linux-kernel - Re: [PATCH 01/16] perf tools: Add id index

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Date:	Wed, 29 Oct 2014 09:55:14 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 01/16] perf tools: Add id index

On Fri, Oct 24, 2014 at 10:25:02AM +0300, Adrian Hunter wrote:
> On 24/10/14 00:08, Arnaldo Carvalho de Melo wrote:
> > Em Thu, Oct 23, 2014 at 01:45:09PM +0300, Adrian Hunter escreveu:
> >> Add an index of the event identifiers.
> >>
> >> This is needed to queue Instruction
> >> Trace samples according to the mmap
> >> buffer from which they were recorded.
> > 
> > This gets difficult to review, I end up having to look at all the
> > patches together to figure out the use cases, to see if this here makes
> > sense...
> > 
> > Can you try to explain like to a seven year old?
> > 
> > Sigh.
> > 
> > 'id' is somethig super vague, what does this identifies? I want to make
> > progress processing these patches, but with so short explanations like
> > the above one, it gets difficult.
> 
> This is the event id (also called the sample id) which is a unique number
> allocated by the kernel to the event created by perf_event_open().  Events
> can include the event id by having a sample type including PERF_SAMPLE_ID or
> PERF_SAMPLE_IDENTIFIER.
> 
> Currently the main use of the event id is to match an event back to the
> evsel to which it belongs i.e. perf_evlist__id2evsel()
> 
> The purpose of this patch is to make it possible to match an event back to
> the mmap from which it was read.  The reason that is useful is because the
> mmap represents a time-ordered context (either for a cpu or for a thread).
> Intel PT decodes trace information on that basis.  In full-trace mode, that
> information can be recorded when the Intel PT trace is read, but in
> sample-mode the Intel PT trace data is embedded in a sample and it is in
> that case that the "id index" is needed.
> 
> So the mmaps are numbered (idx) and the cpu and tid recorded against the id
> by perf_evlist__set_sid_idx() which is called by perf_evlist__mmap_per_evsel().
> 
> That information is recorded on the perf.data file in the new "id index".
> idx, cpu and tid are added to struct perf_sample_id (which is the node of
> evlist's hash table to match ids to evsels).  The information can be
> retrieved using perf_evlist__id2sid().  Note however this all depends on
> having a sample type including PERF_SAMPLE_ID or PERF_SAMPLE_IDENTIFIER,
> otherwise ids are not recorded.
> 
> The "id index" is a synthesized event record which will be created when
> Intel PT sampling is used by calling perf_event__synthesize_id_index().

with above text in changelog:

Acked-by: Jiri Olsa <jolsa@...nel.org>

thanks,
jirka
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