linux-kernel - Re: [PATCH v2 for-4.12-fixes 2/2] sched/fair: Fix O(# total cgroups) in load balance path

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Date:   Fri, 12 May 2017 16:36:31 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
        Chris Mason <clm@...com>, kernel-team@...com
Subject: Re: [PATCH v2 for-4.12-fixes 2/2] sched/fair: Fix O(# total cgroups)
 in load balance path

On 12 May 2017 at 15:16, Tejun Heo <tj@...nel.org> wrote:
> Hello, Vincent.
>
> On Thu, May 11, 2017 at 09:02:22AM +0200, Vincent Guittot wrote:
>> Sorry, what i mean is:
>> When the group entity of a cfs_rq is enqueued, we are sure that either
>> the parents is already enqueued or it will be enqueued in the same
>> sequence. We must be sure that no other branch will be enqueued in the
>> middle of the sequence and will reset tmp_alone_branch.
>> This is true with current implementation but I  wondered it can happen
>> if we del/add the cfs_rq out of order
>>
>> That said i haven't find a use case that break the sequence
>
> Hmm... a cfs_rq can be removed from leaf list iff it's empty and
> dequeued, and enqueueing is always from top down.  If an ancestor is
> already enqueued, it's guaranteed to be on the leaf list; otherwise,
> it's guaranteed to be enqueued beforehand and thus put on the leaf
> list too.  I think it should be fine.

I agree

FWIW
Acked-by: Vincent Guittot <vincent.guittot@...aro.org>

>
> Thanks.
>
> --
> tejun