linux-kernel - Re: [patch] perf_event_open.2 clarify the PERF_FLAG_FD

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Date:	Fri, 09 Jan 2015 14:15:20 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Vince Weaver <vincent.weaver@...ne.edu>
CC:	mtk.manpages@...il.com, xmtk.manpages@...il.com,
	linux-man@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [patch] perf_event_open.2 clarify the PERF_FLAG_FD_* flags

Hi Vince,

On 01/08/2015 06:04 PM, Vince Weaver wrote:
> 
> This change clarifies the behavior of the PERF_FLAG_FD_OUTPUT and
> PERF_FLAG_FD_NO_GROUP flags to perf_event_open(), and removes
> the related FIXME comments.

Patch applied. Thank you!

> While writing tests to validate the behavior of these flags I
> discovered that PERF_FLAG_FD_OUTPUT has been broken since the
> 2.6.35 kernel release.

Sigh!

Cheers,

Michael


> Signed-off-by: Vince Weaver <vincent.weaver@...ne.edu>
> 
> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
> index 8bde791..7181c29 100644
> --- a/man2/perf_event_open.2
> +++ b/man2/perf_event_open.2
> @@ -149,14 +149,17 @@ then
>  .BR execve (2).
>  .TP
>  .BR PERF_FLAG_FD_NO_GROUP
> -.\" FIXME . The following sentence is unclear
> -This flag allows creating an event as part of an event group but
> -having no group leader.
> -It is unclear why this is useful.
> -.\" FIXME . So, why is it useful?
> -.TP
> -.BR PERF_FLAG_FD_OUTPUT
> -This flag reroutes the output from an event to the group leader.
> +This flag tells the event to ignore the
> +.IR group_fd
> +parameter except for the purpose of setting up output redirection
> +using the
> +.B PERF_FLAG_FD_OUTPUT
> +flag.
> +.TP
> +.BR PERF_FLAG_FD_OUTPUT " (broken since Linux 2.6.35)."
> +This flag re-routes the event's sampled output to instead
> +be included in the mmap buffer of the event specified by
> +.IR group_fd .
>  .TP
>  .BR PERF_FLAG_PID_CGROUP " (since Linux 2.6.39)."
>  This flag activates per-container system-wide monitoring.
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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