linux-kernel - Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting

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Date:   Wed, 29 Apr 2020 13:30:17 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Giovanni Gherdovich <ggherdovich@...e.cz>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>, Len Brown <lenb@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency
 invariant accounting

On Tue, Apr 28, 2020 at 3:25 PM Giovanni Gherdovich <ggherdovich@...e.cz> wrote:
>
> The product mcnt * arch_max_freq_ratio could be zero if it overflows u64.
>
> For context, a large value for arch_max_freq_ratio would be 5000,
> corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like
> 1500-2000). A large increment frequency for the MPERF counter would be 5GHz
> (the base clock of all CPUs on the market today is less than that). With
> these figures, a CPU would need to go without a scheduler tick for around 8
> days for the u64 overflow to happen. It is unlikely, but the check is
> warranted.
>
> In that case it's also appropriate to disable frequency invariant
> accounting: the feature relies on measures of the clock frequency done at
> every scheduler tick, which need to be "fresh" to be at all meaningful.
>
> Signed-off-by: Giovanni Gherdovich <ggherdovich@...e.cz>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

> ---
>  arch/x86/kernel/smpboot.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 8c89e4d9ad28..4718f29a3065 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -2039,6 +2039,14 @@ static void init_freq_invariance(bool secondary)
>         }
>  }
>
> +static void disable_freq_invariance_workfn(struct work_struct *work)
> +{
> +       static_branch_disable(&arch_scale_freq_key);
> +}
> +
> +static DECLARE_WORK(disable_freq_invariance_work,
> +                   disable_freq_invariance_workfn);
> +
>  DEFINE_PER_CPU(unsigned long, arch_freq_scale) = SCHED_CAPACITY_SCALE;
>
>  void arch_scale_freq_tick(void)
> @@ -2055,14 +2063,18 @@ void arch_scale_freq_tick(void)
>
>         acnt = aperf - this_cpu_read(arch_prev_aperf);
>         mcnt = mperf - this_cpu_read(arch_prev_mperf);
> -       if (!mcnt)
> -               return;
>
>         this_cpu_write(arch_prev_aperf, aperf);
>         this_cpu_write(arch_prev_mperf, mperf);
>
>         acnt <<= 2*SCHED_CAPACITY_SHIFT;
>         mcnt *= arch_max_freq_ratio;
> +       if (!mcnt) {
> +               pr_warn("Scheduler tick missing for long time, disabling scale-invariant accounting.\n");
> +               /* static_branch_disable() acquires a lock and may sleep */
> +               schedule_work(&disable_freq_invariance_work);
> +               return;
> +       }
>
>         freq_scale = div64_u64(acnt, mcnt);
>
> --
> 2.16.4
>