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SubjectRe: [PATCH 2/2] perf_event_open.2: Document write_backward
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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?
> 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 <vincent.weaver@maine.edu>

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/

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