linux-kernel - Re: perf_counters issue with PERF_SAMPLE

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Message-ID: <7c86c4470908121354q33a82914x6cf63eb375b6fb59@mail.gmail.com>
Date:	Wed, 12 Aug 2009 22:54:31 +0200
From:	stephane eranian <eranian@...glemail.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Robert Richter <robert.richter@....com>,
	Paul Mackerras <paulus@...ba.org>,
	Andi Kleen <andi@...stfloor.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <cel@...ibm.com>,
	Corey J Ashford <cjashfor@...ibm.com>,
	Philip Mucci <mucci@...s.utk.edu>,
	Dan Terpstra <terpstra@...s.utk.edu>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: Re: perf_counters issue with PERF_SAMPLE_GROUP

Hi,

Another thing that occurred to me  while using PERF_SAMPLE_GROUP
is about that unique ID which you get out of read() with PERF_FORMAT_ID.

That number if generated by the kernel and it is a globally incrementing
64-bit counter.

An application may not get subsequent numbers even though it is issuing
perf_open_counter() in sequence. Another applications may cause the
number to change.

I believe this is not very convenient because imagine in your signal handler
you parse the GROUP and you want to relate id to the event. You need
to have a lookup table.

I believe it would be more convenient if the tool could pass a 64-bit number
itself when the event is created. It would get it back as part of
PERF_SAMPLE_GROUP.
The id could then be more related to the tool's data structure, no need for a
lookup table, overall more efficient. If I recall, this is what
epoll() can do and
this is very convenient.
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