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

[Balbir Singh <bsingharora@gmail.com>]

On Fri, Nov 20, 2020 at 01:11:06PM -0500, Vineeth Pillai wrote:
> 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@infradead.org>
> > > 
> > > 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@linux.microsoft.com/
> 
> 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..
>

Yes, it does!

Thanks,
Balbir Singh.