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Date:   Fri, 20 Nov 2020 13:11:06 -0500
From:   Vineeth Pillai <viremana@...ux.microsoft.com>
To:     "Singh, Balbir" <bsingharora@...il.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, torvalds@...ux-foundation.org,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
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 <peterz@...radead.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..

Thanks,
Vineeth

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