Re: [PATCH 2/2] perf_event_open.2: Document write_backward

From	Vince Weaver <vincent.weaver@maine.edu>
Newsgroups	linux.kernel
Subject	Re: [PATCH 2/2] perf_event_open.2: Document write_backward
Date	2016-10-21 23:30 +0200
Message-ID	<suPy1-2Jv-7@gated-at.bofh.it> (permalink)
References	<suGl5-4T9-35@gated-at.bofh.it> <suGuJ-4WC-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Fri, 21 Oct 2016, Wang Nan wrote:

>            context_switch :  1,  /* context switch data */
> -
> -          __reserved_1   : 37;
> +          write_backward :  1,  /* Write ring buffer from end to beginning */
> +          __reserved_1   : 36;

This removes a blank line, not sure if intentional or not.

> +.IR "write_backward" " (since Linux 4.6)"

It didn't committed until Linux 4.7 from what I can tell?

> +This makes the resuling event use a backward ring-buffer, which
resulting

> +writes samples from the end of the ring-buffer.
> +
> +It is not allowed to connect events with backward and forward
> +ring-buffer settings together using
> +.B PERF_EVENT_IOC_SET_OUTPUT.
> +
> +Backward ring-buffer is useful when the ring-buffer is overwritable
> +(created by readonly
> +.BR mmap (2)
> +). 

A ring buffer is over-writable when it is mmapped readonly?
Is this a hard requirement?
Can you set the read-backwards bit if not mapped readonly?

Otherwise the documentation seems reasonable.

Reviewed-by: Vince Weaver <vincent.weaver@maine.edu>

Thread

[PATCH 1/2] perf_event_open.2: Document PERF_EVENT_IOC_PAUSE_OUTPUT Wang Nan <wangnan0@huawei.com> - 2016-10-21 13:40 +0200
  [PATCH 2/2] perf_event_open.2: Document write_backward Wang Nan <wangnan0@huawei.com> - 2016-10-21 13:50 +0200
    Re: [PATCH 2/2] perf_event_open.2: Document write_backward Vince Weaver <vincent.weaver@maine.edu> - 2016-10-21 23:30 +0200
      Re: [PATCH 2/2] perf_event_open.2: Document write_backward "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-10-22 12:10 +0200
        Re: [PATCH 2/2] perf_event_open.2: Document write_backward "Wangnan (F)" <wangnan0@huawei.com> - 2016-10-24 08:50 +0200
  Re: [PATCH 1/2] perf_event_open.2: Document  PERF_EVENT_IOC_PAUSE_OUTPUT Vince Weaver <vincent.weaver@maine.edu> - 2016-10-21 23:20 +0200
    Re: [PATCH 1/2] perf_event_open.2: Document  PERF_EVENT_IOC_PAUSE_OUTPUT "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-10-22 12:10 +0200
  Re: [PATCH 1/2] perf_event_open.2: Document  PERF_EVENT_IOC_PAUSE_OUTPUT "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-10-22 12:10 +0200

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