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Date:   Fri, 20 Nov 2020 13:11:06 -0500
From:   Vineeth Pillai <>
To:     "Singh, Balbir" <>,
        "Joel Fernandes (Google)" <>,
        Nishanth Aravamudan <>,
        Julien Desfossez <>,
        Peter Zijlstra <>,
        Tim Chen <>,
        Aaron Lu <>,
        Aubrey Li <>,,
        Phil Auld <>,
        Valentin Schneider <>,
        Mel Gorman <>,
        Pawan Gupta <>,
        Paolo Bonzini <>,,
        Chen Yu <>,
        Christian Brauner <>,
        Agata Gruza <>,
        Antonio Gomez Iglesias <>,,,,,,,,
        Alexandre Chartre <>,,,
        Dhaval Giani <>,
        Junaid Shahid <>,,, Ben Segall <>,
        Josh Don <>, Hao Luo <>,
        Tom Lendacky <>,
        Aubrey Li <>,
        "Paul E. McKenney" <>,
        Tim Chen <>
Subject: Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to
 empty rbtree

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 <>
>> 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:

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..


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