linux-kernel - Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked

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Date:   Tue, 22 May 2018 11:38:52 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "Joel Fernandes (Google.)" <joelaf@...gle.com>,
        linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Todd Kjos <tkjos@...gle.com>, claudio@...dence.eu.com,
        kernel-team@...roid.com, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even
 when kthread kicked

Hi Viresh,
thanks for clarifying...

On 22-May 15:53, Viresh Kumar wrote:
> On 21-05-18, 10:20, Joel Fernandes wrote:
> > On Mon, May 21, 2018 at 06:00:50PM +0100, Patrick Bellasi wrote:
> > > If that's the case, this means that if, for example, during a
> > > frequency switch you get a request to reduce the frequency (e.g.
> > > deadline task passing the 0-lag time) and right after a request to
> > > increase the frequency (e.g. the current FAIR task tick)... you will
> > > enqueue a freq drop followed by a freq increase and actually do two
> > > frequency hops?
> 
> I don't think so.
> 
> Consider the kthread as running currently and has just cleared the
> work_in_progress flag. Sched update comes at that time and we decide
> to reduce the frequency, we queue another work and update next_freq.
> Now if another sched update comes before the kthread finishes its
> previous loop, we will simply update next_freq and return. So when the
> next time kthread runs, it will pick the most recent update.

Mmm... right... looking better at the two execution contexts:

   // A) Frequency update requests
   sugov_update_commit() {
      sg_policy->next_freq = next_freq;
      if (!sg_policy->work_in_progress) {
         sg_policy->work_in_progress = true;
         irq_work_queue(&sg_policy->irq_work);
      }
   }

   // B) Actual frequency updates
   sugov_work() {
      freq = sg_policy->next_freq;
      sg_policy->work_in_progress = false;

      __cpufreq_driver_target(sg_policy->policy, freq, CPUFREQ_RELATION_L);
   }


It's true that A will enqueue only one B at the first next_freq update
and then it will keep just updating the next_freq.
Thus, we should be ensure to have always just one kwork pending in the
queue.
 
> Where is the problem both of you see ?

Perhaps the confusion comes just from the naming of
"work_in_progress", which is confusing since we use it now to
represent that we enqueued a frequency change and we wait for the
kwork to pick it up.

Maybe it can help to rename it to something like kwork_queued or
update_pending, update_queued... ?


-- 
#include <best/regards.h>

Patrick Bellasi