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

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Message-ID: <20180522213113.GA40506@joelaf.mtv.corp.google.com>
Date:   Tue, 22 May 2018 14:41:53 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Todd Kjos <tkjos@...gle.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        kernel-team@...roid.com, Linux PM <linux-pm@...r.kernel.org>,
        "Joel Fernandes (Google.)" <joelaf@...gle.com>
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even
 when kthread kicked

On Tue, May 22, 2018 at 05:27:11PM +0200, Rafael J. Wysocki wrote:
> On Tue, May 22, 2018 at 2:22 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > On Tuesday, May 22, 2018 1:42:05 PM CEST Rafael J. Wysocki wrote:
> >> On Tue, May 22, 2018 at 1:38 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >> > On 22-05-18, 13:31, Rafael J. Wysocki wrote:
> >> >> So below is my (compiled-only) version of the $subject patch, obviously based
> >> >> on the Joel's work.
> >> >>
> >> >> Roughly, what it does is to move the fast_switch_enabled path entirely to
> >> >> sugov_update_single() and take the spinlock around sugov_update_commit()
> >> >> in the one-CPU case too.
> >
> > [cut]
> >
> >> >
> >> > Why do you assume that fast switch isn't possible in shared policy
> >> > cases ? It infact is already enabled for few drivers.
> >
> > I hope that fast_switch is not used with devfs_possible_from_any_cpu set in the
> > one-CPU policy case, as that looks racy even without any patching.
> 
> Which would be the only case in which sugov_update_single() would run
> on a CPU that is not the target.
> 
> And running sugov_update_single() concurrently on two different CPUs
> for the same target is a no-no, as we don't prevent concurrent updates
> from occurring in that path.
> 
> Which means that the original patch from Joel will be sufficient as
> long as we ensure that sugov_update_single() can only run on one CPU
> at a time.

Since target CPU's runqueue lock is held, I don't see how we can run
sugov_update_single concurrently with any other CPU for single policy, so
protecting such race shouldn't be necessary.

Also the "if (work_in_progress)" check I added to the sugov_update_single
doesn't change the behavior of single policy from what it is in mainline
since we were doing the same thing in already sugov_should_update_freq.

thanks,

 - Joel