linux-kernel - Re: [PATCH v2 1/2] sched/fair: Fix how load gets propagated from cfs_rq to its sched

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Date:   Wed, 3 May 2017 17:45:46 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
        Chris Mason <clm@...com>, kernel-team@...com
Subject: Re: [PATCH v2 1/2] sched/fair: Fix how load gets propagated from
 cfs_rq to its sched_entity

Hello, Peter.

On Wed, May 03, 2017 at 08:00:28PM +0200, Peter Zijlstra wrote:
> Just FUDGE2 on its own seems to be the best on my system and is a change
> that makes sense (and something Paul recently pointed out as well).
> 
> The implementation isn't particularly pretty or fast, but should
> illustrate the idea.
> 
> Poking at the whole update_tg_cfs_load() thing only makes it worse after
> that. And while I agree that that code is mind bending; it seems to work
> OK-ish.
> 
> Tejun, Vincent, could you guys have a poke?

So, just preliminary testing.

FUDGE: Does cut down the number of wrong picks by about 70% and p99
       latency by about half; however, the resulting p99 is still
       worse by 5 - 10 times compared to !cgroup case.

FUDGE2: Changes things a lot (load values go wild) but only because
        it's missing scale_load_down().  After adding
        scale_load_down(), it doesn't do much.  For this to work, it
        needs to be always propagated, which btw shouldn't be
        prohibitively expensive given other operations which are
        performed at the same time.

Thanks.

-- 
tejun