linux-kernel - Re: [PATCH] perf record: mmap output file

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Date:	Tue, 15 Oct 2013 07:25:04 -0600
From:	David Ahern <dsahern@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>, Ingo Molnar <mingo@...nel.org>
CC:	acme@...stprotocols.net, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf record: mmap output file - v2

On 10/15/13 1:09 AM, Namhyung Kim wrote:
>> The stat() seems superfluous, here in __cmd_record() we've just checked
>> the output_name and made sure it exists. Can that stat() call ever fail?
>
> AFAICS it's needed to check current file size.  But I think it's better
> to use fstat().

Sure fstat could be used over stat -- if it ends up staying.

>>
>> 3)
>>
>> The rec->bytes_at_mmap_start field feels a bit weird. If I read the code
>> correctly, in every 'perf record' invocation, rec->bytes_written starts at
>> 0 - i.e. we don't have repeat invocations of cmd_record().
>
> rec->bytes_written is updated when it writes to the output file for
> synthesizing COMM/MMAP events (this mmap output is not used at that time).

Ingo: I went through a number of itereations before using the 
bytes_at_mmap_start. One of those was to use the bytes_written counter. 
All failed. Header + synthesized events are written to the file before 
we start farming the ring buffers.

Perhaps a good code cleanup will help figure out why. I needed the 
functionality ASAP for use with perf-trace -a so I stuck with the new 
variable. Since this change is working out well, I will look at a code 
clean up on the next round.

I am traveling to LinuxCon / KVM Forum / Tracing Forum on Friday. 
Perhaps the clean up and followup patch can be done on the long plane 
ride; more likely when I return which means 3.14 material.

> Actually I worried about the mmap offset not being aligned to page
> size.  But it seems that's not a problem.

This code snippet makes sure the mmap offset is a multiple of 64M 
(rec->mmap_size). offset is the argument to mmap; mmap_offset is the 
where we are within the mmap for the next copy:

+		offset = rec->bytes_at_mmap_start + rec->bytes_written;
+		if (offset < (ssize_t) rec->mmap_size) {
+			rec->mmap_offset = offset;
+			offset = 0;
+		} else
+			rec->mmap_offset = 0;

David
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