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

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Date:	Thu, 7 May 2015 19:37:50 +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 01:22:23PM -0300, Arnaldo Carvalho de Melo wrote:
> 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?

Sure, PERF_SAMPLE_ID is required, but attr::sample_id_all is not is it?

We can largely get by without using sample_id_all, as we did for a
while.

That said; sample_id_all has been around for more than 4 years and its
recommended for use; but should we mandate it?
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