linux-kernel - Re: [PATCH v2 08/11] sched: get CPU's activity statistic

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Date:	Wed, 4 Jun 2014 11:23:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 08/11] sched: get CPU's activity statistic

On Wed, Jun 04, 2014 at 09:55:42AM +0100, Morten Rasmussen wrote:
> Both running_avg and runnable_avg are affected by other tasks on the
> same cpus, but in different ways. They are equal if you only have one
> task on a cpu. If you have more, running_avg will give you the true
> requirement of the tasks until the cpu is fully utilized. At which point
> the task running_avg will drop if you add more tasks (the unweighted sum
> of task running_avgs remains constant).
> 
> runnable_avg on the other hand, might be affected as soon as you have
> two task running on the same cpu if they are runnable at the same time.
> That isn't necessarily a bad thing for load-balancing purposes, because
> tasks that are runnable at the same time are likely to be run more
> efficiently by placing them on different cpus. You might view as at sort
> of built in concurrency factor, somewhat similar to what Yuyang is
> proposing. runnable_avg increases rapidly when the cpu is over-utilized.

Agreed.

> > I'm not sure I see how 100% is possible, but yes I agree that runnable
> > can indeed be inflated due to this queueing effect.
> 
> You should only be able to get to 75% worst case for runnable_avg for
> that example. The total running_avg is 50% no matter if the tasks
> overlaps or not.

Yes, 75% is what I ended up with.

> f you had five tasks on one cpu that each have a 25% requirement you can
> get individual task runnable_avgs of up to 100% (cpu unweighted
> runnable_load_avg can get up 500%, I think), but the task running_avgs
> would be 20% each (total of 100%). 

Yeah, more or less so indeed. I had not considered the queueing effects
on runnable_avg yesterday, so good that that got raised.

That does indeed invalidate my: runnable - running := extra cpu required
thing. It ends up being the extra cpu required for 0 latency but gobs of
idle time, which is something else entirely.

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