linux-kernel - [RFC] perf: perf record sets inherit by default

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Message-ID: <AANLkTinsEANHEicdjOaE6jwUa0bVqH5PBwuGQ3_L59hT@mail.gmail.com>
Date:	Tue, 11 May 2010 16:04:17 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	mingo@...e.hu, Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>,
	perfmon2-devel@...ts.sf.net
Subject: [RFC] perf: perf record sets inherit by default

Hi,


I am confused by the inheritance cmd line option of perf record:

$ perf record -h
 usage: perf record [<options>] [<command>]
    or: perf record [<options>] -- <command> [<options>]

    -e, --event <event>   event selector. use 'perf list' to list
available events
        --filter <filter>
                          event filter
    -p, --pid <n>         record events on existing process id
    -t, --tid <n>         record events on existing thread id
    -r, --realtime <n>    collect data with this RT SCHED_FIFO priority
    -R, --raw-samples     collect raw sample records from all opened counters
    -a, --all-cpus        system-wide collection from all CPUs
    -A, --append          append to the output file to do incremental profiling
    -C, --profile_cpu <n>
                          CPU to profile on
    -f, --force           overwrite existing data file (deprecated)
    -c, --count           event period to sample
    -o, --output <file>   output file name
    -i, --inherit         child tasks inherit counters

This leads to believe that by default inheritance in children is off.

However, builtin-record.c says:

static bool                     inherit                         =   true;

If that's the case, what's the point of the -i option?

Another side effect of inheritance is that in per-thread mode,
perf creates as many "sessions" as you have CPUs. So
on a 16-way processor, sampling on cycles, perf creates
16 events and 16 x 2-page sampling buffers. That's a lot of
resources consumed if I am just interested in monitoring
a single-threaded workload.

Am I missing something here?
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