linux-kernel - Re: [PATCH V8 7/8] perf, x86: introduce PERF_RECORD_LOST

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Message-ID: <20150507143939.GW21418@twins.programming.kicks-ass.net>
Date:	Thu, 7 May 2015 16:39:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arnaldo Carvalho de Melo <acme@...radead.org>
Cc:	Kan Liang <kan.liang@...el.com>, mingo@...nel.org,
	eranian@...gle.com, andi@...stfloor.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V8 7/8] perf, x86: introduce PERF_RECORD_LOST_SAMPLES

On Thu, May 07, 2015 at 11:15:20AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Thu, May 07, 2015 at 01:54:46PM +0200, Peter Zijlstra escreveu:
> > On Thu, May 07, 2015 at 01:35:24PM +0200, Peter Zijlstra wrote:
> > > 
> > >  - dropped the @id field from the record, it is already included in the
> > >    @sample_id values.
> > 
> > Hmm, this would force people to use sample_id; which in general is a
> > good idea, but should we really force that on people?
> 
> Well, if there are more than one sample, we need it, right? If there is
> just one, we don't need it, what is different? Am I needing (even more)
> coffee?
> 
> /me goes read some code...

So the question was, do we do:

	/*
	 * struct {
	 *	struct perf_event_header	header;
	 *	u64				id;
	 *	u64				lost;
	 *	struct sample_id		sample_id;
	 * };
	 */
	PERF_RECORD_LOST_SAMPLES

And have the id thing twice if attr.sample_id && PERF_SAMPLE_ID, but
allow decoding if !attr.sample_id.

Or force attr.sample_id && PERF_SAMPLE_ID if there's multiple events and
do away with the extra id field, like:

	/*
	 * struct {
	 *	struct perf_event_header	header;
	 *	u64				lost;
	 *	struct sample_id		sample_id;
	 * };
	 */
	PERF_RECORD_LOST_SAMPLES

Should we force the use of sample_id on people?
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