linux-kernel - Re: 5.6-rc3: WARNING: CPU: 48 PID: 17435 at kernel/sched/fair.c:380 enqueue_task

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Date:   Thu, 5 Mar 2020 14:02:00 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: 5.6-rc3: WARNING: CPU: 48 PID: 17435 at kernel/sched/fair.c:380 enqueue_task_fair+0x328/0x440

On Thu, 5 Mar 2020 at 13:49, Christian Borntraeger
<borntraeger@...ibm.com> wrote:
>
>
>
> On 05.03.20 13:33, Vincent Guittot wrote:
> > Le jeudi 05 mars 2020 à 13:12:39 (+0100), Dietmar Eggemann a écrit :
> >> On 05/03/2020 12:28, Christian Borntraeger wrote:
> >>>
> >>> On 05.03.20 10:30, Vincent Guittot wrote:
> >>>> Le mercredi 04 mars 2020 à 20:59:33 (+0100), Christian Borntraeger a écrit :
> >>>>>
> >>>>> On 04.03.20 20:38, Christian Borntraeger wrote:
> >>>>>>
> >>>>>>
> >>>>>> On 04.03.20 20:19, Dietmar Eggemann wrote:
> >>
> >> [...]
> >>
> >>> It seems to speed up the issue when I do a compile job in parallel on the host:
> >>>
> >>> Do you also need the sysfs tree?
> >>
> >> [   87.932552] CPU23 path=/machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10. on_list=1 nr_running=1 throttled=0 p=[CPU 2/KVM 2662]
> >> [   87.932559] CPU23 path=/machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10. on_list=0 nr_running=3 throttled=0 p=[CPU 2/KVM 2662]
> >> [   87.932562] CPU23 path=/machine.slice/machine-test.slice on_list=1 nr_running=1 throttled=1 p=[CPU 2/KVM 2662]
> >> [   87.932564] CPU23 path=/machine.slice on_list=1 nr_running=0 throttled=0 p=[CPU 2/KVM 2662]
> >> [   87.932566] CPU23 path=/ on_list=1 nr_running=1 throttled=0 p=[CPU 2/KVM 2662]
> >> [   87.951872] CPU23 path=/ on_list=1 nr_running=2 throttled=0 p=[ksoftirqd/23 126]
> >> [   87.987528] CPU23 path=/user.slice on_list=1 nr_running=2 throttled=0 p=[as 6737]
> >> [   87.987533] CPU23 path=/ on_list=1 nr_running=1 throttled=0 p=[as 6737]
> >>
> >> Arrh, looks like 'char path[64]' is too small to hold 'machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10.scope/vcpuX' !
> >>                                                                                                                     ^
> >> But I guess that the 'on_list=0' for 'machine-qemu\x2d18\x2dtest10.scope' could be the missing hint?
> >
> > yes the if (cfs_bandwidth_used()) at the end of enqueue_task_fair is not enough
> > to ensure that all cfs will be added back. It will "work" for the 1st enqueue
> > because the throttled cfs will be added and will reset tmp_alone_branch but not
> > for the next one
> >
> > Compare to the previous proposed fix, we can optimize it a bit with:
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 9ccde775e02e..3b19e508641d 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4035,10 +4035,16 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >                 __enqueue_entity(cfs_rq, se);
> >         se->on_rq = 1;
> >
> > -       if (cfs_rq->nr_running == 1) {
> > +       /*
> > +        * When bandwidth control is enabled, cfs might have been removed because of
> > +        * a parent been throttled but cfs->nr_running > 1. Try to add it
> > +        * unconditionnally.
> > +        */
> > +       if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
>
> This needs a forward declaration for cfs_bandwidth_used, but with that it compiles fine
> and its seems to work fine so far. Will keep it running for while.

ok. Thanks

>
> >                 list_add_leaf_cfs_rq(cfs_rq);
> > +
> > +       if (cfs_rq->nr_running == 1)
> >                 check_enqueue_throttle(cfs_rq);
> > -       }
> >  }
> >
> >  static void __clear_buddies_last(struct sched_entity *se)
>