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

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Date:   Thu, 14 Mar 2019 15:46:06 +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, 10:28, Rafael J. Wysocki 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.

Even if per-cpu iteration has to be done at some place, we are
avoiding function calls here and the code/locking in the notifier
layer as well. Will get more such info into changelog.

> I guess some notifiers need to know what other CPUs there are in the
> policy?  If so, then why?

You mean about the offline CPUs? I mentioned the rationale in 1/7. It
is to avoid bugs where we may end up using a stale value if the CPUs
are offlined/onlined regularly.

> > 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.

24 different parts of the kernel register for transition notifiers and
only 5 required update here, the other 19 don't need to do per-cpu
stuff and they also get benefited by this work. Those routines will
get called only once now, instead of once per every CPU of the policy.

> 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.

I did that because they don't need to use the other fields of the
policy today and that doesn't look likely in near future as well.

I can do that if you want, but not sure why more information should be
provided than required.

-- 
viresh