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

From: Tejun Heo <tj@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Mike Galbraith <efault@gmx.de>, Paul Turner <pjt@google.com>,
	Chris Mason <clm@fb.com>,
	kernel-team@fb.com
Subject: Re: [PATCH v2 1/2] sched/fair: Fix how load gets propagated from cfs_rq to its sched_entity
Date: Wed, 3 May 2017 17:45:46 -0400	[thread overview]
Message-ID: <20170503214546.GA7451@htj.duckdns.org> (raw)
In-Reply-To: <20170503180028.ejf73et3pc4meqji@hirez.programming.kicks-ass.net>

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