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

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Date:	Thu, 7 May 2015 11:21:20 -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 11:15:20AM -0300, Arnaldo Carvalho de Melo escreveu:
> 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...

I.e.:

  [acme@...andy linux]$ perf record -e cycles usleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.013 MB perf.data (8 samples) ]
  [acme@...andy linux]$ 
  [acme@...andy linux]$ perf evlist -v
  cycles: size: 112, { sample_period, sample_freq }: 4000,
  sample_type: IP|TID|TIME|PERIOD, disabled: 1, inherit: 1, mmap: 1,
  comm: 1, freq: 1, enable_on_exec: 1, task: 1, sample_id_all: 1,
  exclude_guest: 1, mmap2: 1, comm_exec: 1
  [acme@...andy linux]$

Just one event, no need for PERF_SAMPLE_ID.

But with > 1 events:

  [acme@...andy linux]$ perf record -e cycles,instructions usleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.014 MB perf.data (8 samples) ]
  [acme@...andy linux]$ perf evlist -v
  cycles: size: 112, { sample_period, sample_freq }: 4000,
  sample_type: IP|TID|TIME|ID|PERIOD, read_format: ID, disabled: 1,
  inherit: 1, mmap: 1, comm: 1, freq: 1, enable_on_exec: 1, task: 1,
  sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1
  instructions: size: 112, config: 1,
  { sample_period, sample_freq }: 4000,
  sample_type: IP|TID|TIME|ID|PERIOD, read_format: ID, disabled: 1,
  inherit: 1, freq: 1, enable_on_exec: 1, sample_id_all: 1,
  exclude_guest: 1
  [acme@...andy linux]$

- Arnaldo
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