linux-kernel - Re: [PATCH v3][manpages 1/2] perf_event_open.2: Document PERF_EVENT_IOC_PAUSE

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Message-ID: <556dbe79-f213-d4f4-094c-e6a4ec2fa8f7@gmail.com>
Date:   Wed, 9 Nov 2016 14:26:15 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Wang Nan <wangnan0@...wei.com>, vincent.weaver@...ne.edu
Cc:     mtk.manpages@...il.com, pi3orama@....com,
        linux-kernel@...r.kernel.org, lizefan@...wei.com,
        linux-man@...r.kernel.org
Subject: Re: [PATCH v3][manpages 1/2] perf_event_open.2: Document
 PERF_EVENT_IOC_PAUSE_OUTPUT

Hello Wang Nan,

On 10/24/2016 08:52 AM, Wang Nan wrote:
> Linux 4.7 (86e7972f690c1017fd086cdfe53d8524e68c661c) introduces
> PERF_EVENT_IOC_PAUSE_OUTPUT feature. Document it.
> 
> Signed-off-by: Wang Nan <wangnan0@...wei.com>
> Reviewed-by: Vince Weaver <vincent.weaver@...ne.edu>
> Cc: Michael Kerrisk <mtk.manpages@...il.com>
> ---
>  man2/perf_event_open.2 | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
> index fade28c..561331c 100644
> --- a/man2/perf_event_open.2
> +++ b/man2/perf_event_open.2
> @@ -1687,6 +1687,15 @@ the
>  .I data_tail
>  value should be written by user space to reflect the last read data.
>  In this case, the kernel will not overwrite unread data.
> +
> +When the mapping is read only (without
> +.BR PROT_WRITE ),
> +setting .I data_tail is not allowed.

Missing line breaks in the preceding line.

> +In this case, the kernel will overwrite data when sample coming, unless

I find that last line hard to understand.
s/sample coming/a sample arrives/?

> +the ring buffer is paused by a
> +.BR PERF_EVENT_IOC_PAUSE_OUTPUT
> +.BR ioctl (2)
> +system call before reading.
>  .TP
>  .IR data_offset " (since Linux 4.1)"
>  .\" commit e8c6deac69629c0cb97c3d3272f8631ef17f8f0f
> @@ -2865,6 +2874,21 @@ The argument is a BPF program file descriptor that was created by
>  a previous
>  .BR bpf (2)
>  system call.
> +.TP
> +.BR PERF_EVENT_IOC_PAUSE_OUTPUT " (since Linux 4.7)"
> +.\" commit 86e7972f690c1017fd086cdfe53d8524e68c661c
> +This allows pausing and resuming the event's ring-buffer. A
> +paused ring-buffer does not prevent generation of samples, but simply
> +discards the samples. The discarded samples are considered lost,
> +causing
> +.BR PERF_RECORD_LOST
> +to be generated when possible.
> +
> +The argument is an integer. A nonzero value pauses the ring-buffer,
> +zero resumes the ring-buffer.
> +
> +Pausing a read only ring buffer before reading from it without having
> +to worry about data being overwritten.

That last sentence seems incomplete. I can't understand what you
mean here?

Thanks,

Michael


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