Subject: Re: [PATCH][manpages 2/2] perf_event_open.2: Document write_backward
To: Wang Nan <wangnan0@huawei.com>, peterz@infradead.org, vince@deater.net
References: <1459147292-239310-5-git-send-email-wangnan0@huawei.com>
 <1459160189-240780-1-git-send-email-wangnan0@huawei.com>
Cc: mtk.manpages@gmail.com, linux-kernel@vger.kernel.org,
        linux-man@vger.kernel.org, pi3orama@163.com, lizefan@huawei.com,
        Vince Weaver <vincent.weaver@maine.edu>
From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <f7f97a4e-a700-ad84-4ad2-d92d286da630@gmail.com>
Date: Fri, 21 Oct 2016 10:57:10 +0200
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Thanks for this patch, Wangnan.

Vince, do you have any comments?

Cheers,

Michael

On 03/28/2016 12:16 PM, Wang Nan wrote:
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> ---
>  man2/perf_event_open.2 | 57 ++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 55 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
> index b232cba..942a410 100644
> --- a/man2/perf_event_open.2
> +++ b/man2/perf_event_open.2
> @@ -234,8 +234,10 @@ struct perf_event_attr {
>            mmap2          :  1,  /* include mmap with inode data */
>            comm_exec      :  1,  /* flag comm events that are due to exec */
>            use_clockid    :  1,  /* use clockid for time fields */
> +          context_switch :  1,  /* context switch data */
> +          write_backward :  1,  /* Write ring buffer from end to beginning */
>  
> -          __reserved_1   : 38;
> +          __reserved_1   : 36;
>  
>      union {
>          __u32 wakeup_events;    /* wakeup every n events */
> @@ -1105,6 +1107,30 @@ field.
>  This can make it easier to correlate perf sample times with
>  timestamps generated by other tools.
>  .TP
> +.IR "write_backward" " (since Linux 4.6)"
> +.\" commit ? (http://lkml.kernel.org/g/1459147292-239310-5-git-send-email-wangnan0@huawei.com)
> +This makes the resuling event use a backward ring-buffer, which
> +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)
> +). In this case,
> +.IR data_tail
> +is useless,
> +.IR data_head
> +points to the head of the most recent sample in a backward
> +ring-buffer. It is easy to iterate over the whole ring-buffer by reading
> +samples one by one because size of a sample can be found from decoding
> +its header. In contract, in a forward overwritable ring-buffer, the only
> +information is the end of the most recent sample which is pointed by
> +.IR data_head,
> +but the size of a sample can't be determined from the end of it.
> +.TP
>  .IR "wakeup_events" ", " "wakeup_watermark"
>  This union sets how many samples
>  .RI ( wakeup_events )
> @@ -1634,7 +1660,9 @@ And vice versa:
>  .TP
>  .I data_head
>  This points to the head of the data section.
> -The value continuously increases, it does not wrap.
> +The value continuously increases (or decrease if
> +.IR write_backward
> +is set), it does not wrap.
>  The value needs to be manually wrapped by the size of the mmap buffer
>  before accessing the samples.
>  
> @@ -2581,6 +2609,24 @@ Starting with Linux 3.18,
>  .B POLL_HUP
>  is indicated if the event being monitored is attached to a different
>  process and that process exits.
> +.SS Reading from overwritable ring-buffer
> +Reader is unable to update
> +.IR data_tail
> +if the mapping is not
> +.BR PROT_WRITE .
> +In this case, kernel will overwrite data without considering whether
> +they are read or not, so ring-buffer is overwritable and
> +behaves like a flight recorder. To read from an overwritable
> +ring-buffer, setting
> +.IR write_backward
> +is suggested, or it would be hard to find a proper position to start
> +decoding. In addition, ring-buffer should be paused before reading
> +through
> +.BR ioctl (2)
> +with
> +.B PERF_EVENT_IOC_PAUSE_OUTPUT
> +to avoid racing between kernel and reader. Ring-buffer should be resumed
> +after finish reading.
>  .SS rdpmc instruction
>  Starting with Linux 3.4 on x86, you can use the
>  .\" commit c7206205d00ab375839bd6c7ddb247d600693c09
> @@ -2693,6 +2739,13 @@ The file descriptors must all be on the same CPU.
>  
>  The argument specifies the desired file descriptor, or \-1 if
>  output should be ignored.
> +
> +Two events with different
> +.IR write_backward
> +settings are not allowed to be connected together using
> +.B PERF_EVENT_IOC_SET_OUTPUT.
> +.B EINVAL
> +is returned in this case.
>  .TP
>  .BR PERF_EVENT_IOC_SET_FILTER " (since Linux 2.6.33)"
>  .\" commit 6fb2915df7f0747d9044da9dbff5b46dc2e20830
> 


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