linux-kernel - Re: [PATCH 00/13] perf_events: add support for sampling taken branches (v3)

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Message-ID: <4F1ED780.3010305@gmail.com>
Date:	Tue, 24 Jan 2012 09:08:32 -0700
From:	David Ahern <dsahern@...il.com>
To:	Stephane Eranian <eranian@...gle.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, acme@...radead.org,
	robert.richter@....com, ming.m.lin@...el.com, andi@...stfloor.org,
	asharma@...com, ravitillo@....gov, vweaver1@...s.utk.edu
Subject: Re: [PATCH 00/13] perf_events: add support for sampling taken branches
 (v3)


On 01/24/2012 08:39 AM, Stephane Eranian wrote:
> Hi,
> 
> The branch stack sampling patch exposes a flaw in the sampling
> buffer format as currently exported by the kernel.
> 
> In the current format, sample records (RECORD_SAMPLE) are NOT
> self-describing. That means that by looking at the fixed size header, it
> is not possible to determine which event caused the sample to be recorded
> and what's in the body of the variable length sample.
> 
> Such introspection is only possible once we know the event unique id
> (PERF_SAMPLE_ID). But to get the event ID, we need to parse the
> sample. But, given that a sample has a variable length, there is no
> predefined position for that ID in the sample. You have a chicken
> and egg problem here. There is no room left in the fixed size header
> to fit this in.

I brought this up last Fall as well. As I recall the response is to move
each sample_type based stream into its own data file and then merge the
data files while processing.

David
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