linux-kernel - Re: [RFC][PATCH v2 2/2] cpufreq: schedutil: Avoid decreasing frequency of busy CPUs

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Date:   Tue, 21 Mar 2017 13:30:37 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC][PATCH v2 2/2] cpufreq: schedutil: Avoid decreasing
 frequency of busy CPUs

On Tue, Mar 21, 2017 at 7:40 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 20-03-17, 22:46, Rafael J. Wysocki wrote:
>> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
>
>> +static void sugov_update_commit(struct sugov_cpu *sg_cpu,
>> +                             struct sugov_policy *sg_policy,
>> +                             u64 time, unsigned int next_freq)
>>  {
>>       struct cpufreq_policy *policy = sg_policy->policy;
>>
>> +     if (sugov_cpu_is_busy(sg_cpu) && next_freq < sg_policy->next_freq)
>> +             next_freq = sg_policy->next_freq;
>> +
>
> In the earlier version you said that we want to be opportunistic and
> don't want to do heavy computation and so check only for current CPU.
>
> But in this version, all those computations are already done by now.
> Why shouldn't we check all CPUs in the policy now? I am asking as we
> will still have the same problem, we are trying to work-around if the
> current CPU isn't busy but others sharing the policy are.

This isn't the way I'm looking at that.

This is an easy (and relatively cheap) check to make for the *current*
*CPU* and our frequency selection algorithm turns out to have
problems, so it would be kind of unreasonable to not use the
opportunity to fix up the value coming from it - if we can do that
easily enough.

For the other CPUs in the policy that would require extra
synchronization etc., so not that easy any more.

> Also, why not return directly from within the if block? To run
> trace_cpu_frequency()?

Yes.

> I don't remember exactly, but why don't we run that for !fast-switch
> case?

That's an interesting question.

We do that in the fast switch case, because otherwise utilities get
confused if the frequency is not updated for a long enough time.

I'm not really sure why they don't get confused in the other case,
though.  [In that case the core calls trace_cpu_frequency() for us,
but only if we actually run the async work.]

It looks like it wouldn't hurt to always run trace_cpu_frequency()
when we want to bail out early for next_freq == sg_policy->next_freq.

Let me prepare a patch for that. :-)

> We can simplify the code a bit if we check for no freq change at
> the top of the routine.

Right.

Thanks,
Rafael