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Message-ID: <cab6918c-7b52-923c-4274-f92e9f0a5cd2@gmail.com>
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,
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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
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