linux-kernel - Re: [PATCH 5/7] x86/tsc: Update cpufreq transition notifier to handle multiple CPUs

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Message-ID: <CAJZ5v0i=QoToW0WCaL50t0qe5sKSe6J3W+EnFTvYuVCp6MvZDA@mail.gmail.com>
Date:   Thu, 14 Mar 2019 10:33:04 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.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>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/7] x86/tsc: Update cpufreq transition notifier to handle
 multiple CPUs

On Thu, Mar 14, 2019 at 7:43 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> The cpufreq core currently calls the cpufreq transition notifier
> callback once for each affected CPU. This is going to change soon and
> the cpufreq core will call the callback only once for each cpufreq
> policy. The callback must look at the newly added field in struct
> cpufreq_freqs, "cpus", which contains policy->related_cpus (both
> online/offline CPUs) and perform per-cpu actions for them if any.
>
> This patch updates time_cpufreq_notifier() to use the new "cpus" field,
> update per-cpu data for all the related CPUs and call set_cyc2ns_scale()
> for all online related_cpus.
>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  arch/x86/kernel/tsc.c | 31 ++++++++++++++++++++-----------
>  1 file changed, 20 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 3fae23834069..587a6aa72f38 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -956,28 +956,37 @@ 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
> +       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(freq->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, freq->cpus)

This needs to iterate over policy->cpus or you change the behavior.

Not that it will matter a lot (x86 in one CPU per policy anyway in the
vast majority of cases), but it is a change nevertheless.  Moreover,
I'm not even sure if doing that for offline CPUs makes sense.

> +                               cpu_data(cpu).loops_per_jiffy = lpj;
> +               }
> +
> +               for_each_cpu_and(cpu, freq->cpus, cpu_online_mask)
> +                       set_cyc2ns_scale(tsc_khz, cpu, rdtsc());
>         }
>
>         return 0;
> --