On Fri, Mar 15, 2019 at 09:51:25AM -0400, Phil Auld wrote:
> On Fri, Mar 15, 2019 at 11:33:57AM +0100 Peter Zijlstra wrote:
> > On Fri, Mar 15, 2019 at 11:11:50AM +0100, Peter Zijlstra wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index ea74d43924b2..b71557be6b42 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -4885,6 +4885,8 @@ static enum hrtimer_restart
> > > sched_cfs_slack_timer(struct hrtimer *timer)
> > > return HRTIMER_NORESTART;
> > > }
> > >
> > > +extern const u64 max_cfs_quota_period;
> > > +
> > > static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
> > > {
> > > struct cfs_bandwidth *cfs_b =
> > > @@ -4892,6 +4894,7 @@ static enum hrtimer_restart
> > > sched_cfs_period_timer(struct hrtimer *timer)
> > > unsigned long flags;
> > > int overrun;
> > > int idle = 0;
> > > + int count = 0;
> > >
> > > raw_spin_lock_irqsave(&cfs_b->lock, flags);
> > > for (;;) {
> > > @@ -4899,6 +4902,28 @@ static enum hrtimer_restart
> > > sched_cfs_period_timer(struct hrtimer *timer)
> > > if (!overrun)
> > > break;
> > >
> > > + if (++count > 3) {
> > > + u64 new, old = ktime_to_ns(cfs_b->period);
> > > +
> > > + new = (old * 147) / 128; /* ~115% */
> > > + new = min(new, max_cfs_quota_period);
> >
> > Also, we can still engineer things to come unstuck; if we explicitly
> > configure period at 1e9 and then set a really small quota and then
> > create this insane amount of cgroups you have..
> >
> > this code has no room to manouvre left.
> >
> > Do we want to do anything about that? Or leave it as is, don't do that
> > then?
> >
>
> If the period is 1s it would be hard to make this loop fire repeatedly. I
> don't think
> it's that dependent on the quota other than getting some rqs throttled. The
> small quota
> would also mean fewer of them would get unthrottled per distribute call.
> You'd probably
> need _significantly_ more cgroups than my insane 2500 to hit it.
>
> Right now it settles out with a new period of ~12-15ms. So ~200,000 cgroups?
>
> Ben and I talked a little about this in another thread. I think hitting this
> is enough of
> an edge case that this approach will make the problem go away. The only
> alternative we
> came up with to reduce the time taken in unthrottle involved a fair bit of
> complexity
> added to the every day code paths. And might not help if the children all
> had their
> own quota/period settings active.
Ah right. I forgot that part. And yes, I remember what was proposed to
avoid the tree walk, that wouldn't have been pretty.
