linux-kernel - Re: Null pointer crash at find_idlest

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Date:   Wed, 4 Dec 2019 10:16:36 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Patrick Bellasi <Patrick.Bellasi@....com>,
        Ingo Molnar <mingo@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Null pointer crash at find_idlest_group on db845c w/ linus/master

On Wed, Dec 4, 2019 at 2:09 AM Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> Le Wednesday 04 Dec 2019 à 09:42:17 (+0000), Qais Yousef a écrit :
> > On 12/04/19 09:06, Vincent Guittot wrote:
> > > Hi John,
> > >
> > > On Tue, 3 Dec 2019 at 20:15, John Stultz <john.stultz@...aro.org> wrote:
> > > >
> > > > With today's linus/master on db845c running android, I'm able to
> > > > fairly easily reproduce the following crash. I've not had a chance to
> > > > bisect it yet, but I'm suspecting its connected to Vincent's recent
> > > > rework.
> > >
> > > Does the crash happen randomly or after a specific action ?
> > > I have a db845 so I can try to reproduce it locally.
> >
> > Isn't there a chance we use local_sgs without initializing it in that function?
>
> Normally not because the cpu belongs to its sched_domain
>
> Now, we test that a group has at least one allowed CPU for the task so we
> could skip the local group with the correct/wrong p->cpus_ptr
>
> The path is used for fork/exec ibut also for wakeup path for b.L when the task doesn't fit in the CPUs
>
> So we can probably imagine a scenario where we change task affinity while
> sleeping. If the wakeup happens on a CPU that belongs to the group that is not
> allowed, we can imagine that we skip the local_group
>
> John,
>
> Could you try the fix below ?
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e..bcd216d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8417,6 +8417,10 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>         if (!idlest)
>                 return NULL;
>
> +       /* The local group has been skipped because of cpu affinity */
> +       if (!local)
> +               return idlest;
> +
>         /*
>          * If the local group is idler than the selected idlest group
>          * don't try and push the task.

This patch does seem to solve the issue for me! Thanks so much!

Tested-by: John Stultz <john.stultz@...aro.org>
-john