linux-kernel - Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

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Date:	Wed, 10 Feb 2016 15:26:31 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Steve Muckle <steve.muckle@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux PM list <linux-pm@...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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

On Wed, Feb 10, 2016 at 3:03 PM, Juri Lelli <juri.lelli@....com> wrote:
> On 10/02/16 14:23, Rafael J. Wysocki wrote:
>> On Wed, Feb 10, 2016 at 1:33 PM, Juri Lelli <juri.lelli@....com> wrote:
>> > Hi Rafael,
>> >
>> > On 09/02/16 21:05, Rafael J. Wysocki wrote:
>> >
>> > [...]
>> >
>> >> +/**
>> >> + * cpufreq_update_util - Take a note about CPU utilization changes.
>> >> + * @util: Current utilization.
>> >> + * @max: Utilization ceiling.
>> >> + *
>> >> + * This function is called by the scheduler on every invocation of
>> >> + * update_load_avg() on the CPU whose utilization is being updated.
>> >> + */
>> >> +void cpufreq_update_util(unsigned long util, unsigned long max)
>> >> +{
>> >> +     struct update_util_data *data;
>> >> +
>> >> +     rcu_read_lock();
>> >> +
>> >> +     data = rcu_dereference(*this_cpu_ptr(&cpufreq_update_util_data));
>> >> +     if (data && data->func)
>> >> +             data->func(data, cpu_clock(smp_processor_id()), util, max);
>> >
>> > Are util and max used anywhere?
>>
>> They aren't yet, but they will be.
>>
>> Maybe not in this cycle (it it takes too much time to integrate the
>> preliminary changes), but we definitely are going to use those
>> numbers.
>>
>
> Oh OK. However, I was under the impression that this set was only
> proposing a way to get rid of timers and use the scheduler as heartbeat
> for cpufreq governors. The governors' sample based approach wouldn't
> change, though. Am I wrong in assuming this?

Your assumption is correct.

The sample-based approach doesn't change at this time, simply to avoid
making too many changes in one go.

The next step, as I'm seeing it, would be to use the
scheduler-provided utilization in the governor computations instead of
the load estimation made by governors themselves.

> Also, is linux-pm/bleeding-edge the one I want to fetch to try this set out?

You can get it from there, but possibly with some changes unrelated to cpufreq.

You can also pull from the pm-cpufreq-test branch to get the cpufreq
changes only.

Apart from that, I'm going resend the $subject set with updated patch
[1/3] for completeness.

Thanks,
Rafael