linux-kernel - Re: [PATCH 4/5] perf, x86: Add INST

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Date:	Thu, 14 Aug 2014 19:47:56 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Andi Kleen <ak@...ux.intel.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 4/5] perf, x86: Add INST_RETIRED.ALL workarounds

[+perf tool maintainers]

On Thu, Aug 14, 2014 at 4:30 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>
> I understand all your points, but there's no alternative.
> The only other way would be to disable INST_RETIRED.ALL.
>
You cannot do that either. INST_RETIRED:ALL is important.
I assume the bug applies whether or not the event is used
with a filter.

I think we need to ensure that by looking at the perf.data file,
one can reconstruct the total number of inst_Retired:all
occurrences for the run. With a fixed period, one would do
num_samples * fixed_period. I know the Gooda tool does
that. It is used to estimate the number of events captured
vs. the number of events occurring.

So what I am saying is that on BDW, we need to have
perf force PERF_SAMPLE_PERIOD even with a fixed
period is passed. The kernel cannot simply add
PERF_SAMPLE_PERIOD to the event sample_format
because the tool would get confused by the sample
record size otherwise.

I understand this is not pretty. We'd need to have
a on the evsel just before making the syscall. That
callback would be for x86 and it would check the
CPUID to force PERF_SAMPLE_PERIOD. That's
my thinking but there may be a better way of doing
this.
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