[2/2] perf_event_open.2: Document write_backward

Message ID 580DADD9.8020807@huawei.com
State New
Headers show

Commit Message

Wang Nan Oct. 24, 2016, 6:44 a.m.
On 2016/10/22 18:05, Michael Kerrisk (man-pages) wrote:
> On 10/21/2016 11:25 PM, Vince Weaver wrote:

>> 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.

> Maybe it would be better to keep it. I don't feel too strongly about

> this though.

>

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

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

> Yes, that's my recollection too.

>

>>> +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?


I'd like to explain over-writable ring buffer in patch 1/1 like this:


The ring buffer become over-writable because there's no way to tell kernel
the positioin of the last read data when mmaped read only.

>> Can you set the read-backwards bit if not mapped readonly?


I don't understand why we need read-backwards.

Mapped with PROT_WRITE is the *default* setting. In this case user program
like perf is able to tell the reading position to kernel through writing to
'data_tail'. In this case kernel won't overwrite unread data, it reads
forwardly.

Or do you think the naming is confusing? The name of 'write_backward' is
kernel-centric, means adjust kernel behavior. kernel *write* data, so I
call it 'write_backward'. The name 'read-backwards' is user-centric,
because user 'read' data.

Thank you.

Patch

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