linux-kernel - Re: [PATCH] sched/fair: initialize throttle

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Message-ID: <20160621211054.GV30154@twins.programming.kicks-ass.net>
Date:	Tue, 21 Jun 2016 23:10:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] sched/fair: initialize throttle_count for new
 task-groups lazily

On Thu, Jun 16, 2016 at 03:57:01PM +0300, Konstantin Khlebnikov wrote:
> Cgroup created inside throttled group must inherit current throttle_count.
> Broken throttle_count allows to nominate throttled entries as a next buddy,
> later this leads to null pointer dereference in pick_next_task_fair().
> 
> This patch initialize cfs_rq->throttle_count at first enqueue: laziness
> allows to skip locking all rq at group creation. Lazy approach also allows
> to skip full sub-tree scan at throttling hierarchy (not in this patch).

You're talking about taking rq->lock in alloc_fair_sched_group(), right?

We're about to go do that anyway... But I suppose for backports this
makes sense. Doing it at creation time also avoids the issues Ben
raised, right?