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

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Date:	Tue, 7 Jul 2015 17:36:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.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

On Tue, Jul 07, 2015 at 10:44:37AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Jul 07, 2015 at 10:25:52AM -0300, Arnaldo Carvalho de Melo escreveu:
> > Em Tue, Jul 07, 2015 at 11:36:39AM +0300, Adrian Hunter escreveu:
> > > To make the event useful for privileged users also, if the
> > > context is cpu-wide then the event will also provide the
> > > next or previous pid/tid.
> > 
> > <SNIP>
> >   
> > > +	/*
> > > +	 * Records a context switch in or out (flagged by
> > > +	 * PERF_RECORD_MISC_SWITCH_OUT).  next_prev_pid and next_prev_tid are
> > > +	 * (u32)-1 unless the context is cpu-wide, in which case they are the
> > 
> > Why carry those extra 8 bytes for non priviledged users, all the time
> > with -1?
> 
> > Can't userspace cope with this, i.e. we should be able to look for those
> > fields when the context is CPU wide, and to not look for them otherwise,
> > no?
> 
> 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.
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