From bea10c8c40e75f4808bf6ba9337e866c2e498d8c Mon Sep 17 00:00:00 2001 From: Vince Weaver Date: Thu, 19 Dec 2013 12:09:29 -0500 Subject: [PATCH] perf_event_open.2. Clarify issues with the exclusive bit Warn that using the perf_event_open "exclusive" bit, while it might seem like a good idea, might lead to all 0 results in some common usage cases. Reported-by: Sudhanshu Goswami Signed-off-by: Vince Weaver Signed-off-by: Michael Kerrisk --- man2/perf_event_open.2 | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2 index 6d42c4171..ebda7000d 100644 --- a/man2/perf_event_open.2 +++ b/man2/perf_event_open.2 @@ -775,6 +775,13 @@ it should be the only group using the CPU's counters. In the future this may allow monitoring programs to support PMU features that need to run alone so that they do not disrupt other hardware counters. + +Note that many unexpected situations may prevent events with the +.I exclusive +bit set from ever running. +This includes any users running a system-wide +measurement as well as any kernel use of the performance counters +(including the commonly enabled NMI Watchdog Timer interface). .TP .IR "exclude_user" If this bit is set, the count excludes events that happen in user space.