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SubjectRe: [PATCH] sched/core: Fix forceidle balancing
On Thu, Mar 31, 2022 at 03:00:40PM -0400, Joel Fernandes wrote:
> Hi,
>
> By the way, might be slightly related - we still see crashes with
> pick_task_fair() in our kernel even with this change:
> https://lkml.org/lkml/2020/11/17/2137

Please as to not use lkml.org. Please use something with a MsgID in like
lore.

> Is it possible that when doing pick_task_fair() especially on a remote
> CPU, both the "cfs_rq->curr" and the rbtree's "left" be NULL with core
> scheduling? In this case, se will be NULL and can cause crashes right?
> I think the code assumes this can never happen.
>
> +Guenter Roeck kindly debugged pick_task_fair() in a crash as
> follows. Copying some details he mentioned in a bug report:
>
> Assembler/source:
>
> 25: e8 4f 11 00 00 call 0x1179 ; se =
> pick_next_entity(cfs_rq, curr);
> 2a:* 48 8b 98 60 01 00 00 mov 0x160(%rax),%rbx ; trapping
> instruction [cfs_rq = group_cfs_rq(se);]
> 31: 48 85 db test %rbx,%rbx
> 34: 75 d1 jne 0x7
> 36: 48 89 c7 mov %rax,%rdi
>
> At 2a: RAX = se == NULL after pick_next_entity(). Looking closely into
> pick_next_entity(), it can indeed return NULL if curr is NULL and if
> left in pick_next_entity() is NULL. Per line 7:, curr is in %r14 and
> indeed 0.
>
> Thoughts?

It is possible for ->curr and ->leftmost to be NULL, but then we should
also be having ->nr_running == 0 and not call pick in the first place.
Because picking a task from no tasks doesn't make much sense.

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