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

Michael Kerrisk (man-pages)
2016-10-21T08:57:18Z

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/