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perf tools: Update ioctl documentation for PERF_IOC_FLAG_GROUP

The ioctl interface of perf event fd receives 3 arguments to control
event group behavior but it lacked documentation.

Signed-off-by: Namhyung Kim <>
Cc: Namhyung Kim <>
Cc: Paul Mackerras <>
Cc: Peter Zijlstra <>
Signed-off-by: Arnaldo Carvalho de Melo <>
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Namhyung Kim Arnaldo Carvalho de Melo
Namhyung Kim authored and Arnaldo Carvalho de Melo committed May 31, 2012
1 parent aa5cdd3 commit a59e64a13a927fb7530bef39e9f5e7de8268137e
Showing with 4 additions and 3 deletions.
  1. +4 −3 tools/perf/design.txt
@@ -409,14 +409,15 @@ Counters can be enabled and disabled in two ways: via ioctl and via
prctl. When a counter is disabled, it doesn't count or generate
events but does continue to exist and maintain its count value.
-An individual counter or counter group can be enabled with
+An individual counter can be enabled with
+ ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);
or disabled with
+ ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
+For a counter group, pass PERF_IOC_FLAG_GROUP as the third argument.
Enabling or disabling the leader of a group enables or disables the
whole group; that is, while the group leader is disabled, none of the
counters in the group will count. Enabling or disabling a member of a

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