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

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Date:	Mon, 2 Feb 2015 11:05:12 +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 11:52:26AM +0200, Adrian Hunter wrote:
> On 02/02/15 11:15, Jiri Olsa wrote:
> > 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.
> 
> Why not make it the same as all the other data. i.e. find the start and size
> via the index? And then just lump all the data together?

thats what I suggested

> 
> > 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.
> 
> Would it need to be last? Logically it should precede the data that depends
> on it.

i suggested this as a workaround for having features at the end of the file
while keeping the current perf data header

jirka
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