linux-kernel - Re: [PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown

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Message-ID: <CAJZ5v0gQ8zJfeeyqH=GK=VFb=_ziGX1NgFnTGH1G3uqz5Yqb6w@mail.gmail.com>
Date:   Wed, 29 Apr 2020 13:30:43 +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 2/2] x86, sched: Bail out of frequency invariance if turbo
 frequency is unknown

On Tue, Apr 28, 2020 at 3:25 PM Giovanni Gherdovich <ggherdovich@...e.cz> wrote:
>
> There may be CPUs that support turbo boost but don't declare any turbo
> ratio, i.e. their MSR_TURBO_RATIO_LIMIT is all zeroes. In that condition
> scale-invariant calculations can't be performed.
>
> Signed-off-by: Giovanni Gherdovich <ggherdovich@...e.cz>
> Suggested-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

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

> ---
>  arch/x86/kernel/smpboot.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 4718f29a3065..ab2a0df7d1fb 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1991,9 +1991,11 @@ static bool intel_set_max_freq_ratio(void)
>         /*
>          * Some hypervisors advertise X86_FEATURE_APERFMPERF
>          * but then fill all MSR's with zeroes.
> +        * Some CPUs have turbo boost but don't declare any turbo ratio
> +        * in MSR_TURBO_RATIO_LIMIT.
>          */
> -       if (!base_freq) {
> -               pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
> +       if (!base_freq || !turbo_freq) {
> +               pr_debug("Couldn't determine cpu base or turbo frequency, necessary for scale-invariant accounting.\n");
>                 return false;
>         }
>
> --
> 2.16.4
>