linux-kernel - Re: [PATCH 0/7] cpufreq: Call transition notifier only once for each policy

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Date:   Thu, 14 Mar 2019 15:48:44 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, Borislav Petkov <bp@...en8.de>,
        "David S. Miller" <davem@...emloft.net>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kvm@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        sparclinux@...r.kernel.org
Subject: Re: [PATCH 0/7] cpufreq: Call transition notifier only once for each
 policy

On 14-03-19, 10:40, Rafael J. Wysocki wrote:
> On Thu, Mar 14, 2019 at 10:28 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Thu, Mar 14, 2019 at 7:43 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > >
> > > Currently we call the cpufreq transition notifiers once for each CPU of
> > > the policy->cpus cpumask, which isn't that efficient.
> >
> > Why isn't it efficient?
> >
> > Transitions are per-policy anyway, so if something needs to be done
> > for each CPU in the policy, it doesn't matter too much which part of
> > the code carries out the iteration.
> >
> > I guess some notifiers need to know what other CPUs there are in the
> > policy?  If so, then why?
> >
> > > This patchset tries to simplify that by adding another field in struct cpufreq_freqs,
> > > cpus, so the callback has all the information available with a single
> > > call for each policy.
> >
> > Well, you can argue that the core is simplified by it somewhat, but
> > the notifiers aren't.  They actually get more complex, conceptually
> > too, because they now need to worry about offline vs online CPUs etc.
> >
> > Also I wonder why you decided to pass a cpumask in freqs instead of
> > just passing a policy pointer.  If you change things from per-CPU to
> > per-policy, passing the whole policy seems more natural.
> 
> It also looks to me like all that needs to be one patch, or you have
> the ugly transition situation in which notifiers are still invoked for
> each CPU, but they assume to be invoked once per policy.

I assumed that calling something like set_cyc2ns_scale() in x86
multiple times for each CPU shouldn't be that bad even if the
frequency changes only once, but such things may actually have
side-effects. I should merged them all.

-- 
viresh