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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 5 Mar 2020 14:02:00 +0100
Message-ID: <CAKfTPtBpY+KCJU7KR+Z=cq6L7cy3uWqaFT1PpKjmDj_2Wy+48A@mail.gmail.com>
Subject: Re: 5.6-rc3: WARNING: CPU: 48 PID: 17435 at kernel/sched/fair.c:380 enqueue_task_fair+0x328/0x440
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, 5 Mar 2020 at 13:49, Christian Borntraeger
<borntraeger@de.ibm.com> wrote:
>
>
>
> On 05.03.20 13:33, Vincent Guittot wrote:
> > Le jeudi 05 mars 2020 =C3=A0 13:12:39 (+0100), Dietmar Eggemann a =C3=
=A9crit :
> >> On 05/03/2020 12:28, Christian Borntraeger wrote:
> >>>
> >>> On 05.03.20 10:30, Vincent Guittot wrote:
> >>>> Le mercredi 04 mars 2020 =C3=A0 20:59:33 (+0100), Christian Borntrae=
ger a =C3=A9crit :
> >>>>>
> >>>>> 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=3D/machine.slice/machine-test.slice/machine-=
qemu\x2d18\x2dtest10. on_list=3D1 nr_running=3D1 throttled=3D0 p=3D[CPU 2/K=
VM 2662]
> >> [   87.932559] CPU23 path=3D/machine.slice/machine-test.slice/machine-=
qemu\x2d18\x2dtest10. on_list=3D0 nr_running=3D3 throttled=3D0 p=3D[CPU 2/K=
VM 2662]
> >> [   87.932562] CPU23 path=3D/machine.slice/machine-test.slice on_list=
=3D1 nr_running=3D1 throttled=3D1 p=3D[CPU 2/KVM 2662]
> >> [   87.932564] CPU23 path=3D/machine.slice on_list=3D1 nr_running=3D0 =
throttled=3D0 p=3D[CPU 2/KVM 2662]
> >> [   87.932566] CPU23 path=3D/ on_list=3D1 nr_running=3D1 throttled=3D0=
 p=3D[CPU 2/KVM 2662]
> >> [   87.951872] CPU23 path=3D/ on_list=3D1 nr_running=3D2 throttled=3D0=
 p=3D[ksoftirqd/23 126]
> >> [   87.987528] CPU23 path=3D/user.slice on_list=3D1 nr_running=3D2 thr=
ottled=3D0 p=3D[as 6737]
> >> [   87.987533] CPU23 path=3D/ on_list=3D1 nr_running=3D1 throttled=3D0=
 p=3D[as 6737]
> >>
> >> Arrh, looks like 'char path[64]' is too small to hold 'machine.slice/m=
achine-test.slice/machine-qemu\x2d18\x2dtest10.scope/vcpuX' !
> >>                                                                       =
                                              ^
> >> But I guess that the 'on_list=3D0' for 'machine-qemu\x2d18\x2dtest10.s=
cope' could be the missing hint?
> >
> > yes the if (cfs_bandwidth_used()) at the end of enqueue_task_fair is no=
t enough
> > to ensure that all cfs will be added back. It will "work" for the 1st e=
nqueue
> > 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 sc=
hed_entity *se, int flags)
> >                 __enqueue_entity(cfs_rq, se);
> >         se->on_rq =3D 1;
> >
> > -       if (cfs_rq->nr_running =3D=3D 1) {
> > +       /*
> > +        * When bandwidth control is enabled, cfs might have been remov=
ed because of
> > +        * a parent been throttled but cfs->nr_running > 1. Try to add =
it
> > +        * unconditionnally.
> > +        */
> > +       if (cfs_rq->nr_running =3D=3D 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 =3D=3D 1)
> >                 check_enqueue_throttle(cfs_rq);
> > -       }
> >  }
> >
> >  static void __clear_buddies_last(struct sched_entity *se)
>