linux-kernel - Re: [PATCH] perf, tools: Add PERF

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Date:	Tue, 9 Sep 2014 18:08:39 -0700
From:	Brendan Gregg <brendan.d.gregg@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Jiri Olsa <jolsa@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, peterz@...radead.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] perf, tools: Add PERF_PID

On Tue, Sep 9, 2014 at 5:03 PM, Andi Kleen <andi@...stfloor.org> wrote:
> From: Andi Kleen <ak@...ux.intel.com>
>
> It's currently difficult to filter out perf itself using a filter.
> This can give cascading effects during IO tracing when the IO
> perf does itself causes more trace output.
>
> The best way to filter is to use the pid. But it's difficult to get the pid
> of perf without using hacks.
>
> Add a PERF_PID meta variable to the perf filter that contains the current pid.
>
> With this patch the following works
>
> % perf record -e syscalls:sys_enter_write -a --filter 'common_pid != PERF_PID' ...
>
> This won't work for more complex perf pipe lines with multiple processes,
> but at least solves the problem nicely for a single perf.

Thank you, this will be handy. I tested it and it works.

It will mean I can avoid hacks like this:
https://github.com/brendangregg/perf-tools/blob/d30e8a1a9f136d532ba78c6d48aedb99dc316be9/syscount#L144

I think it's unrelated to this patch, but the very next test I tried
didn't work as expected:

# perf record -e 'syscalls:sys_enter_*' --filter 'common_pid !=
PERF_PID' -a sleep 5

It only filters the last event from the wildcard expansion, leaving
write() unfiltered. Looks like it's because parse_filter() is only
setting the filter for perf_evlist__last(), instead of doing an
evlist__for_each(). How best to deal with this can be addressed in a
separate patch later.

Brendan
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