On Wed, Jul 26, 2017 at 10:50:13AM -0700, Paul E. McKenney wrote:
> On Wed, Jul 26, 2017 at 09:54:32AM -0700, David Miller wrote:
> > From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
> > Date: Wed, 26 Jul 2017 08:49:00 -0700
> >
> > > On Wed, Jul 26, 2017 at 04:33:40PM +0100, Jonathan Cameron wrote:
> > >> Didn't leave it long enough. Still bad on 4.10-rc7 just took over
> > >> an hour to occur.
> > >
> > > And it is quite possible that SOFTLOCKUP_DETECTOR=y and HZ_PERIODIC=y
> > > are just greatly reducing the probability of the problem rather than
> > > completely preventing it.
> > >
> > > Still, hopefully useful information, thank you for the testing!
> >
> > I guess that invalidates my idea to test reverting recent changes to
> > the tick-sched.c code... :-/
> >
> > In NO_HZ_IDLE mode, what is really supposed to happen on a completely
> > idle system?
> >
> > All the cpus enter the idle loop, have no timers programmed, and they
> > all just go to sleep until an external event happens.
> >
> > What ensures that grace periods get processed in this regime?
>
> There are several different situations with different mechanisms:
>
> 1. No grace period is in progress and no RCU callbacks are pending
> anywhere in the system. In this case, some other event would
> need to start a grace period, so RCU just stays idle until that
> happens, possibly indefinitely. According to the battery-powered
> embedded guys, this is a feature, not a bug. ;-)
>
> 2. No grace period is in progress, but there is at least one RCU
> callback somewhere in the system. In this case, the mechanism
> depends on CONFIG_RCU_FAST_NO_HZ:
>
> CONFIG_RCU_FAST_NO_HZ=n: The CPU on which the callback is
> queued will return "true" in response to the call to
> rcu_needs_cpu() that is made shortly before that CPU
> enters idle. This will cause the scheduling-clock
> interrupt to remain on, despite the CPU being idle,
> which will in turn allow RCU's state machine to continue
> running out of softirq, triggered by the scheduling-clock
> interrupts.
>
> CONFIG_RCU_FAST_NO_HZ=y: The CPU on which the callback is queued
> will return "false" in response to the call to
> rcu_needs_cpu() that is made shortly before that CPU
> enters idle. However, it will also request a next event
> about six seconds in the future if all callbacks do
> nothing but free memory (kfree_rcu()), or about four
> jiffies in the future if at least one callback does
> something more than just free memory.
>
> There is also a rcu_prepare_for_idle() function that
> is invoked later in the idle-entry process in this case
> which will wake up the grace-period kthread if need be.
>
> 3. A grace period is in progress. In this case the grace-period
> kthread is either currently running (in which case there will be
> at least one non-idle CPU) or is in a timed wait for its next
> scan for idle/offline CPUs (such CPUs need the grace-period
> kthread to report quiescent states on their behalf). In this
> latter case, the timer subsystem will post a next event that
> will be the wakeup time for the grace-period kthread, or some
> earlier event.
>
> This is where we have been seeing trouble, if for no other
> reason because RCU CPU stall warnings only happen when there
> is a grace period in progress.
>
> That is the theory, anyway...
>
> And when I enabled CONFIG_SOFTLOCKUP_DETECTOR, I still see failures.
> I did 24 half-hour rcutorture runs on the TREE01 scenario, and two of them
> saw RCU CPU stall warnings with starvation of the grace-period kthread.
> I just now started another test but without CONFIG_SOFTLOCKUP_DETECTOR
> to see if it makes a significance difference for my testing. I do have
> CONFIG_RCU_FAST_NO_HZ=y in my runs.
And without CONFIG_SOFTLOCKUP_DETECTOR, I see five runs of 24 with RCU
CPU stall warnings. So it seems likely that CONFIG_SOFTLOCKUP_DETECTOR
really is having an effect.
Thanx, Paul
