Re: [PATCH RFC] sched/fair: Penalty the cfs task which executes mwait/hlt

From: Peter Zijlstra <peterz@infradead.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Wanpeng Li <kernellwp@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	KarimAllah <karahmed@amazon.de>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Ingo Molnar <mingo@kernel.org>,
	Ankur Arora <ankur.a.arora@oracle.com>,
	christopher.s.hall@intel.com, hubert.chrzaniuk@intel.com,
	len.brown@intel.com, thomas.lendacky@amd.com, rjw@rjwysocki.net
Subject: Re: [PATCH RFC] sched/fair: Penalty the cfs task which executes mwait/hlt
Date: Mon, 13 Jan 2020 13:35:58 +0100	[thread overview]
Message-ID: <20200113123558.GF2827@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <ee2b6da2-be8c-2540-29e9-ffbb9fdfd3fc@redhat.com>

On Mon, Jan 13, 2020 at 12:52:20PM +0100, Paolo Bonzini wrote:
> On 13/01/20 11:43, Peter Zijlstra wrote:
> > So the very first thing we need to get sorted is that MPERF/TSC ratio
> > thing. TurboStat does it, but has 'funny' hacks on like:
> > 
> >   b2b34dfe4d9a ("tools/power turbostat: KNL workaround for %Busy and Avg_MHz")
> > 
> > and I imagine that there's going to be more exceptions there. You're
> > basically going to have to get both Intel and AMD to commit to this.
> > 
> > IFF we can get concensus on MPERF/TSC, then yes, that is a reasonable
> > way to detect a VCPU being idle I suppose. I've added a bunch of people
> > who seem to know about this.
> > 
> > Anyone, what will it take to get MPERF/TSC 'working' ?
> 
> Do we really need MPERF/TSC for this use case, or can we just track
> APERF as well and do MPERF/APERF to compute the "non-idle" time?

So MPERF runs at fixed frequency (when !IDLE and typically the same
frequency as TSC), APERF runs at variable frequency (when !IDLE)
depending on DVFS state.

So APERF/MPERF gives the effective frequency of the core, but since both
stop during IDLE, it will not be a good indication of IDLE.

Otoh, TSC doesn't stop in idle (.oO this depends on
X86_FEATURE_CONSTANT_TSC) and therefore the MPERF/TSC ratio gives how
much !idle time there was between readings.