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

On 10/21/2016 11:25 PM, Vince Weaver wrote:

On Fri, 21 Oct 2016, Wang Nan wrote:

quoted

           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.

quoted

+.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.

quoted

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

resulting

quoted

+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?

Wang Nan, could you perhaps clarify this in the next version of the patch?

Otherwise the documentation seems reasonable.

Reviewed-by: Vince Weaver <redacted>

Thanks for reviewing both patches, Vince. Wang Nan, please include the
Reviewed-by: in the next patch iteration.

Cheers,

Michael

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