linux-kernel - Re: [PATCH V3 1/4] perf: Add PERF_RECORD

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Date:	Wed, 8 Jul 2015 10:28:05 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Adrian Hunter <adrian.hunter@...el.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Jiri Olsa <jolsa@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	mathieu.poirier@...aro.org, Pawel Moll <pawel.moll@....com>
Subject: Re: [PATCH V3 1/4] perf: Add PERF_RECORD_SWITCH to indicate context
 switches

Em Wed, Jul 08, 2015 at 12:52:40AM +0200, Peter Zijlstra escreveu:
> On Tue, Jul 07, 2015 at 01:13:59PM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Tue, Jul 07, 2015 at 05:36:14PM +0200, Peter Zijlstra escreveu:
> > > > To help userspace in places where all it has is the union perf_event, we
> > > > can reuse one bit in misc to state that, i.e.

> > > >   #define PERF_RECORD_MISC_SWITCH_NEXT_PREV_PID 14
 
> > > > For instance.

> > > The other option would be a separate RECORD type, which might be
> > > simpler.

> > Humm, do we really need it?

> > I think this is just us wanting to, since we are going to add a new
> > record, to make it more useful for other, not right now needed,
> > situations, i.e. if the user is priviledged, there are two other options
> > to get his info, right?
 
> I was just thinking that 2 records, each with a fixed layout would be
> easier to parse than 1 record with variable layout.
 
> The record space is immense, so from that point it really doesn't
> matter.

We could do a land grab at some point there, if/when we find some reason
for that... :-)
 
> Do whatever is easiest, less mistakes get made etc. :-)
 
> No real preference either way, as long we we've thought about it.

Right, I just don't want to have two u32 carrying -1 for no reason.

- Arnaldo
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