Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree

[Vineeth Pillai]

Hi Balbir,

On 11/20/20 5:15 AM, Singh, Balbir wrote:
> On 18/11/20 10:19 am, Joel Fernandes (Google) wrote:
> > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >
> > pick_next_entity() is passed curr == NULL during core-scheduling. Due to
> > this, if the rbtree is empty, the 'left' variable is set to NULL within
> > the function. This can cause crashes within the function.
> >
> > This is not an issue if put_prev_task() is invoked on the currently
> > running task before calling pick_next_entity(). However, in core
> > scheduling, it is possible that a sibling CPU picks for another RQ in
> > the core, via pick_task_fair(). This remote sibling would not get any
> > opportunities to do a put_prev_task().
> >
> > Fix it by refactoring pick_task_fair() such that pick_next_entity() is
> > called with the cfs_rq->curr. This will prevent pick_next_entity() from
> > crashing if its rbtree is empty.
> >
> > Also this fixes another possible bug where update_curr() would not be
> > called on the cfs_rq hierarchy if the rbtree is empty. This could effect
> > cross-cpu comparison of vruntime.
> It is not clear from the changelog as to what does put_prev_task() do to prevent
> the crash from occuring? Why did we pass NULL as curr in the first place to
> pick_next_entity?
A little more context on this crash in v8 is here:
https://lwn.net/ml/linux-kernel/8230ada7-839f-2335-9a55-b09f6a813e91@xxxxxxxxxxxxxxxxxxx/

The issue here arises from the fact that, we try to pick task for a
sibling while sibling is running a task. Running tasks are not in the
cfs_rq and pick_next_entity can return NULL if there is only one cfs
task in the cfs_rq. This would not happen normally because
put_prev_task is called before pick_task and put_prev_task adds the
task back to cfs_rq. But for coresched, pick_task is called on a
remote sibling's cfs_rq without calling put_prev_task and this can
lead to pick_next_entity returning NULL.

The initial logic of passing NULL would work fine as long as we call
put_prev_task before calling pick_task_fair. But for coresched, we
call pick_task_fair on siblings while the task is running and would
not be able to call put_prev_task. So this refactor of the code fixes
the crash by explicitly passing curr.

Hope this clarifies..

Thanks,
Vineeth