linux-kernel - Re: [PATCH V2 12/15] perf tools: allow non-matching sample types

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Message-ID: <CABPqkBQar4SKq_A+=ShdjJgYfxW90yuD=M=WQ2HjFwR_bKxKkg@mail.gmail.com>
Date:	Wed, 3 Jul 2013 08:44:25 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	David Ahern <dsahern@...il.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH V2 12/15] perf tools: allow non-matching sample types

On Tue, Jul 2, 2013 at 9:09 AM, Adrian Hunter <adrian.hunter@...el.com> wrote:
> On 01/07/13 21:53, David Ahern wrote:
>> On 7/1/13 3:32 AM, Adrian Hunter wrote:
>>> Snip
>>>
>>>>
>>>> While this works for a combined S/W and tracepoint events session, I do not
>>>> like promoting sample types to the minimum compatible level for all events
>>>> in the session. perf needs to allow each event to have its own sample_type
>>>> and not force a minimal compatibility.
>>>
>>> Why?  The impact is small. The kernel API is completely unchanged.
>>
>> I'd like to see libperf become a stable, usable library - usable by more
>> than the perf binary and its builtin commands. I have already done this once
>> for a daemon, and it was a PITA to get the specific use functional without
>> memory leaks/growth in the libperf part.
>>
>> With respect to this specific patch it means appropriate flexibility in the
>> data collected for events. ie., each event can have its own sample_type. For
>> example if the tracepoint already contains task information TID is not
>> needed - and IP may not be wanted either. The code processing the samples
>> should not require all events to have some minimum data format - that just
>> wastes buffer space.
>
> It would be more compelling to provide a use-case where that "waste"
> actually makes any difference.
>
In my opinion, it's not so much of the "wasted" space than on the ease of use
for tools. With your change, tools would have to know something about the order
in which sample_type is laid out. And it just happens that it is not
as trivial as
expected. It is NOT the bit position order in sample_type. So this is more error
prone.

I prefer your IDENTIFIER solution better, yet it also implies that this flag is
special. It provides the event ID at the first position in the sample's body.
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