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

From: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
To: Giovanni Gherdovich <ggherdovich@suse.cz>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@suse.de>, Len Brown <lenb@kernel.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	x86@kernel.org, linux-pm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Mel Gorman <mgorman@techsingularity.net>,
	Doug Smythies <dsmythies@telus.net>,
	Like Xu <like.xu@linux.intel.com>,
	Neil Rickert <nwr10cst-oslnx@yahoo.com>,
	Chris Wilson <chris@chris-wilson.co.uk>
Subject: Re: [PATCH 1/4] x86, sched: Bail out of frequency invariance if base frequency is unknown
Date: Wed, 22 Apr 2020 10:15:47 -0700	[thread overview]
Message-ID: <20200422171547.GA11942@ranerica-svr.sc.intel.com> (raw)
In-Reply-To: <20200416054745.740-2-ggherdovich@suse.cz>

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@suse.cz>
> 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