linux-kernel - [patch] perf_event_open.2: Clarify PERF_EVENT_IOC

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Date:	Thu, 12 Feb 2015 12:14:26 -0500 (EST)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
cc:	linux-man@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: [patch] perf_event_open.2: Clarify PERF_EVENT_IOC_REFRESH behavior


Currently the PERF_EVENT_IOC_REFRESH ioctl, when applied to a group
leader, will refresh all children.  Also if a refresh value of 0
is chosen then the refresh becomes infinite (never runs out).
Back in 2011 PAPI was relying on these behaviors but I was told
that both were unsupported and subject to being removed at any time.
   (See https://lkml.org/lkml/2011/5/24/337 )
However the behavior has not been changed.  

This patch updates the manpage to still list the behavior as
unsupported, but removes the innacurate description of it 
only being a problem with 2.6 kernels.

Signed-off-by: Vince Weaver <vincent.weaver@...ne.edu>

diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
index 39c8d8c..76136e2 100644
--- a/man2/perf_event_open.2
+++ b/man2/perf_event_open.2
@@ -2353,6 +2353,12 @@ once the counter reaches 0
 is indicated and
 the underlying event is disabled.
 
+Refreshing an event group leader refreshes all siblings and
+refreshing with a parameter of 0 currently enables infinite
+refreshes;
+these behaviors are unsupported and should not be relied on.
+.\" See https://lkml.org/lkml/2011/5/24/337
+
 Starting with Linux 3.18,
 .\" commit 179033b3e064d2cd3f5f9945e76b0a0f0fbf4883
 .B POLL_HUP
@@ -2805,11 +2811,6 @@ Prior to Linux 2.6.35,
 .B PERF_FORMAT_GROUP
 did not work with attached processes.
 
-In older Linux 2.6 versions,
-refreshing an event group leader refreshed all siblings,
-and refreshing with a parameter of 0 enabled infinite refresh.
-This behavior is unsupported and should not be relied on.
-
 There is a bug in the kernel code between
 Linux 2.6.36 and Linux 3.0 that ignores the
 "watermark" field and acts as if a wakeup_event

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