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

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Message-ID: <5152C83F.6060509@linux.vnet.ibm.com>
Date:	Wed, 27 Mar 2013 15:51:51 +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,

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?


@Vincent: Yes the comment above needs to be fixed. A busy buddy is a CPU
with *high rq utilization*, as far as the equation goes.

Regards
Preeti U Murthy

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