linux-kernel - Re: [PATCH v4 7/7] cpufreq: schedutil: New governor based on scheduler utilization data

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Date:	Wed, 16 Mar 2016 22:42:43 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Michael Turquette <mturquette@...libre.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 7/7] cpufreq: schedutil: New governor based on scheduler utilization data

On Wednesday, March 16, 2016 06:35:41 PM Peter Zijlstra wrote:
> On Wed, Mar 16, 2016 at 03:59:18PM +0100, Rafael J. Wysocki wrote:
> 
> > +static unsigned int get_next_freq(struct cpufreq_policy *policy,
> > +				  unsigned long util, unsigned long max)
> > +{
> > +	unsigned int freq = arch_scale_freq_invariant() ?
> > +				policy->cpuinfo.max_freq : policy->cur;
> > +
> > +	return (freq + (freq >> 2)) * util / max;
> > +}
> > +
> > +static void sugov_update_single(struct update_util_data *hook, u64 time,
> > +				unsigned long util, unsigned long max)
> > +{
> > +	struct sugov_cpu *sg_cpu = container_of(hook, struct sugov_cpu, update_util);
> > +	struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> > +	struct cpufreq_policy *policy = sg_policy->policy;
> > +	unsigned int next_f;
> > +
> > +	if (!sugov_should_update_freq(sg_policy, time))
> > +		return;
> > +
> > +	next_f = util <= max ?
> > +		get_next_freq(policy, util, max) : policy->cpuinfo.max_freq;
> 
> I'm not sure that is correct, would not something like this be more
> accurate?
> 
> 	if (util > max)
> 		util = max;
> 	next_f = get_next_freq(policy, util, max);
> 
> After all, if we clip util we will still only increment to the next freq
> with our multiplication factor.
> 
> Hmm, or was this meant to deal with the DL/RT stuff?

Yes, it was.

> Would then not something like:
> 
> 	/* ULONG_MAX is used to force max_freq for Real-Time policies */
> 	if (util == ULONG_MAX) {
> 		next_f = policy->cpuinfo.max_freq;
> 	} else {
> 		if (util > max)

That cannot happen given the way CFS deals with max before passing it
to cpufreq_update_util().

> 			util = max;
> 		next_f = get_next_freq(policy, util, max);
> 	}
> 
> Be clearer?
> 
> > +	sugov_update_commit(sg_policy, time, next_f);
> > +}

So essentially I can replace the util > max check with the util == ULONG_MAX one
(here and in some other places) if that helps to understand the code, but
functionally that won't change anything.