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

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Date:	Thu, 23 Jul 2015 09:00:37 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	Andy Lutomirski <luto@...capital.net>
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 8:52 AM, Andy Lutomirski <luto@...capital.net> wrote:
> 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.
>
You get the same with cycles and ref-cycles. They can both run on
fixed-counters.
So you can always schedule them. If you cannot, then it means you are already
measuring them.

The only case I can see where there is a benefit is if you have a
competing system-wide
and per-thread sessions and the former is already using all the
generic counters + fixed
and you come in with a per-thread event to measure cycles or
ref-cycles. That would be
rejected but aperf/mperf would not. But that would only work if you
are counting. There
would be no benefits for sampling mode.
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