Re: perf.data file format specification draft

[Namhyung Kim <namhyung@kernel.org>]

Hi Arnaldo and Andi,

On Thu, May 14, 2015 at 10:11:29AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Thu, May 14, 2015 at 02:25:17PM +0200, Andi Kleen escreveu:
> > Hi,
> > 
> > Since there are more and more consumers I started a description of the
> > on-disk perf.data format. This does not replace the kernel perf event
> > description or the manpage, but describes the parts that perf record
> > adds.
> > 
> > So far it is still has some gaps and needs review. Eventually this should
> > become part of the perf documentation.
> > 
> > Steven, would be good if you could fill in some details on how trace
> > data works. 
> 
> I guess that would be Frédéric, and also I think this is a good
> opportunity to remove some stuff that seem to be collected but unused,
> namely a kallsyms copy and maybe something else.

Let me try to describe.  The tracing_data_get() does the work and it
records the following for HEADER_TRACING_DATA in order:

 * tracing header data
   - file magic bytes (including someone's birthday :) )
   - file version (it's 0.5 - the only thing I can see in diff with trace-cmd
     (version 6) is "saved-cmdline" file data which is unnecessary for perf
   - byte order, size of long and page size of the system

 * tracing header files
   - $tracefs/events/header_page and $tracefs/events/header_event
   - describe ftrace raw buffer format which is unnecessary for perf unless
     it reads the raw buffer directly (like my ftrace integration work?)

 * ftrace event files
   - format file for each event in $tracefs/events/ftrace directory
   - this is same as below but precedes other events, not sure why it's needed

 * (normal) event files
   - format file for each tracepoint event

 * /proc/kallsyms
   - for kernel symbol resolution, unnecessary for perf

 * tracing printk formats
   - for trace_printk?  unnecessary for perf

The last two can go away at least.

Thanks,
Namhyung


> 
> > Adrian, would be good if you could fill in the missing bits for
> > auxtrace/itrace.
> > Everyone else, please review and add missing information.
> 
> Thanks for doing this work!
> 
> IIRC there is a presentation written by Jiri where parts of this is
> documented, lemme try to find it...
> 
> - Arnaldo
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