Re: [PATCH 1/4] x86, sched: Bail out of frequency invariance if base frequency is unknown
From: Ricardo Neri
Date: Wed Apr 22 2020 - 13:15:10 EST
On Thu, Apr 16, 2020 at 07:47:42AM +0200, Giovanni Gherdovich 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")
> ---
> 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;
> + }
It may be possible that MSR_TURBO_RATIO_LIMIT is also all-zeros. In
such case, turbo_freq will be also zero. If that is the case,
arch_max_freq_ratio will be zero and we will see a division by zero
exception in arch_scale_freq_tick() because mcnt is multiplied by
arch_max_freq_ratio().
Hence, you should also check for !turbo_freq.
Thanks and BR,
Ricardo