >
> > swait_event_interruptible(rdp->nocb_wq,
> >
> > READ_ONCE(rdp->nocb_follower_head));
> >
> > So what I'm saying is that since we need to maintain ordering between
> > `*tail = rdp->nocb_gp_head;`
> > and wake up, we need to change the smp_mb__after_atomic() to smp_mb().
> > Because
> > smp_mb__after_atomic() wouldn't guarantee
> >
> > So this is what I'm proposing.
>
> Good eyes!
>
> Hmmm... What do I do about this in mainline? Ah, I introduced a
> ->nocb_lock in mainline to prevent this from happening. In that same
> commit that you didn't want to use because it is hard to backport. ;-)
>
> So yes, but there might well be other misorderings fixed by the
> hard-to-backport commit that your change below does not cover. Still I
> do agree that you need full ordering at that point.
Another approach would be to backport only the ->nocb_lock portions
of that patch. This would still potentially leave failure-to-wake
(as opposed to misordering-on-wake) issues, but it should cover all
of the misordering-on-wake issues.
Thanx, Paul
> > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > linux-4.9.37/kernel/rcu/tree_plugin.h
> > --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> > 06:42:41.0 -0700
> > +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
> > -0700
> > @@ -2076,7 +2076,7 @@
> > /* Append callbacks to follower's "done" list. */
> > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > *tail = rdp->nocb_gp_head;
> > - smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > + smp_mb(); /* Store *tail before wakeup. */
> > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > /*
> > * List was empty, wake up the follower.
> >
> > Thanks,
> > David
> >
> > From: Paul E. McKenney
> > Sent: Friday, July 27, 2018 4:47 PM
> > To: David Chen
> > Cc: linux-kernel@vger.kernel.org
> > Subject: Re: RCU nocb list not reclaiming causing OOM
> >
> >
> > On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> > > Hi Paul,
> > >
> > > Thanks for the advice.
> > > The bug is kind of hard to hit, so I can't say for certain yet.
> >
> > Well, you can always remove the "tail == >nocb_follower_head" as an
> > extra belt-and-suspenders safety net. I am not putting that in mainline,
> > but in the privacy of your own copy of the kernel, I don't see any really
> > serious problem with it. (As long as you aren't going for absolute maximum
> > performance, but even then there are other more important tuning actions
> > and code changes you could make.)
> >
> > > Though after another look at the code, I found out the
> > > `smp_mb__after_atomic();`
> > > seems to be only a compiler barrier on x86.
> >
> > Yes, and that is because the locked xchg instruction used on x86 to
> > implement xchg() already provides full ordering. ;-)
> >
> > > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > > *tail = rdp->nocb_gp_head;
> > > smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > > swake_up(>nocb_wq);
> > >
> > > But that wouldn't be enough right? Because from 6b5fc3a13318, it stated
> > > that
> > > wakeup operation don't guarantee ordering. And when the follower wakes
> > > up, it checks
> > > for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;`
> > > which doesn't
> > > have LOCK prefix. So it's possible for follower to wake up and see the
> > > list is empty and go
> > > back to sleep.
> >
Re: RCU nocb list not reclaiming causing OOM
On Sat, Jul 28, 2018 at 12:07:19AM +, David Chen wrote:
> Hi Paul,
>
> I wasn't talking about the xchg() though.
>
> The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
> rdp->nocb_gp_head;`
> it's stated so in the comment. And I do think we need ordering between
> `*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking on
> head not tail.
>
> swait_event_interruptible(rdp->nocb_wq,
>
> READ_ONCE(rdp->nocb_follower_head));
>
> So what I'm saying is that since we need to maintain ordering between `*tail
> = rdp->nocb_gp_head;`
> and wake up, we need to change the smp_mb__after_atomic() to smp_mb(). Because
> smp_mb__after_atomic() wouldn't guarantee
>
> So this is what I'm proposing.
Good eyes!
Hmmm... What do I do about this in mainline? Ah, I introduced a
->nocb_lock in mainline to prevent this from happening. In that same
commit that you didn't want to use because it is hard to backport. ;-)
So yes, but there might well be other misorderings fixed by the
hard-to-backport commit that your change below does not cover. Still I
do agree that you need full ordering at that point.
Thanx, Paul
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
> -0700
> @@ -2076,7 +2076,7 @@
> /* Append callbacks to follower's "done" list. */
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> - smp_mb__after_atomic(); /* Store *tail before wakeup. */
> + smp_mb(); /* Store *tail before wakeup. */
> if (rdp != my_rdp && tail == >nocb_follower_head) {
> /*
>* List was empty, wake up the follower.
>
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 27, 2018 4:47 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> > Hi Paul,
> >
> > Thanks for the advice.
> > The bug is kind of hard to hit, so I can't say for certain yet.
>
> Well, you can always remove the "tail == >nocb_follower_head" as an
> extra belt-and-suspenders safety net. I am not putting that in mainline,
> but in the privacy of your own copy of the kernel, I don't see any really
> serious problem with it. (As long as you aren't going for absolute maximum
> performance, but even then there are other more important tuning actions
> and code changes you could make.)
>
> > Though after another look at the code, I found out the
> > `smp_mb__after_atomic();`
> > seems to be only a compiler barrier on x86.
>
> Yes, and that is because the locked xchg instruction used on x86 to
> implement xchg() already provides full ordering. ;-)
>
> > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > *tail = rdp->nocb_gp_head;
> > smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > swake_up(>nocb_wq);
> >
> > But that wouldn't be enough right? Because from 6b5fc3a13318, it stated that
> > wakeup operation don't guarantee ordering. And when the follower wakes up,
> > it checks
> > for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;`
> > which doesn't
> > have LOCK prefix. So it's possible for follower to wake up and see the list
> > is empty and go
> > back to sleep.
>
> Again, xchg() is defined to provide full ordering against all operations
> before and after it. Each architecture is required to do whatever is
> necessary to implement that full ordering, and x86 need only provide
> its "lock xchg" instruction.
>
> The smp_mb__after_atomic() has effect only after atomic read-modify-write
> operations that do not return a value, for example, atomic_inc().
> If you use it after a value-returning atomic read-modify-write operation
> like xchg(), all you do is needlessly slow things down on platforms
> that provide non-empty smp_mb__after_atomic() definitions. So again,
> smp_mb__after_atomic() after xchg() is pointless.
>
> Please take a look at Documentation/core-api/a
Re: RCU nocb list not reclaiming causing OOM
On Sat, Jul 28, 2018 at 12:07:19AM +, David Chen wrote:
> Hi Paul,
>
> I wasn't talking about the xchg() though.
>
> The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
> rdp->nocb_gp_head;`
> it's stated so in the comment. And I do think we need ordering between
> `*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking on
> head not tail.
>
> swait_event_interruptible(rdp->nocb_wq,
>
> READ_ONCE(rdp->nocb_follower_head));
>
> So what I'm saying is that since we need to maintain ordering between `*tail
> = rdp->nocb_gp_head;`
> and wake up, we need to change the smp_mb__after_atomic() to smp_mb(). Because
> smp_mb__after_atomic() wouldn't guarantee
>
> So this is what I'm proposing.
Good eyes!
Hmmm... What do I do about this in mainline? Ah, I introduced a
->nocb_lock in mainline to prevent this from happening. In that same
commit that you didn't want to use because it is hard to backport. ;-)
So yes, but there might well be other misorderings fixed by the
hard-to-backport commit that your change below does not cover. Still I
do agree that you need full ordering at that point.
Thanx, Paul
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
> -0700
> @@ -2076,7 +2076,7 @@
> /* Append callbacks to follower's "done" list. */
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> - smp_mb__after_atomic(); /* Store *tail before wakeup. */
> + smp_mb(); /* Store *tail before wakeup. */
> if (rdp != my_rdp && tail == >nocb_follower_head) {
> /*
>* List was empty, wake up the follower.
>
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 27, 2018 4:47 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> > Hi Paul,
> >
> > Thanks for the advice.
> > The bug is kind of hard to hit, so I can't say for certain yet.
>
> Well, you can always remove the "tail == >nocb_follower_head" as an
> extra belt-and-suspenders safety net. I am not putting that in mainline,
> but in the privacy of your own copy of the kernel, I don't see any really
> serious problem with it. (As long as you aren't going for absolute maximum
> performance, but even then there are other more important tuning actions
> and code changes you could make.)
>
> > Though after another look at the code, I found out the
> > `smp_mb__after_atomic();`
> > seems to be only a compiler barrier on x86.
>
> Yes, and that is because the locked xchg instruction used on x86 to
> implement xchg() already provides full ordering. ;-)
>
> > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > *tail = rdp->nocb_gp_head;
> > smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > if (rdp != my_rdp && tail == >nocb_follower_head) {
> > swake_up(>nocb_wq);
> >
> > But that wouldn't be enough right? Because from 6b5fc3a13318, it stated that
> > wakeup operation don't guarantee ordering. And when the follower wakes up,
> > it checks
> > for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;`
> > which doesn't
> > have LOCK prefix. So it's possible for follower to wake up and see the list
> > is empty and go
> > back to sleep.
>
> Again, xchg() is defined to provide full ordering against all operations
> before and after it. Each architecture is required to do whatever is
> necessary to implement that full ordering, and x86 need only provide
> its "lock xchg" instruction.
>
> The smp_mb__after_atomic() has effect only after atomic read-modify-write
> operations that do not return a value, for example, atomic_inc().
> If you use it after a value-returning atomic read-modify-write operation
> like xchg(), all you do is needlessly slow things down on platforms
> that provide non-empty smp_mb__after_atomic() definitions. So again,
> smp_mb__after_atomic() after xchg() is pointless.
>
> Please take a look at Documentation/core-api/a
Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
I wasn't talking about the xchg() though.
The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
rdp->nocb_gp_head;`
it's stated so in the comment. And I do think we need ordering between
`*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking on
head not tail.
swait_event_interruptible(rdp->nocb_wq,
READ_ONCE(rdp->nocb_follower_head));
So what I'm saying is that since we need to maintain ordering between `*tail =
rdp->nocb_gp_head;`
and wake up, we need to change the smp_mb__after_atomic() to smp_mb(). Because
smp_mb__after_atomic() wouldn't guarantee
So this is what I'm proposing.
diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
linux-4.9.37/kernel/rcu/tree_plugin.h
--- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12 06:42:41.0
-0700
+++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
-0700
@@ -2076,7 +2076,7 @@
/* Append callbacks to follower's "done" list. */
tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
*tail = rdp->nocb_gp_head;
- smp_mb__after_atomic(); /* Store *tail before wakeup. */
+ smp_mb(); /* Store *tail before wakeup. */
if (rdp != my_rdp && tail == >nocb_follower_head) {
/*
* List was empty, wake up the follower.
Thanks,
David
From: Paul E. McKenney
Sent: Friday, July 27, 2018 4:47 PM
To: David Chen
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> Hi Paul,
>
> Thanks for the advice.
> The bug is kind of hard to hit, so I can't say for certain yet.
Well, you can always remove the "tail == >nocb_follower_head" as an
extra belt-and-suspenders safety net. I am not putting that in mainline,
but in the privacy of your own copy of the kernel, I don't see any really
serious problem with it. (As long as you aren't going for absolute maximum
performance, but even then there are other more important tuning actions
and code changes you could make.)
> Though after another look at the code, I found out the
> `smp_mb__after_atomic();`
> seems to be only a compiler barrier on x86.
Yes, and that is because the locked xchg instruction used on x86 to
implement xchg() already provides full ordering. ;-)
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> if (rdp != my_rdp && tail == >nocb_follower_head) {
> swake_up(>nocb_wq);
>
> But that wouldn't be enough right? Because from 6b5fc3a13318, it stated that
> wakeup operation don't guarantee ordering. And when the follower wakes up, it
> checks
> for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;`
> which doesn't
> have LOCK prefix. So it's possible for follower to wake up and see the list
> is empty and go
> back to sleep.
Again, xchg() is defined to provide full ordering against all operations
before and after it. Each architecture is required to do whatever is
necessary to implement that full ordering, and x86 need only provide
its "lock xchg" instruction.
The smp_mb__after_atomic() has effect only after atomic read-modify-write
operations that do not return a value, for example, atomic_inc().
If you use it after a value-returning atomic read-modify-write operation
like xchg(), all you do is needlessly slow things down on platforms
that provide non-empty smp_mb__after_atomic() definitions. So again,
smp_mb__after_atomic() after xchg() is pointless.
Please take a look at Documentation/core-api/atomic_ops.rst in the
Linux-kernel source tree for more information. Or get a v4.17 kernel
source tree and check this using the memory model (tools/memory-model
in that version).
Thanx, Paul
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 27, 2018 3:31 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 27, 2018 at 07:07:46PM +, David Chen wrote:
> > Hi Paul,
> >
> > I'd like to opinion again on this subject.
> >
> > So we are going to backport this patch:
> > 6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
>
> Does this one solve the problem, or are you still seeing hangs?
> If you are no longer seeing hangs, my advice is "hands off keyboard",
> though some would no doubt point out that I should follow tha
Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
I wasn't talking about the xchg() though.
The smp_mb__after_atomic() is not for xchg(), it's for `*tail =
rdp->nocb_gp_head;`
it's stated so in the comment. And I do think we need ordering between
`*tail = rdp->nocb_gp_head;` and wake up, because the waiter is checking on
head not tail.
swait_event_interruptible(rdp->nocb_wq,
READ_ONCE(rdp->nocb_follower_head));
So what I'm saying is that since we need to maintain ordering between `*tail =
rdp->nocb_gp_head;`
and wake up, we need to change the smp_mb__after_atomic() to smp_mb(). Because
smp_mb__after_atomic() wouldn't guarantee
So this is what I'm proposing.
diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
linux-4.9.37/kernel/rcu/tree_plugin.h
--- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12 06:42:41.0
-0700
+++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 16:23:41.349044259
-0700
@@ -2076,7 +2076,7 @@
/* Append callbacks to follower's "done" list. */
tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
*tail = rdp->nocb_gp_head;
- smp_mb__after_atomic(); /* Store *tail before wakeup. */
+ smp_mb(); /* Store *tail before wakeup. */
if (rdp != my_rdp && tail == >nocb_follower_head) {
/*
* List was empty, wake up the follower.
Thanks,
David
From: Paul E. McKenney
Sent: Friday, July 27, 2018 4:47 PM
To: David Chen
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> Hi Paul,
>
> Thanks for the advice.
> The bug is kind of hard to hit, so I can't say for certain yet.
Well, you can always remove the "tail == >nocb_follower_head" as an
extra belt-and-suspenders safety net. I am not putting that in mainline,
but in the privacy of your own copy of the kernel, I don't see any really
serious problem with it. (As long as you aren't going for absolute maximum
performance, but even then there are other more important tuning actions
and code changes you could make.)
> Though after another look at the code, I found out the
> `smp_mb__after_atomic();`
> seems to be only a compiler barrier on x86.
Yes, and that is because the locked xchg instruction used on x86 to
implement xchg() already provides full ordering. ;-)
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> if (rdp != my_rdp && tail == >nocb_follower_head) {
> swake_up(>nocb_wq);
>
> But that wouldn't be enough right? Because from 6b5fc3a13318, it stated that
> wakeup operation don't guarantee ordering. And when the follower wakes up, it
> checks
> for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;`
> which doesn't
> have LOCK prefix. So it's possible for follower to wake up and see the list
> is empty and go
> back to sleep.
Again, xchg() is defined to provide full ordering against all operations
before and after it. Each architecture is required to do whatever is
necessary to implement that full ordering, and x86 need only provide
its "lock xchg" instruction.
The smp_mb__after_atomic() has effect only after atomic read-modify-write
operations that do not return a value, for example, atomic_inc().
If you use it after a value-returning atomic read-modify-write operation
like xchg(), all you do is needlessly slow things down on platforms
that provide non-empty smp_mb__after_atomic() definitions. So again,
smp_mb__after_atomic() after xchg() is pointless.
Please take a look at Documentation/core-api/atomic_ops.rst in the
Linux-kernel source tree for more information. Or get a v4.17 kernel
source tree and check this using the memory model (tools/memory-model
in that version).
Thanx, Paul
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 27, 2018 3:31 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 27, 2018 at 07:07:46PM +, David Chen wrote:
> > Hi Paul,
> >
> > I'd like to opinion again on this subject.
> >
> > So we are going to backport this patch:
> > 6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
>
> Does this one solve the problem, or are you still seeing hangs?
> If you are no longer seeing hangs, my advice is "hands off keyboard",
> though some would no doubt point out that I should follow tha
Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> Hi Paul,
>
> Thanks for the advice.
> The bug is kind of hard to hit, so I can't say for certain yet.
Well, you can always remove the "tail == >nocb_follower_head" as an
extra belt-and-suspenders safety net. I am not putting that in mainline,
but in the privacy of your own copy of the kernel, I don't see any really
serious problem with it. (As long as you aren't going for absolute maximum
performance, but even then there are other more important tuning actions
and code changes you could make.)
> Though after another look at the code, I found out the
> `smp_mb__after_atomic();`
> seems to be only a compiler barrier on x86.
Yes, and that is because the locked xchg instruction used on x86 to
implement xchg() already provides full ordering. ;-)
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> if (rdp != my_rdp && tail == >nocb_follower_head) {
> swake_up(>nocb_wq);
>
> But that wouldn't be enough right? Because from 6b5fc3a13318, it stated that
> wakeup operation don't guarantee ordering. And when the follower wakes up, it
> checks
> for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;`
> which doesn't
> have LOCK prefix. So it's possible for follower to wake up and see the list
> is empty and go
> back to sleep.
Again, xchg() is defined to provide full ordering against all operations
before and after it. Each architecture is required to do whatever is
necessary to implement that full ordering, and x86 need only provide
its "lock xchg" instruction.
The smp_mb__after_atomic() has effect only after atomic read-modify-write
operations that do not return a value, for example, atomic_inc().
If you use it after a value-returning atomic read-modify-write operation
like xchg(), all you do is needlessly slow things down on platforms
that provide non-empty smp_mb__after_atomic() definitions. So again,
smp_mb__after_atomic() after xchg() is pointless.
Please take a look at Documentation/core-api/atomic_ops.rst in the
Linux-kernel source tree for more information. Or get a v4.17 kernel
source tree and check this using the memory model (tools/memory-model
in that version).
Thanx, Paul
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 27, 2018 3:31 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 27, 2018 at 07:07:46PM +, David Chen wrote:
> > Hi Paul,
> >
> > I'd like to opinion again on this subject.
> >
> > So we are going to backport this patch:
> > 6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
>
> Does this one solve the problem, or are you still seeing hangs?
> If you are no longer seeing hangs, my advice is "hands off keyboard",
> though some would no doubt point out that I should follow that advice
> more myself. ;-)
>
> > But the other one:
> > 8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
> > It doesn't apply cleanly, and I'm not too comfortable porting it myself.
>
> Yeah, that one is a bit on the non-trivial side, no two ways about it.
>
> > So I'm wondering if I use the following change to always wake up follower
> > thread
> > regardless if the list was empty or not, just to be on the safe side. Do
> > you think
> > this change is reasonable? Do you see any problem it might cause?
> >
> > Thanks,
> > David
> >
> > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > linux-4.9.37/kernel/rcu/tree_plugin.h
> > --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> > 06:42:41.0 -0700
> > +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 11:57:03.582134519
> > -0700
> > @@ -2077,7 +2077,7 @@
> > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > *tail = rdp->nocb_gp_head;
> > smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > - if (rdp != my_rdp && tail == >nocb_follower_head) {
> > + if (rdp != my_rdp) {
>
> This will burn a bit of extra CPU time, but it should be fine other than
> that.
>
> Thanx, Paul
>
> > /*
> > * List was empty, wake up the follower.
> >
Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 11:16:39PM +, David Chen wrote:
> Hi Paul,
>
> Thanks for the advice.
> The bug is kind of hard to hit, so I can't say for certain yet.
Well, you can always remove the "tail == >nocb_follower_head" as an
extra belt-and-suspenders safety net. I am not putting that in mainline,
but in the privacy of your own copy of the kernel, I don't see any really
serious problem with it. (As long as you aren't going for absolute maximum
performance, but even then there are other more important tuning actions
and code changes you could make.)
> Though after another look at the code, I found out the
> `smp_mb__after_atomic();`
> seems to be only a compiler barrier on x86.
Yes, and that is because the locked xchg instruction used on x86 to
implement xchg() already provides full ordering. ;-)
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> if (rdp != my_rdp && tail == >nocb_follower_head) {
> swake_up(>nocb_wq);
>
> But that wouldn't be enough right? Because from 6b5fc3a13318, it stated that
> wakeup operation don't guarantee ordering. And when the follower wakes up, it
> checks
> for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;`
> which doesn't
> have LOCK prefix. So it's possible for follower to wake up and see the list
> is empty and go
> back to sleep.
Again, xchg() is defined to provide full ordering against all operations
before and after it. Each architecture is required to do whatever is
necessary to implement that full ordering, and x86 need only provide
its "lock xchg" instruction.
The smp_mb__after_atomic() has effect only after atomic read-modify-write
operations that do not return a value, for example, atomic_inc().
If you use it after a value-returning atomic read-modify-write operation
like xchg(), all you do is needlessly slow things down on platforms
that provide non-empty smp_mb__after_atomic() definitions. So again,
smp_mb__after_atomic() after xchg() is pointless.
Please take a look at Documentation/core-api/atomic_ops.rst in the
Linux-kernel source tree for more information. Or get a v4.17 kernel
source tree and check this using the memory model (tools/memory-model
in that version).
Thanx, Paul
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 27, 2018 3:31 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 27, 2018 at 07:07:46PM +, David Chen wrote:
> > Hi Paul,
> >
> > I'd like to opinion again on this subject.
> >
> > So we are going to backport this patch:
> > 6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
>
> Does this one solve the problem, or are you still seeing hangs?
> If you are no longer seeing hangs, my advice is "hands off keyboard",
> though some would no doubt point out that I should follow that advice
> more myself. ;-)
>
> > But the other one:
> > 8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
> > It doesn't apply cleanly, and I'm not too comfortable porting it myself.
>
> Yeah, that one is a bit on the non-trivial side, no two ways about it.
>
> > So I'm wondering if I use the following change to always wake up follower
> > thread
> > regardless if the list was empty or not, just to be on the safe side. Do
> > you think
> > this change is reasonable? Do you see any problem it might cause?
> >
> > Thanks,
> > David
> >
> > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > linux-4.9.37/kernel/rcu/tree_plugin.h
> > --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> > 06:42:41.0 -0700
> > +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 11:57:03.582134519
> > -0700
> > @@ -2077,7 +2077,7 @@
> > tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> > *tail = rdp->nocb_gp_head;
> > smp_mb__after_atomic(); /* Store *tail before wakeup. */
> > - if (rdp != my_rdp && tail == >nocb_follower_head) {
> > + if (rdp != my_rdp) {
>
> This will burn a bit of extra CPU time, but it should be fine other than
> that.
>
> Thanx, Paul
>
> > /*
> > * List was empty, wake up the follower.
> >
Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
Thanks for the advice.
The bug is kind of hard to hit, so I can't say for certain yet.
Though after another look at the code, I found out the `smp_mb__after_atomic();`
seems to be only a compiler barrier on x86.
tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
*tail = rdp->nocb_gp_head;
smp_mb__after_atomic(); /* Store *tail before wakeup. */
if (rdp != my_rdp && tail == >nocb_follower_head) {
swake_up(>nocb_wq);
But that wouldn't be enough right? Because from 6b5fc3a13318, it stated that
wakeup operation don't guarantee ordering. And when the follower wakes up, it
checks
for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;` which
doesn't
have LOCK prefix. So it's possible for follower to wake up and see the list is
empty and go
back to sleep.
Thanks,
David
From: Paul E. McKenney
Sent: Friday, July 27, 2018 3:31 PM
To: David Chen
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 07:07:46PM +, David Chen wrote:
> Hi Paul,
>
> I'd like to opinion again on this subject.
>
> So we are going to backport this patch:
> 6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
Does this one solve the problem, or are you still seeing hangs?
If you are no longer seeing hangs, my advice is "hands off keyboard",
though some would no doubt point out that I should follow that advice
more myself. ;-)
> But the other one:
> 8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
> It doesn't apply cleanly, and I'm not too comfortable porting it myself.
Yeah, that one is a bit on the non-trivial side, no two ways about it.
> So I'm wondering if I use the following change to always wake up follower
> thread
> regardless if the list was empty or not, just to be on the safe side. Do you
> think
> this change is reasonable? Do you see any problem it might cause?
>
> Thanks,
> David
>
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 11:57:03.582134519
> -0700
> @@ -2077,7 +2077,7 @@
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> - if (rdp != my_rdp && tail == >nocb_follower_head) {
> + if (rdp != my_rdp) {
This will burn a bit of extra CPU time, but it should be fine other than
that.
Thanx, Paul
> /*
> * List was empty, wake up the follower.
> * Memory barriers supplied by atomic_long_add().
>
>
> From: David Chen
> Sent: Friday, July 20, 2018 5:12 PM
> To: paul...@linux.vnet.ibm.com
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> Hi Paul,
>
> Ok, I'll try those patches.
>
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 20, 2018 4:32:12 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 20, 2018 at 11:05:52PM +, David Chen wrote:
> > Hi Paul,
> >
> > We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows
> > too
> > large, and not getting reclaimed, causing the system to OOM.
> >
> > Printing the culprit rcu_sched_data:
> >
> > nocb_q_count = {
> > counter = 32369635
> > },
> > nocb_follower_head = 0x88ae901c0a00,
> > nocb_follower_tail = 0x88af1538b8d8,
> > nocb_kthread = 0x88b06d29,
> >
> > As you can see here, the nocb_follower_head is not empty, so in theory, the
> > nocb_kthread shouldn't go to sleep. However, if dump the stack of the
> > kthread:
> >
> > crash> bt 0x88b06d29
> > PID: 21 TASK: 88b06d29 CPU: 3 COMMAND: "rcuos/1"
> > #0 [afc9020b7dc0] __schedule at 8d8789dc
> > #1 [afc9020b7e38] schedule at 8d878e76
> > #2 [afc9020b7e50] rcu_nocb_kthread at 8d112337
> > #3 [afc9020b7ec8] kthread at 8d0c6ce7
> > #4 [afc9020b7f50] ret_from_fork at 8d87d755
> >
> > And if we dis the address at 8d112337:
> >
> > /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.0
Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
Thanks for the advice.
The bug is kind of hard to hit, so I can't say for certain yet.
Though after another look at the code, I found out the `smp_mb__after_atomic();`
seems to be only a compiler barrier on x86.
tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
*tail = rdp->nocb_gp_head;
smp_mb__after_atomic(); /* Store *tail before wakeup. */
if (rdp != my_rdp && tail == >nocb_follower_head) {
swake_up(>nocb_wq);
But that wouldn't be enough right? Because from 6b5fc3a13318, it stated that
wakeup operation don't guarantee ordering. And when the follower wakes up, it
checks
for nocb_follower_head, which is assigned by `*tail = rdp->nocb_gp_head;` which
doesn't
have LOCK prefix. So it's possible for follower to wake up and see the list is
empty and go
back to sleep.
Thanks,
David
From: Paul E. McKenney
Sent: Friday, July 27, 2018 3:31 PM
To: David Chen
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 07:07:46PM +, David Chen wrote:
> Hi Paul,
>
> I'd like to opinion again on this subject.
>
> So we are going to backport this patch:
> 6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
Does this one solve the problem, or are you still seeing hangs?
If you are no longer seeing hangs, my advice is "hands off keyboard",
though some would no doubt point out that I should follow that advice
more myself. ;-)
> But the other one:
> 8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
> It doesn't apply cleanly, and I'm not too comfortable porting it myself.
Yeah, that one is a bit on the non-trivial side, no two ways about it.
> So I'm wondering if I use the following change to always wake up follower
> thread
> regardless if the list was empty or not, just to be on the safe side. Do you
> think
> this change is reasonable? Do you see any problem it might cause?
>
> Thanks,
> David
>
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 11:57:03.582134519
> -0700
> @@ -2077,7 +2077,7 @@
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> - if (rdp != my_rdp && tail == >nocb_follower_head) {
> + if (rdp != my_rdp) {
This will burn a bit of extra CPU time, but it should be fine other than
that.
Thanx, Paul
> /*
> * List was empty, wake up the follower.
> * Memory barriers supplied by atomic_long_add().
>
>
> From: David Chen
> Sent: Friday, July 20, 2018 5:12 PM
> To: paul...@linux.vnet.ibm.com
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> Hi Paul,
>
> Ok, I'll try those patches.
>
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 20, 2018 4:32:12 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 20, 2018 at 11:05:52PM +, David Chen wrote:
> > Hi Paul,
> >
> > We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows
> > too
> > large, and not getting reclaimed, causing the system to OOM.
> >
> > Printing the culprit rcu_sched_data:
> >
> > nocb_q_count = {
> > counter = 32369635
> > },
> > nocb_follower_head = 0x88ae901c0a00,
> > nocb_follower_tail = 0x88af1538b8d8,
> > nocb_kthread = 0x88b06d29,
> >
> > As you can see here, the nocb_follower_head is not empty, so in theory, the
> > nocb_kthread shouldn't go to sleep. However, if dump the stack of the
> > kthread:
> >
> > crash> bt 0x88b06d29
> > PID: 21 TASK: 88b06d29 CPU: 3 COMMAND: "rcuos/1"
> > #0 [afc9020b7dc0] __schedule at 8d8789dc
> > #1 [afc9020b7e38] schedule at 8d878e76
> > #2 [afc9020b7e50] rcu_nocb_kthread at 8d112337
> > #3 [afc9020b7ec8] kthread at 8d0c6ce7
> > #4 [afc9020b7f50] ret_from_fork at 8d87d755
> >
> > And if we dis the address at 8d112337:
> >
> > /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.0
Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 07:07:46PM +, David Chen wrote:
> Hi Paul,
>
> I'd like to opinion again on this subject.
>
> So we are going to backport this patch:
> 6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
Does this one solve the problem, or are you still seeing hangs?
If you are no longer seeing hangs, my advice is "hands off keyboard",
though some would no doubt point out that I should follow that advice
more myself. ;-)
> But the other one:
> 8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
> It doesn't apply cleanly, and I'm not too comfortable porting it myself.
Yeah, that one is a bit on the non-trivial side, no two ways about it.
> So I'm wondering if I use the following change to always wake up follower
> thread
> regardless if the list was empty or not, just to be on the safe side. Do you
> think
> this change is reasonable? Do you see any problem it might cause?
>
> Thanks,
> David
>
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 11:57:03.582134519
> -0700
> @@ -2077,7 +2077,7 @@
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> - if (rdp != my_rdp && tail == >nocb_follower_head) {
> + if (rdp != my_rdp) {
This will burn a bit of extra CPU time, but it should be fine other than
that.
Thanx, Paul
> /*
>* List was empty, wake up the follower.
>* Memory barriers supplied by atomic_long_add().
>
>
> From: David Chen
> Sent: Friday, July 20, 2018 5:12 PM
> To: paul...@linux.vnet.ibm.com
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> Hi Paul,
>
> Ok, I'll try those patches.
>
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 20, 2018 4:32:12 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 20, 2018 at 11:05:52PM +, David Chen wrote:
> > Hi Paul,
> >
> > We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows
> > too
> > large, and not getting reclaimed, causing the system to OOM.
> >
> > Printing the culprit rcu_sched_data:
> >
> > nocb_q_count = {
> > counter = 32369635
> > },
> > nocb_follower_head = 0x88ae901c0a00,
> > nocb_follower_tail = 0x88af1538b8d8,
> > nocb_kthread = 0x88b06d29,
> >
> > As you can see here, the nocb_follower_head is not empty, so in theory, the
> > nocb_kthread shouldn't go to sleep. However, if dump the stack of the
> > kthread:
> >
> > crash> bt 0x88b06d29
> > PID: 21 TASK: 88b06d29 CPU: 3 COMMAND: "rcuos/1"
> > #0 [afc9020b7dc0] __schedule at 8d8789dc
> > #1 [afc9020b7e38] schedule at 8d878e76
> > #2 [afc9020b7e50] rcu_nocb_kthread at 8d112337
> > #3 [afc9020b7ec8] kthread at 8d0c6ce7
> > #4 [afc9020b7f50] ret_from_fork at 8d87d755
> >
> > And if we dis the address at 8d112337:
> >
> > /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.07142017.el7.centos.x86_64/kernel/rcu/tree_plugin.h:
> > 2106
> > 0x8d11232d : test %rax,%rax
> > 0x8d112330 : jne 0x8d112355
> >
> > 0x8d112332 : callq 0x8d878e40
> >
> > 0x8d112337 : lea -0x40(%rbp),%rsi
> >
> > So the kthread is blocked at swait_event_interruptible in the
> > nocb_follower_wait.
> > This contradict with the fact that nocb_follower_head was not empty. So I
> > wonder if this is caused by the lack of memory barrier in the place shown
> > below.
> > If the head is set to NULL after doing xchg, it will overwrite the head set
> > by leader. This caused the kthread to sleep the next iteration, and the
> > leader
> > won't wake him up as the tail doesn't point to head.
> >
> > Please tell me what do you think.
> >
> > Thanks,
> > David
> >
> > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > linux-4.9.37/kernel/rcu/tree_plu
Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 27, 2018 at 07:07:46PM +, David Chen wrote:
> Hi Paul,
>
> I'd like to opinion again on this subject.
>
> So we are going to backport this patch:
> 6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
Does this one solve the problem, or are you still seeing hangs?
If you are no longer seeing hangs, my advice is "hands off keyboard",
though some would no doubt point out that I should follow that advice
more myself. ;-)
> But the other one:
> 8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
> It doesn't apply cleanly, and I'm not too comfortable porting it myself.
Yeah, that one is a bit on the non-trivial side, no two ways about it.
> So I'm wondering if I use the following change to always wake up follower
> thread
> regardless if the list was empty or not, just to be on the safe side. Do you
> think
> this change is reasonable? Do you see any problem it might cause?
>
> Thanks,
> David
>
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 11:57:03.582134519
> -0700
> @@ -2077,7 +2077,7 @@
> tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
> *tail = rdp->nocb_gp_head;
> smp_mb__after_atomic(); /* Store *tail before wakeup. */
> - if (rdp != my_rdp && tail == >nocb_follower_head) {
> + if (rdp != my_rdp) {
This will burn a bit of extra CPU time, but it should be fine other than
that.
Thanx, Paul
> /*
>* List was empty, wake up the follower.
>* Memory barriers supplied by atomic_long_add().
>
>
> From: David Chen
> Sent: Friday, July 20, 2018 5:12 PM
> To: paul...@linux.vnet.ibm.com
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> Hi Paul,
>
> Ok, I'll try those patches.
>
> Thanks,
> David
>
> From: Paul E. McKenney
> Sent: Friday, July 20, 2018 4:32:12 PM
> To: David Chen
> Cc: linux-kernel@vger.kernel.org
> Subject: Re: RCU nocb list not reclaiming causing OOM
>
>
> On Fri, Jul 20, 2018 at 11:05:52PM +, David Chen wrote:
> > Hi Paul,
> >
> > We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows
> > too
> > large, and not getting reclaimed, causing the system to OOM.
> >
> > Printing the culprit rcu_sched_data:
> >
> > nocb_q_count = {
> > counter = 32369635
> > },
> > nocb_follower_head = 0x88ae901c0a00,
> > nocb_follower_tail = 0x88af1538b8d8,
> > nocb_kthread = 0x88b06d29,
> >
> > As you can see here, the nocb_follower_head is not empty, so in theory, the
> > nocb_kthread shouldn't go to sleep. However, if dump the stack of the
> > kthread:
> >
> > crash> bt 0x88b06d29
> > PID: 21 TASK: 88b06d29 CPU: 3 COMMAND: "rcuos/1"
> > #0 [afc9020b7dc0] __schedule at 8d8789dc
> > #1 [afc9020b7e38] schedule at 8d878e76
> > #2 [afc9020b7e50] rcu_nocb_kthread at 8d112337
> > #3 [afc9020b7ec8] kthread at 8d0c6ce7
> > #4 [afc9020b7f50] ret_from_fork at 8d87d755
> >
> > And if we dis the address at 8d112337:
> >
> > /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.07142017.el7.centos.x86_64/kernel/rcu/tree_plugin.h:
> > 2106
> > 0x8d11232d : test %rax,%rax
> > 0x8d112330 : jne 0x8d112355
> >
> > 0x8d112332 : callq 0x8d878e40
> >
> > 0x8d112337 : lea -0x40(%rbp),%rsi
> >
> > So the kthread is blocked at swait_event_interruptible in the
> > nocb_follower_wait.
> > This contradict with the fact that nocb_follower_head was not empty. So I
> > wonder if this is caused by the lack of memory barrier in the place shown
> > below.
> > If the head is set to NULL after doing xchg, it will overwrite the head set
> > by leader. This caused the kthread to sleep the next iteration, and the
> > leader
> > won't wake him up as the tail doesn't point to head.
> >
> > Please tell me what do you think.
> >
> > Thanks,
> > David
> >
> > diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> > linux-4.9.37/kernel/rcu/tree_plu
Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
I'd like to opinion again on this subject.
So we are going to backport this patch:
6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
But the other one:
8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
It doesn't apply cleanly, and I'm not too comfortable porting it myself.
So I'm wondering if I use the following change to always wake up follower thread
regardless if the list was empty or not, just to be on the safe side. Do you
think
this change is reasonable? Do you see any problem it might cause?
Thanks,
David
diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
linux-4.9.37/kernel/rcu/tree_plugin.h
--- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12 06:42:41.0
-0700
+++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 11:57:03.582134519
-0700
@@ -2077,7 +2077,7 @@
tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
*tail = rdp->nocb_gp_head;
smp_mb__after_atomic(); /* Store *tail before wakeup. */
- if (rdp != my_rdp && tail == >nocb_follower_head) {
+ if (rdp != my_rdp) {
/*
* List was empty, wake up the follower.
* Memory barriers supplied by atomic_long_add().
From: David Chen
Sent: Friday, July 20, 2018 5:12 PM
To: paul...@linux.vnet.ibm.com
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
Ok, I'll try those patches.
Thanks,
David
From: Paul E. McKenney
Sent: Friday, July 20, 2018 4:32:12 PM
To: David Chen
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 20, 2018 at 11:05:52PM +, David Chen wrote:
> Hi Paul,
>
> We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows too
> large, and not getting reclaimed, causing the system to OOM.
>
> Printing the culprit rcu_sched_data:
>
> nocb_q_count = {
> counter = 32369635
> },
> nocb_follower_head = 0x88ae901c0a00,
> nocb_follower_tail = 0x88af1538b8d8,
> nocb_kthread = 0x88b06d29,
>
> As you can see here, the nocb_follower_head is not empty, so in theory, the
> nocb_kthread shouldn't go to sleep. However, if dump the stack of the kthread:
>
> crash> bt 0x88b06d29
> PID: 21 TASK: 88b06d29 CPU: 3 COMMAND: "rcuos/1"
> #0 [afc9020b7dc0] __schedule at 8d8789dc
> #1 [afc9020b7e38] schedule at 8d878e76
> #2 [afc9020b7e50] rcu_nocb_kthread at 8d112337
> #3 [afc9020b7ec8] kthread at 8d0c6ce7
> #4 [afc9020b7f50] ret_from_fork at 8d87d755
>
> And if we dis the address at 8d112337:
>
> /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.07142017.el7.centos.x86_64/kernel/rcu/tree_plugin.h:
> 2106
> 0x8d11232d : test %rax,%rax
> 0x8d112330 : jne 0x8d112355
>
> 0x8d112332 : callq 0x8d878e40
>
> 0x8d112337 : lea -0x40(%rbp),%rsi
>
> So the kthread is blocked at swait_event_interruptible in the
> nocb_follower_wait.
> This contradict with the fact that nocb_follower_head was not empty. So I
> wonder if this is caused by the lack of memory barrier in the place shown
> below.
> If the head is set to NULL after doing xchg, it will overwrite the head set
> by leader. This caused the kthread to sleep the next iteration, and the leader
> won't wake him up as the tail doesn't point to head.
>
> Please tell me what do you think.
>
> Thanks,
> David
>
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-20 15:25:57.311206343
> -0700
> @@ -2149,6 +2149,7 @@
> BUG_ON(!list);
> trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, "WokeNonEmpty");
> WRITE_ONCE(rdp->nocb_follower_head, NULL);
> + smp_mb();
> tail = xchg(>nocb_follower_tail,
>>nocb_follower_head);
The xchg() operation implies full memory barriers both before and after,
so adding the smp_mb() before would have no effect.
But let me take a look at post-4.9 changes to this code...
I suggest trying out the following commit:
6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
If that one doesn't help, the following might be worth trying, but probably
a lot harder to backport:
8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
Please let me know how it goes!
Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
I'd like to opinion again on this subject.
So we are going to backport this patch:
6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
But the other one:
8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
It doesn't apply cleanly, and I'm not too comfortable porting it myself.
So I'm wondering if I use the following change to always wake up follower thread
regardless if the list was empty or not, just to be on the safe side. Do you
think
this change is reasonable? Do you see any problem it might cause?
Thanks,
David
diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
linux-4.9.37/kernel/rcu/tree_plugin.h
--- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12 06:42:41.0
-0700
+++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-27 11:57:03.582134519
-0700
@@ -2077,7 +2077,7 @@
tail = xchg(>nocb_follower_tail, rdp->nocb_gp_tail);
*tail = rdp->nocb_gp_head;
smp_mb__after_atomic(); /* Store *tail before wakeup. */
- if (rdp != my_rdp && tail == >nocb_follower_head) {
+ if (rdp != my_rdp) {
/*
* List was empty, wake up the follower.
* Memory barriers supplied by atomic_long_add().
From: David Chen
Sent: Friday, July 20, 2018 5:12 PM
To: paul...@linux.vnet.ibm.com
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
Hi Paul,
Ok, I'll try those patches.
Thanks,
David
From: Paul E. McKenney
Sent: Friday, July 20, 2018 4:32:12 PM
To: David Chen
Cc: linux-kernel@vger.kernel.org
Subject: Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 20, 2018 at 11:05:52PM +, David Chen wrote:
> Hi Paul,
>
> We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows too
> large, and not getting reclaimed, causing the system to OOM.
>
> Printing the culprit rcu_sched_data:
>
> nocb_q_count = {
> counter = 32369635
> },
> nocb_follower_head = 0x88ae901c0a00,
> nocb_follower_tail = 0x88af1538b8d8,
> nocb_kthread = 0x88b06d29,
>
> As you can see here, the nocb_follower_head is not empty, so in theory, the
> nocb_kthread shouldn't go to sleep. However, if dump the stack of the kthread:
>
> crash> bt 0x88b06d29
> PID: 21 TASK: 88b06d29 CPU: 3 COMMAND: "rcuos/1"
> #0 [afc9020b7dc0] __schedule at 8d8789dc
> #1 [afc9020b7e38] schedule at 8d878e76
> #2 [afc9020b7e50] rcu_nocb_kthread at 8d112337
> #3 [afc9020b7ec8] kthread at 8d0c6ce7
> #4 [afc9020b7f50] ret_from_fork at 8d87d755
>
> And if we dis the address at 8d112337:
>
> /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.07142017.el7.centos.x86_64/kernel/rcu/tree_plugin.h:
> 2106
> 0x8d11232d : test %rax,%rax
> 0x8d112330 : jne 0x8d112355
>
> 0x8d112332 : callq 0x8d878e40
>
> 0x8d112337 : lea -0x40(%rbp),%rsi
>
> So the kthread is blocked at swait_event_interruptible in the
> nocb_follower_wait.
> This contradict with the fact that nocb_follower_head was not empty. So I
> wonder if this is caused by the lack of memory barrier in the place shown
> below.
> If the head is set to NULL after doing xchg, it will overwrite the head set
> by leader. This caused the kthread to sleep the next iteration, and the leader
> won't wake him up as the tail doesn't point to head.
>
> Please tell me what do you think.
>
> Thanks,
> David
>
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h 2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-20 15:25:57.311206343
> -0700
> @@ -2149,6 +2149,7 @@
> BUG_ON(!list);
> trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, "WokeNonEmpty");
> WRITE_ONCE(rdp->nocb_follower_head, NULL);
> + smp_mb();
> tail = xchg(>nocb_follower_tail,
>>nocb_follower_head);
The xchg() operation implies full memory barriers both before and after,
so adding the smp_mb() before would have no effect.
But let me take a look at post-4.9 changes to this code...
I suggest trying out the following commit:
6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
If that one doesn't help, the following might be worth trying, but probably
a lot harder to backport:
8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
Please let me know how it goes!
Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 20, 2018 at 11:05:52PM +, David Chen wrote:
> Hi Paul,
>
> We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows too
> large, and not getting reclaimed, causing the system to OOM.
>
> Printing the culprit rcu_sched_data:
>
> nocb_q_count = {
> counter = 32369635
> },
> nocb_follower_head = 0x88ae901c0a00,
> nocb_follower_tail = 0x88af1538b8d8,
> nocb_kthread = 0x88b06d29,
>
> As you can see here, the nocb_follower_head is not empty, so in theory, the
> nocb_kthread shouldn't go to sleep. However, if dump the stack of the kthread:
>
> crash> bt 0x88b06d29
> PID: 21 TASK: 88b06d29 CPU: 3 COMMAND: "rcuos/1"
> #0 [afc9020b7dc0] __schedule at 8d8789dc
> #1 [afc9020b7e38] schedule at 8d878e76
> #2 [afc9020b7e50] rcu_nocb_kthread at 8d112337
> #3 [afc9020b7ec8] kthread at 8d0c6ce7
> #4 [afc9020b7f50] ret_from_fork at 8d87d755
>
> And if we dis the address at 8d112337:
>
> /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.07142017.el7.centos.x86_64/kernel/rcu/tree_plugin.h:
> 2106
> 0x8d11232d : test %rax,%rax
> 0x8d112330 : jne0x8d112355
>
> 0x8d112332 : callq 0x8d878e40
>
> 0x8d112337 : lea-0x40(%rbp),%rsi
>
> So the kthread is blocked at swait_event_interruptible in the
> nocb_follower_wait.
> This contradict with the fact that nocb_follower_head was not empty. So I
> wonder if this is caused by the lack of memory barrier in the place shown
> below.
> If the head is set to NULL after doing xchg, it will overwrite the head set
> by leader. This caused the kthread to sleep the next iteration, and the leader
> won't wake him up as the tail doesn't point to head.
>
> Please tell me what do you think.
>
> Thanks,
> David
>
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-20 15:25:57.311206343
> -0700
> @@ -2149,6 +2149,7 @@
> BUG_ON(!list);
> trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, "WokeNonEmpty");
> WRITE_ONCE(rdp->nocb_follower_head, NULL);
> + smp_mb();
> tail = xchg(>nocb_follower_tail, >nocb_follower_head);
The xchg() operation implies full memory barriers both before and after,
so adding the smp_mb() before would have no effect.
But let me take a look at post-4.9 changes to this code...
I suggest trying out the following commit:
6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
If that one doesn't help, the following might be worth trying, but probably
a lot harder to backport:
8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
Please let me know how it goes!
Thanx, Paul
commit 6b5fc3a1331810db407c9e0e673dc1837afdc9d0
Author: Paul E. McKenney
Date: Fri Apr 28 20:11:09 2017 -0700
rcu: Add memory barriers for NOCB leader wakeup
Wait/wakeup operations do not guarantee ordering on their own. Instead,
either locking or memory barriers are required. This commit therefore
adds memory barriers to wake_nocb_leader() and nocb_leader_wait().
Signed-off-by: Paul E. McKenney
Tested-by: Krister Johansen
Cc: # 4.6.x
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0b1042545116..573fbe9640a0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1810,6 +1810,7 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool
force)
if (READ_ONCE(rdp_leader->nocb_leader_sleep) || force) {
/* Prior smp_mb__after_atomic() orders against prior enqueue. */
WRITE_ONCE(rdp_leader->nocb_leader_sleep, false);
+ smp_mb(); /* ->nocb_leader_sleep before swake_up(). */
swake_up(_leader->nocb_wq);
}
}
@@ -2064,6 +2065,7 @@ static void nocb_leader_wait(struct rcu_data *my_rdp)
* nocb_gp_head, where they await a grace period.
*/
gotcbs = false;
+ smp_mb(); /* wakeup before ->nocb_head reads. */
for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_follower) {
rdp->nocb_gp_head = READ_ONCE(rdp->nocb_head);
if (!rdp->nocb_gp_head)
Re: RCU nocb list not reclaiming causing OOM
On Fri, Jul 20, 2018 at 11:05:52PM +, David Chen wrote:
> Hi Paul,
>
> We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows too
> large, and not getting reclaimed, causing the system to OOM.
>
> Printing the culprit rcu_sched_data:
>
> nocb_q_count = {
> counter = 32369635
> },
> nocb_follower_head = 0x88ae901c0a00,
> nocb_follower_tail = 0x88af1538b8d8,
> nocb_kthread = 0x88b06d29,
>
> As you can see here, the nocb_follower_head is not empty, so in theory, the
> nocb_kthread shouldn't go to sleep. However, if dump the stack of the kthread:
>
> crash> bt 0x88b06d29
> PID: 21 TASK: 88b06d29 CPU: 3 COMMAND: "rcuos/1"
> #0 [afc9020b7dc0] __schedule at 8d8789dc
> #1 [afc9020b7e38] schedule at 8d878e76
> #2 [afc9020b7e50] rcu_nocb_kthread at 8d112337
> #3 [afc9020b7ec8] kthread at 8d0c6ce7
> #4 [afc9020b7f50] ret_from_fork at 8d87d755
>
> And if we dis the address at 8d112337:
>
> /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.07142017.el7.centos.x86_64/kernel/rcu/tree_plugin.h:
> 2106
> 0x8d11232d : test %rax,%rax
> 0x8d112330 : jne0x8d112355
>
> 0x8d112332 : callq 0x8d878e40
>
> 0x8d112337 : lea-0x40(%rbp),%rsi
>
> So the kthread is blocked at swait_event_interruptible in the
> nocb_follower_wait.
> This contradict with the fact that nocb_follower_head was not empty. So I
> wonder if this is caused by the lack of memory barrier in the place shown
> below.
> If the head is set to NULL after doing xchg, it will overwrite the head set
> by leader. This caused the kthread to sleep the next iteration, and the leader
> won't wake him up as the tail doesn't point to head.
>
> Please tell me what do you think.
>
> Thanks,
> David
>
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h
> linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h2017-07-12
> 06:42:41.0 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h 2018-07-20 15:25:57.311206343
> -0700
> @@ -2149,6 +2149,7 @@
> BUG_ON(!list);
> trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, "WokeNonEmpty");
> WRITE_ONCE(rdp->nocb_follower_head, NULL);
> + smp_mb();
> tail = xchg(>nocb_follower_tail, >nocb_follower_head);
The xchg() operation implies full memory barriers both before and after,
so adding the smp_mb() before would have no effect.
But let me take a look at post-4.9 changes to this code...
I suggest trying out the following commit:
6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")
If that one doesn't help, the following might be worth trying, but probably
a lot harder to backport:
8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")
Please let me know how it goes!
Thanx, Paul
commit 6b5fc3a1331810db407c9e0e673dc1837afdc9d0
Author: Paul E. McKenney
Date: Fri Apr 28 20:11:09 2017 -0700
rcu: Add memory barriers for NOCB leader wakeup
Wait/wakeup operations do not guarantee ordering on their own. Instead,
either locking or memory barriers are required. This commit therefore
adds memory barriers to wake_nocb_leader() and nocb_leader_wait().
Signed-off-by: Paul E. McKenney
Tested-by: Krister Johansen
Cc: # 4.6.x
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0b1042545116..573fbe9640a0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1810,6 +1810,7 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool
force)
if (READ_ONCE(rdp_leader->nocb_leader_sleep) || force) {
/* Prior smp_mb__after_atomic() orders against prior enqueue. */
WRITE_ONCE(rdp_leader->nocb_leader_sleep, false);
+ smp_mb(); /* ->nocb_leader_sleep before swake_up(). */
swake_up(_leader->nocb_wq);
}
}
@@ -2064,6 +2065,7 @@ static void nocb_leader_wait(struct rcu_data *my_rdp)
* nocb_gp_head, where they await a grace period.
*/
gotcbs = false;
+ smp_mb(); /* wakeup before ->nocb_head reads. */
for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_follower) {
rdp->nocb_gp_head = READ_ONCE(rdp->nocb_head);
if (!rdp->nocb_gp_head)
