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

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Message-ID: <1572088.N2D3R5Hx9f@aspire.rjw.lan>
Date:   Tue, 22 May 2018 13:26:55 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Joel Fernandes (Google.)" <joelaf@...gle.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>,
        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>
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked

On Tuesday, May 22, 2018 12:02:24 PM CEST Rafael J. Wysocki wrote:
> On Mon, May 21, 2018 at 6:13 PM, Joel Fernandes <joel@...lfernandes.org> wrote:
> > On Mon, May 21, 2018 at 10:29:52AM +0200, 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
> >> said I have added Looks-good-to: tags to a couple of commits. :-)
> >
> > Cool, I'll covert them to Acks :-)
> 
> So it looks like I should expect an update of this patch, right?
> 
> Or do you prefer the current one to be applied and work on top of it?

Well, sorry, I can't apply this one as it is racy.