linux-kernel - Re: [PATCH RFC V2 1/1] x86, perf: Add a freq pmu driver

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Message-ID: <CALCETrUK5ktvpk_831MyGYnxj0kkykSQTHoqWB_7S9j=DKxOug@mail.gmail.com>
Date:	Thu, 23 Jul 2015 08:52:50 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Kan Liang <kan.liang@...el.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Mark Rutland <mark.rutland@....com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH RFC V2 1/1] x86, perf: Add a freq pmu driver

On Thu, Jul 23, 2015 at 8:44 AM, Stephane Eranian <eranian@...gle.com> wrote:
> I understand the value of the tsc and smi events. It is not
> clear to me what aperf/mperf buys you over cycles and ref-cycles:
>
> $ perf stat -a -e msr/aperf/,msr/mperf/,cycles,ref-cycles -C 1 -I 1000 sleep 10
> #           time             counts unit events
>      2.000361718         14,826,353      msr/aperf/
>      2.000361718         11,865,170      msr/mperf/
>      2.000361718         17,170,101      cycles
>      2.000361718         13,629,675      ref-cycles
>
> Only the ratio aperf/mperf is defined, here 1.25 and the ratio
> cycles/ref-cycles is 1.25 as well. So what is a situation where
> aperf/mperf provides better info than cycles/ref-cycles?
> The SDM also says aperf/mperf only defined when running in C0 mode.

They're free-running and always on, which means that you can never
fail to schedule them.

--Andy
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