linux-kernel - Re: [PATCH 14/42] perf record: Add --index option for building index table

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Date:	Mon, 2 Feb 2015 10:15:54 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Stephane Eranian <eranian@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 14/42] perf record: Add --index option for building index
 table

On Mon, Feb 02, 2015 at 10:34:50AM +0200, Adrian Hunter wrote:

SNIP

> > but how about bump up the header version for this feature? ;-)
> > 
> > currently it's:
> > 
> > struct perf_file_header {
> >         u64                             magic;
> >         u64                             size;
> >         u64                             attr_size;
> >         struct perf_file_section        attrs;
> >         struct perf_file_section        data;
> >         /* event_types is ignored */
> >         struct perf_file_section        event_types;
> >         DECLARE_BITMAP(adds_features, HEADER_FEAT_BITS);
> > };
> > 
> > 
> > - we already store attrs as a FEATURE so we could omit that
> > - your patch stores only synthesized data into 'data' section (-1 idx)
> >   this could be stored into separate file and get merged with the rest
> > - new header version would have 'features' section, so the features
> >   position wouldnt depend on the 'data' end as of now and we could
> >   easily store after all data is merged:
> > 
> > struct perf_file_header {
> >         u64                             magic;
> >         u64                             size;
> >         u64                             attr_size;
> >         struct perf_file_section        features;
> >         DECLARE_BITMAP(adds_features, HEADER_FEAT_BITS);
> > };
> > 
> > 
> > thoughts?
> 
> How come the features are being written before the sample data anyway?
> I would have expected:
> 	- write the data (update the index in memory)
> 	- write the features (including index)
>

I think the problem is that the only way how to get features offset
right now is via perf_file_header::data.offset + perf_file_headerdata.size,
and we still use this section to carry 'sythesized' data, so it needs
to have correct size.

I guess we could workaround that by storing the 'perf_file_header::data'
as the last data section. That would require to treat it the same way as
all other data sections, but we could keep current header layout.

jirka
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