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

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Message-ID: <20150507162223.GF7862@kernel.org>
Date:	Thu, 7 May 2015 13:22:23 -0300
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Peter Zijlstra <peterz@...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

Em Thu, May 07, 2015 at 04:39:39PM +0200, Peter Zijlstra escreveu:
> 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?

If we have more than one event we _need_ PERF_SAMPLE_ID, to
disambiguate, if we don't, then the lost events are just for that one,
no?

- Arnaldo
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