linux-kernel - Re: [RFC PATCH v3 3/6] sched: pack small tasks

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Date:	Fri, 26 Apr 2013 16:02:27 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, mingo@...nel.org,
	linux@....linux.org.uk, pjt@...gle.com, santosh.shilimkar@...com,
	morten.rasmussen@....com, chander.kashyap@...aro.org,
	cmetcalf@...era.com, tony.luck@...el.com, alex.shi@...el.com,
	paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	len.brown@...el.com, arjan@...ux.intel.com,
	amit.kucheria@...aro.org, corbet@....net
Subject: Re: [RFC PATCH v3 3/6] sched: pack small tasks

Hi Peter,

On 04/26/2013 03:48 PM, Peter Zijlstra wrote:
> On Wed, Mar 27, 2013 at 03:51:51PM +0530, Preeti U Murthy wrote:
>> Hi,
>>
>> On 03/26/2013 05:56 PM, Peter Zijlstra wrote:
>>> On Fri, 2013-03-22 at 13:25 +0100, Vincent Guittot wrote:
>>>> +static bool is_buddy_busy(int cpu)
>>>> +{
>>>> +       struct rq *rq = cpu_rq(cpu);
>>>> +
>>>> +       /*
>>>> +        * A busy buddy is a CPU with a high load or a small load with
>>>> a lot of
>>>> +        * running tasks.
>>>> +        */
>>>> +       return (rq->avg.runnable_avg_sum >
>>>> +                       (rq->avg.runnable_avg_period / (rq->nr_running
>>>> + 2)));
>>>> +}
>>>
>>> Why does the comment talk about load but we don't see it in the
>>> equation. Also, why does nr_running matter at all? I thought we'd
>>> simply bother with utilization, if fully utilized we're done etc..
>>>
>>
>> Peter, lets say the run-queue has 50% utilization and is running 2
>> tasks. And we wish to find out if it is busy. We would compare this
>> metric with the cpu power, which lets say is 100.
>>
>> rq->util * 100 < cpu_of(rq)->power.
>>
>> In the above scenario would we declare the cpu _not_busy? Or would we do
>> the following:
>>
>> (rq->util * 100) * #nr_running <  cpu_of(rq)->power and conclude that it
>> is just enough _busy_ to not take on more processes?
> 
> That is just confused... ->power doesn't have anything to do with a per-cpu
> measure. ->power is a inter-cpu measure of relative compute capacity.

Ok.

> 
> Mixing in nr_running confuses things even more; it doesn't matter how many
> tasks it takes to push utilization up to 100%; once its there the cpu simply
> cannot run more.

True, this is from the perspective of the CPU. But will not the tasks on
this CPU get throttled if, you find the utilization of this CPU < 100%
and decide to put more tasks on it?

Regards
Preeti U Murthy


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