linux-kernel - Re: [PATCH RESEND v9 04/10] sched: Make sched entity usage tracking scale-invariant

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Message-ID: <20150219170236.GB22059@e105550-lin.cambridge.arm.com>
Date:	Thu, 19 Feb 2015 17:02:36 +0000
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>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"kamalesh@...ux.vnet.ibm.com" <kamalesh@...ux.vnet.ibm.com>,
	"riel@...hat.com" <riel@...hat.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH RESEND v9 04/10] sched: Make sched entity usage tracking
 scale-invariant

On Thu, Feb 19, 2015 at 04:34:42PM +0000, Peter Zijlstra wrote:
> On Thu, Jan 15, 2015 at 11:09:24AM +0100, Vincent Guittot wrote:
> > From: Morten Rasmussen <morten.rasmussen@....com>
> > 
> > Apply frequency scale-invariance correction factor to usage tracking.
> > Each segment of the running_load_avg geometric series is now scaled by the
> > current frequency so the utilization_avg_contrib of each entity will be
> > invariant with frequency scaling. 
> 
> > As a result, utilization_load_avg which is
> > the sum of utilization_avg_contrib, becomes invariant too. So the usage level
> > that is returned by get_cpu_usage, stays relative to the max frequency as the
> > cpu_capacity which is is compared against.
> 
> > Then, we want the keep the load tracking values in a 32bits type, which implies
> > that the max value of {runnable|running}_avg_sum must be lower than
> > 2^32/88761=48388 (88761 is the max weigth of a task). 
> 
> > As LOAD_AVG_MAX = 47742,
> > arch_scale_freq_capacity must return a value less than
> > (48388/47742) << SCHED_CAPACITY_SHIFT = 1037 (SCHED_SCALE_CAPACITY = 1024).
> > So we define the range to [0..SCHED_SCALE_CAPACITY] in order to avoid overflow.
> 
> > cc: Paul Turner <pjt@...gle.com>
> > cc: Ben Segall <bsegall@...gle.com>
> > 
> > Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/fair.c | 21 ++++++++++++++-------
> >  1 file changed, 14 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index a96affd..a5039da 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -2266,6 +2266,8 @@ static u32 __compute_runnable_contrib(u64 n)
> >  	return contrib + runnable_avg_yN_sum[n];
> >  }
> >  
> > +unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
> 
> There is no actual function definition in this patch; this would hinder
> linking, no?

The function definition already lives further down in fair.c. I can move
it up here if you prefer?

> 
> 
> >  		if (running)
> > +			sa->running_avg_sum += delta_w * scale_freq
> > +				>> SCHED_CAPACITY_SHIFT;
> 
> 
> >  		if (running)
> > +			sa->running_avg_sum += runnable_contrib * scale_freq
> > +				>> SCHED_CAPACITY_SHIFT;
> 
> >  	if (running)
> > +		sa->running_avg_sum += delta * scale_freq
> > +			>> SCHED_CAPACITY_SHIFT;
> 
> If you're going to respin; please but {} around that.

Sure.
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