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

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Date:	Wed, 22 Jun 2016 11:10:46 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Peter Zijlstra <peterz@...radead.org>
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 22.06.2016 00:10, Peter Zijlstra wrote:
> 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?

Yes, all will be fine. But for 8192-cores this will be disaster =)
throttle_count must be initialized after linking tg into lists. obviously.

-- 
Konstantin