linux-kernel - Re: [PATCH V2] cpufreq: Call transition notifier only once for each policy

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Message-ID: <CAJZ5v0h5k72Oer0v6iaqfzNU6fNfoPK+YLsKxi6ZE14P_rWD1Q@mail.gmail.com>
Date:   Mon, 18 Mar 2019 12:09:07 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        sparclinux@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V2] cpufreq: Call transition notifier only once for each policy

On Mon, Mar 18, 2019 at 11:54 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Mar 18, 2019 at 08:05:14AM +0530, Viresh Kumar wrote:
> > On 15-03-19, 13:29, Peter Zijlstra wrote:
> > > On Fri, Mar 15, 2019 at 02:43:07PM +0530, Viresh Kumar wrote:
> > > > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > > > index 3fae23834069..cff8779fc0d2 100644
> > > > --- a/arch/x86/kernel/tsc.c
> > > > +++ b/arch/x86/kernel/tsc.c
> > > > @@ -956,28 +956,38 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
> > > >                           void *data)
> > > >  {
> > > >   struct cpufreq_freqs *freq = data;
> > > > - unsigned long *lpj;
> > > > -
> > > > - lpj = &boot_cpu_data.loops_per_jiffy;
> > > > -#ifdef CONFIG_SMP
> > > > - if (!(freq->flags & CPUFREQ_CONST_LOOPS))
> > > > -         lpj = &cpu_data(freq->cpu).loops_per_jiffy;
> > > > -#endif
> > > > + struct cpumask *cpus = freq->policy->cpus;
> > > > + bool boot_cpu = !IS_ENABLED(CONFIG_SMP) || freq->flags & CPUFREQ_CONST_LOOPS;
> > > > + unsigned long lpj;
> > > > + int cpu;
> > > >
> > > >   if (!ref_freq) {
> > > >           ref_freq = freq->old;
> > > > -         loops_per_jiffy_ref = *lpj;
> > > >           tsc_khz_ref = tsc_khz;
> > > > +
> > > > +         if (boot_cpu)
> > > > +                 loops_per_jiffy_ref = boot_cpu_data.loops_per_jiffy;
> > > > +         else
> > > > +                 loops_per_jiffy_ref = cpu_data(cpumask_first(cpus)).loops_per_jiffy;
> > > >   }
> > > > +
> > > >   if ((val == CPUFREQ_PRECHANGE  && freq->old < freq->new) ||
> > > >                   (val == CPUFREQ_POSTCHANGE && freq->old > freq->new)) {
> > > > -         *lpj = cpufreq_scale(loops_per_jiffy_ref, ref_freq, freq->new);
> > > > -
> > > > +         lpj = cpufreq_scale(loops_per_jiffy_ref, ref_freq, freq->new);
> > > >           tsc_khz = cpufreq_scale(tsc_khz_ref, ref_freq, freq->new);
> > > > +
> > > >           if (!(freq->flags & CPUFREQ_CONST_LOOPS))
> > > >                   mark_tsc_unstable("cpufreq changes");
> > > >
> > > > -         set_cyc2ns_scale(tsc_khz, freq->cpu, rdtsc());
> > > > +         if (boot_cpu) {
> > > > +                 boot_cpu_data.loops_per_jiffy = lpj;
> > > > +         } else {
> > > > +                 for_each_cpu(cpu, cpus)
> > > > +                         cpu_data(cpu).loops_per_jiffy = lpj;
> > > > +         }
> > > > +
> > > > +         for_each_cpu(cpu, cpus)
> > > > +                 set_cyc2ns_scale(tsc_khz, cpu, rdtsc());
> > >
> > > This code doesn't make sense, the rdtsc() _must_ be called on the CPU in
> > > question.
> >
> > You mean rdtsc() must be locally on that CPU? The cpufreq core never guaranteed
> > that and it was left for the notifier to do. This patch doesn't change the
> > behavior at all, just that it moves the for-loop to the notifier instead of the
> > cpufreq core.
>
> Yuck..
>
> Rafael; how does this work in practise? Earlier you said that on x86 the
> policies typically have a single cpu in them anyway.

Yes.

> Is the freq change also notified from _that_ cpu?

May not be, depending on what CPU runs the work item/thread changing
the freq.  It generally is not guaranteed to always be the same as the
target CPU.

> I don't think I have old enough hardware around anymore to test any of
> this. This was truly ancient p6 era stuff IIRC.
>
> Because in that case, I'm all for not doing the changes to this notifier
> Viresh is proposing but simply adding something like:
>
>
>         WARN_ON_ONCE(cpumask_weight(cpuc) != 1);
>         WARN_ON_ONCE(cpumask_first(cpuc) != smp_processor_id());
>
> And leave it at that.

That may not work I'm afraid.