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    SubjectRe: [PATCH v3][manpages 1/2] perf_event_open.2: Document PERF_EVENT_IOC_PAUSE_OUTPUT
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    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@huawei.com>
    > Reviewed-by: Vince Weaver <vincent.weaver@maine.edu>
    > Cc: Michael Kerrisk <mtk.manpages@gmail.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/

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