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

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Message-ID: <20140604094447.GI29593@e103034-lin>
Date:	Wed, 4 Jun 2014 10:44:47 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
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 10:23:13AM +0100, Peter Zijlstra wrote:
> > 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.

Agreed, but I think it is still a useful estimate of the required
compute capacity. If there is a significant difference between runnable
and running on a cpu, the current mix of tasks is not good for latency.
However, we need to treat it as a worst case estimate and not necessarily
try to move exactly runnable-running worth of tasks to another cpu.

So far I haven't been able to come up with something better.
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