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

[Ricardo Neri <ricardo.neri-calderon@linux.intel.com>]

On Thu, Apr 23, 2020 at 10:06:04AM +0200, Giovanni Gherdovich wrote:
> On Wed, 2020-04-22 at 10:15 -0700, Ricardo Neri wrote:
> > 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().
> 
> Thanks Ricardo for clarifying this.
> 
> Follow-up question: when I see an all-zeros MSR_TURBO_RATIO_LIMIT, can I
> assume the CPU doesn't support turbo boost? Or is it possible that such a CPU
> has turbo boost, just the turbo ratios aren't declared in the MSR?
> 
> Some context: this feature (called "frequency invariance") wants to know
> what's the max clock freq a CPU can have at any time (it needs it for some
> scheduler calculations). This is hard to know precisely, because turbo can
> kick in at any time and depends on many factors.  So it settles for an
> "average maximum frequency", which I decided the 4 cores turbo is a good
> estimate for. Now, if an all-zeros MSR_TURBO_RATIO_LIMIT means "turbo boost
> unsupported", this is actually the easy case because then I know exactly what
> the max freq is (base frequency). If, on the other hand, an all-zeros MSR
> means "there may or may not be turbo, and you don't know how much" then I must
> disable frequency invariance.

I'd say that there can be cases in which the CPU has turbo boost and yet the
turbo ratios are not declared in MSR_TURBO_RATIO_LIMIT. Hence, frequency
invariance should be disabled.

Thanks and BR,
Ricardo