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SubjectRe: Null pointer crash at find_idlest_group on db845c w/ linus/master
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@linaro.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.

>
> AFAICS we define local_sgs on the stack but not always could be populated with
> the right values. I can't see tmp_sgs being used in the function too. Could
> this cause the/a problem?
>
> 8377 struct sg_lb_stats local_sgs, tmp_sgs;
> .
> .
> .
> 8399 if (local_group) {
> 8400 sgs = &local_sgs;
> 8401 local = group;
> 8402 } else {
> 8403 sgs = &tmp_sgs;
> 8404 }
> 8405
> 8406 update_sg_wakeup_stats(sd, group, sgs, p);
>
> Cheers
>
> --
> Qais Youef

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