linux-kernel - Re: [patch 3/3] sched: move sched_avg_update() to update_cpu

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Date:	Mon, 16 Aug 2010 21:31:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"chris@...stnet.net" <chris@...stnet.net>,
	"debian00@...ceadsl.fr" <debian00@...ceadsl.fr>,
	"hpa@...or.com" <hpa@...or.com>,
	"jonathan.protzenko@...il.com" <jonathan.protzenko@...il.com>,
	"mans@...sr.com" <mans@...sr.com>,
	"psastudio@...l.ru" <psastudio@...l.ru>,
	"rjw@...k.pl" <rjw@...k.pl>,
	"stephan.eicher@....de" <stephan.eicher@....de>,
	"sxxe@....de" <sxxe@....de>,
	"thomas@...hlinux.org" <thomas@...hlinux.org>,
	"venki@...gle.com" <venki@...gle.com>,
	"wonghow@...il.com" <wonghow@...il.com>
Subject: Re: [patch 3/3] sched: move sched_avg_update() to update_cpu_load()

On Mon, 2010-08-16 at 10:46 -0700, Suresh Siddha wrote:

> There is no guarantee that the original cpu won't be doing this in
> parallel with nohz idle load balancing cpu.

Hmm, true.. bugger.

> > > Fix it by moving the sched_avg_update() to more appropriate update_cpu_load()
> > > where the CFS load gets updated aswell.
> > 
> > Right, except it breaks things a bit, at the very least you really need
> > that update right before reading it, otherwise you can end up with >100%
> > fractions, which are odd indeed ;-)
> 
> with the patch, the update always happens before reading it. isn't it?
> 
> update now happens during the scheduler tick (or during nohz load
> balancing tick). And the load balancer gets triggered with the tick.
> So the update (at the tick) should happen before reading it (used by
> load balancing triggered by the tick). Am I missing something?

We run the load-balancer in softirq context, on -rt that's a task, and
we could have ran other (more important) RT tasks between the hardirq
and the softirq running, which would increase the rt_avg and could thus
result in >100%.

But I think we can simply retain the sched_avg_update(rq) in
sched_rt_avg_update(), that is ran with rq->lock held and should be
enough to avoid that case.

We can retain the other bit of you patch, moving sched_avg_update() from
scale_rt_power() to update_cpu_load(), since that is only concerned with
lowering the average when there is no actual activity.


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