Re: [PATCH 1/4] x86, sched: Bail out of frequency invariance if base frequency is unknown

[Rafael J. Wysocki]

On Thu, Apr 16, 2020 at 7:48 AM Giovanni Gherdovich <ggherdovich@xxxxxxx> wrote:
>
> Some hypervisors such as VMWare ESXi 5.5 advertise support for
> X86_FEATURE_APERFMPERF but then fill all MSR's with zeroes. In particular,
> MSR_PLATFORM_INFO set to zero tricks the code that wants to know the base
> clock frequency of the CPU (highest non-turbo frequency), producing a
> division by zero when computing the ratio turbo_freq/base_freq necessary
> for frequency invariant accounting.
>
> It is to be noted that even if MSR_PLATFORM_INFO contained the appropriate
> data, APERF and MPERF are constantly zero on ESXi 5.5, thus freq-invariance
> couldn't be done in principle (not that it would make a lot of sense in a
> VM anyway). The real problem is advertising X86_FEATURE_APERFMPERF. This
> appears to be fixed in more recent versions: ESXi 6.7 doesn't advertise
> that feature.
>
> Signed-off-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

Please feel free to add

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

to all patches in the series and I'm expecting them to be routed through tip.

Thanks!

> ---
>  arch/x86/kernel/smpboot.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index fe3ab9632f3b..3a318ec9bc17 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1985,6 +1985,15 @@ static bool intel_set_max_freq_ratio(void)
>         return false;
>
>  out:
> +       /*
> +        * Some hypervisors advertise X86_FEATURE_APERFMPERF
> +        * but then fill all MSR's with zeroes.
> +        */
> +       if (!base_freq) {
> +               pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
> +               return false;
> +       }
> +
>         arch_turbo_freq_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE,
>                                         base_freq);
>         arch_set_max_freq_ratio(turbo_disabled());
> --
> 2.16.4
>