linux-kernel - Re: [RFCv3 PATCH 14/48] arm: Frequency invariant scheduler load-tracking support

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Message-ID: <20150324094118.GA18994@e105550-lin.cambridge.arm.com>
Date:	Tue, 24 Mar 2015 09:41:18 +0000
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [RFCv3 PATCH 14/48] arm: Frequency invariant scheduler
 load-tracking support

On Mon, Mar 23, 2015 at 01:39:44PM +0000, Peter Zijlstra wrote:
> On Wed, Feb 04, 2015 at 06:30:51PM +0000, Morten Rasmussen wrote:
> > +/* cpufreq callback function setting current cpu frequency */
> > +void arch_scale_set_curr_freq(int cpu, unsigned long freq)
> > +{
> > +	atomic_long_set(&per_cpu(cpu_curr_freq, cpu), freq);
> > +}
> > +
> > +/* cpufreq callback function setting max cpu frequency */
> > +void arch_scale_set_max_freq(int cpu, unsigned long freq)
> > +{
> > +	atomic_long_set(&per_cpu(cpu_max_freq, cpu), freq);
> > +}
> > +
> > +unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
> > +{
> > +	unsigned long curr = atomic_long_read(&per_cpu(cpu_curr_freq, cpu));
> > +	unsigned long max = atomic_long_read(&per_cpu(cpu_max_freq, cpu));
> > +
> > +	if (!curr || !max)
> > +		return SCHED_CAPACITY_SCALE;
> > +
> > +	return (curr * SCHED_CAPACITY_SCALE) / max;
> > +}
> 
> so I've no idea how many cycles an (integer) division takes on ARM; but
> doesn't it make sense to do this division (once) in
> arch_scale_set_curr_freq() instead of every time we need the result?

It does. In fact I have already prepared that fix for v4. Integer
division is expensive ARM and we call this function a lot.
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