Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional frequency invariant accounting

From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>,
	Len Brown <lenb@kernel.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Mel Gorman <mgorman@techsingularity.net>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Juri Lelli <juri.lelli@arm.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional frequency invariant accounting
Date: Wed, 16 May 2018 11:07:46 +0200	[thread overview]
Message-ID: <CAJZ5v0gFJQK4FbLx=cOpefSAp2idknC5v7yFm41q+kytK05p_g@mail.gmail.com> (raw)
In-Reply-To: <20180516072929.GN12235@hirez.programming.kicks-ass.net>

On Wed, May 16, 2018 at 9:29 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, May 16, 2018 at 09:16:40AM +0200, Peter Zijlstra wrote:
>> On Tue, May 15, 2018 at 09:49:03PM -0700, Srinivas Pandruvada wrote:
>> > intel_pstate has two operating modes: active and passive. In "active"
>> > mode, the in-built scaling governor is used and in "passive" mode,
>> > the driver can be used with any governor like "schedutil". In "active"
>> > mode the utilization values from schedutil is not used and there is
>> > a requirement from high performance computing use cases, not to read
>> > any APERF/MPERF MSRs. In this case no need to use CPU cycles for
>> > frequency invariant accounting by reading APERF/MPERF MSRs.
>> > With this change frequency invariant account is only enabled in
>> > "passive" mode.
>>
>> WTH is active/passive? Is passive when we select performance governor?
>
> Bah, I cannot read it seems. active is when we use the intel_pstate
> governor and passive is when we use schedutil and only use intel_pstate
> as a driver.
>
>> Also; you have to explain why using APERF/MPERF is bad in that case. Why
>> do they care if we read those MSRs during the tick?
>
> That still stands.. this needs to be properly explained.

I guess this is from the intel_pstate perspective only.

The active mode is only used with HWP, so intel_pstate doesn't look at
the utilization (in any form) in the passive mode today.

Still, there are other reasons for PELT to be scale-invariant, so ...