linux-kernel - Re: [PATCH -tip 03/32] sched/fair: Fix pick_task

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Date:   Fri, 20 Nov 2020 21:15:28 +1100
From:   "Singh, Balbir" <bsingharora@...il.com>
To:     "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>,
        Vineeth Pillai <viremana@...ux.microsoft.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

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?

The patch looks functionally correct as in, it passes curr as the reference
to pick_next_entity() for left and entity_before comparisons.

Balbir Singh