linux-kernel - Re: [PATCH 2/2] perf_event_open.2: Document write

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Date:   Sat, 22 Oct 2016 12:05:55 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Vince Weaver <vincent.weaver@...ne.edu>,
        Wang Nan <wangnan0@...wei.com>
Cc:     mtk.manpages@...il.com, pi3orama@....com,
        linux-kernel@...r.kernel.org, linux-man@...r.kernel.org,
        lizefan@...wei.com
Subject: 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:
> 
>>            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@...ne.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/