linux-kernel - Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests

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Message-ID: <CAJZ5v0hNgvd=0dKstXRsbHhtrc6m+ugS5kjPOq2B8k55ioX0MA@mail.gmail.com>
Date:   Wed, 9 May 2018 10:05:09 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>,
        Juri Lelli <juri.lelli@...hat.com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Claudio Scordino <claudio@...dence.eu.com>,
        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>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Joel Fernandes <joelaf@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests

On Wed, May 9, 2018 at 9:01 AM, Joel Fernandes <joel@...lfernandes.org> wrote:
> On Wed, May 09, 2018 at 12:24:49PM +0530, Viresh Kumar wrote:
>> On 09-05-18, 08:45, Juri Lelli wrote:
>> > On 08/05/18 21:54, Joel Fernandes wrote:
>> > Isn't this potentially introducing unneeded irq pressure (and doing the
>> > whole wakeup the kthread thing), while the already active kthread could
>> > simply handle multiple back-to-back requests before going to sleep?
>>
>> And then we may need more instances of the work item and need to store
>> a different value of next_freq with each work item, as we can't use
>> the common one anymore as there would be races around accessing it ?
>
> Exactly. I think it also doesn't make sense to over write an already
> committed request either so better to store them separate (?). After the
> "commit", that previous request is done..

Why is it?

In the non-fast-switch case the "commit" only means queuing up an
irq_work.  Which BTW is one of the reasons for having work_in_progress
even if your kthread can handle multiple work items in one go.

You may try to clear work_in_progress in sugov_irq_work() instead of
in sugov_work(), though.

BTW, I'm not sure if the comment in sugov_irq_work() still applies.  Juri?