linux-kernel - Re: [RFC PATCH v2 3/6] sched: pack small tasks

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Date:	Fri, 21 Dec 2012 14:47:12 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Alex Shi <alex.shi@...el.com>, len.brown@...el.com,
	tony.luck@...el.com, linux@....linux.org.uk, peterz@...radead.org,
	arjan@...ux.intel.com, linux-kernel@...r.kernel.org,
	cmetcalf@...era.com, viresh.kumar@...aro.org,
	preeti@...ux.vnet.ibm.com, linaro-dev@...ts.linaro.org,
	tglx@...utronix.de, chander.kashyap@...aro.org, pjt@...gle.com,
	mingo@...nel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH v2 3/6] sched: pack small tasks

Hi Vincent,

On Thu, Dec 13, 2012 at 11:11:11AM +0100, Vincent Guittot wrote:
> On 13 December 2012 03:17, Alex Shi <alex.shi@...el.com> wrote:
> > On 12/12/2012 09:31 PM, 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->nr_running) >
> >
> > If nr_running a bit big, rq->avg.runnable_avg_sum << rq->nr_running is
> > zero. you will get the wrong decision.
> 
> yes, I'm going to do that like below instead:
> return (rq->avg.runnable_avg_sum > (rq->avg.runnable_avg_period >>
> rq->nr_running));

Doesn't it consider nr_running too much?  It seems current is_buddy_busy
returns false on a cpu that has 1 task runs 40% cputime, but returns true
on a cpu that has 3 tasks runs 10% cputime each or for 2 tasks of 15%
cputime each, right?

I don't know what is correct, but just guessing that in a cpu's point
of view it'd be busier if it has a higher runnable_avg_sum than a
higher nr_running IMHO.


> 
> >
> >> +                     rq->avg.runnable_avg_period);
> >> +}
> >> +
> >> +static bool is_light_task(struct task_struct *p)
> >> +{
> >> +     /* A light task runs less than 25% in average */
> >> +     return ((p->se.avg.runnable_avg_sum << 1) <
> >> +                     p->se.avg.runnable_avg_period);
> >
> > 25% may not suitable for big machine.
> 
> Threshold is always an issue, which threshold should be suitable for
> big machine ?
> 
> I'm wondering if i should use the imbalance_pct value for computing
> the threshold

Anyway, I wonder how 'sum << 1' computes 25%.  Shouldn't it be << 2 ?

Thanks,
Namhyung
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