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Message-ID: <4F22D8D9.3010108@rgmadvisors.com>
Date: Fri, 27 Jan 2012 11:03:21 -0600
From: Andrew Steets <asteets@rgmadvisors.com>
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To: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>, Paul Mackerras <paulus@samba.org>,
        Ingo Molnar <mingo@elte.hu>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Subject: perf: prctl(PR_TASK_PERF_EVENTS_DISABLE) has no effect
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prctl(PR_TASK_PERF_EVENTS_DISABLE) doesn't appear to disable perf event
counters.  Here is a demonstration program:

#include <linux/prctl.h>

void loop_events_disabled() {
     volatile int x;
     int i;
     for (i = 0; i < 1000000000; i++)
         x = i;
     return;
}

void loop_events_enabled() {
     volatile int x;
     int i;
     for (i = 0; i < 1000000000; i++)
         x = i;
     return;
}

int main(int argc, char **argv) {
     prctl(PR_TASK_PERF_EVENTS_ENABLE);
     loop_events_enabled();
     prctl(PR_TASK_PERF_EVENTS_DISABLE);
     loop_events_disabled();
}

I would not expect to see loop_events_disabled() show up in the profile
as reported by perf report, but it does.

$ perf record ./a.out
$ perf report -n --stdio
# Events: 3K cycles
#
# Overhead  Samples    Command      Shared Object                Symbol
# ........ ..........  .......  .................  ....................
#
     51.80%       1679    a.out  a.out              [.] loop_events_enabled
     48.07%       1578    a.out  a.out              [.] loop_events_disabled
      0.03%          5    a.out  [kernel.kallsyms]  [k] intel_pmu_enable_all
      0.03%          1    a.out  [kernel.kallsyms]  [k] sched_clock
      0.03%          1    a.out  [kernel.kallsyms]  [k] ktime_get
      0.03%          1    a.out  [kernel.kallsyms]  [k] update_cpu_load
      0.01%          1    a.out  [sunrpc]           [k] generic_match

I have tested this on several kernels including 3.3rc1.  Can anyone tell
me if I'm using this wrong or if this is a bug?

-Andrew

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