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

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Date:   Mon, 21 May 2018 11:57:51 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        "Joel Fernandes (Google.)" <joelaf@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.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>,
        Todd Kjos <tkjos@...gle.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        kernel-team@...roid.com, Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even
 when kthread kicked

On 21/05/18 10:29, Rafael J. Wysocki wrote:
> On Mon, May 21, 2018 at 7:14 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > On 18-05-18, 11:55, Joel Fernandes (Google.) wrote:
> >> From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> >>
> >> Currently there is a chance of a schedutil cpufreq update request to be
> >> dropped if there is a pending update request. This pending request can
> >> be delayed if there is a scheduling delay of the irq_work and the wake
> >> up of the schedutil governor kthread.
> >>
> >> A very bad scenario is when a schedutil request was already just made,
> >> such as to reduce the CPU frequency, then a newer request to increase
> >> CPU frequency (even sched deadline urgent frequency increase requests)
> >> can be dropped, even though the rate limits suggest that its Ok to
> >> process a request. This is because of the way the work_in_progress flag
> >> is used.
> >>
> >> This patch improves the situation by allowing new requests to happen
> >> even though the old one is still being processed. Note that in this
> >> approach, if an irq_work was already issued, we just update next_freq
> >> and don't bother to queue another request so there's no extra work being
> >> done to make this happen.
> >
> > Now that this isn't an RFC anymore, you shouldn't have added below
> > paragraph here. It could go to the comments section though.
> >
> >> I had brought up this issue at the OSPM conference and Claudio had a
> >> discussion RFC with an alternate approach [1]. I prefer the approach as
> >> done in the patch below since it doesn't need any new flags and doesn't
> >> cause any other extra overhead.
> >>
> >> [1] https://patchwork.kernel.org/patch/10384261/
> >>
> >> LGTMed-by: Viresh Kumar <viresh.kumar@...aro.org>
> >> LGTMed-by: Juri Lelli <juri.lelli@...hat.com>
> >
> > Looks like a Tag you just invented ? :)
> 
> Yeah.
> 
> The LGTM from Juri can be converned into an ACK silently IMO.  That

Sure! :)

Thanks,

- Juri