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 23:52:46 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        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 11:41 PM, Joel Fernandes <joel@...lfernandes.org> wrote:
> 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.

If dvfs_possible_from_any_cpu is set, any CPU can run
sugov_update_single(), but the kthread will only run on the target
itself.  So another CPU running sugov_update_single() for the target
may be racing with the target's kthread.

> 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.

No, it doesn't, which doesn't mean that this is all OK. :-)