linux-kernel - Re: [PATCH 2/2] sched/fair: Always propagate runnable_load

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Date:   Tue, 25 Apr 2017 11:49:41 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Vincent Guittot <vincent.guittot@...aro.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 2/2] sched/fair: Always propagate runnable_load_avg

Hello,

On Tue, Apr 25, 2017 at 02:59:18PM +0200, Vincent Guittot wrote:
> >> So you are changing the purpose of propagate_entity_load_avg which
> >> aims to propagate load_avg/util_avg changes only when a task migrate
> >> and you also want to propagate the enqueue/dequeue in the parent
> >> cfs_rq->runnable_load_avg

Yeah, it always propagates runnable_load_avg and load_avg/util_avg too
on migrations.

> > In fact you want that sched_entity load_avg reflects
> > cfs_rq->runnable_load_avg and not cfs_rq->avg.load_avg

Yes, that's how it gets changed.  The load balancer assumes that the
root's runnable_load_avg is the total sum of all currently active
tasks.  Nesting cfs_rq's shouldn't change that and how it should be
mapped is clearly defined (scaled recursively till it reaches the
root), which is what the code calculates.  The change in
cfs_rq->avg.load_avg's behavior is to reflect that immediate
propagation as load_avg and runnable_load_avg are tightly coupled.

While it does change a nested cfs_rq's load_avg behavior.  It sheds of
the extra layer of averaging and directly reflects the scaled load
avgs of its members, which are already time averaged.  I could have
missed something but couldn't spot anything which can break from this.

> I have run a quick test with your patches and schbench on my platform.
> I haven't been able to reproduce your regression but my platform is
> quite different from yours (only 8 cores without SMT)
> But most importantly, the parent cfs_rq->runnable_load_avg never
> reaches 0 (or almost 0) when it is idle. Instead, it still has a
> runnable_load_avg (this is not due to rounding computation) whereas
> runnable_load_avg should be 0

Heh, let me try that out.  Probably a silly mistake somewhere.

> Just to be curious, Is your regression still there if you disable
> SMT/hyperthreading on your paltform?

Will try that too.  I can't see why HT would change it because I see
single CPU queues misevaluated.  Just in case, you need to tune the
test params so that it doesn't load the machine too much and that
there are some non-CPU intensive workloads going on to purturb things
a bit.  Anyways, I'm gonna try disabling HT.

Thanks.

-- 
tejun