linux-kernel - Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow tracing units

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Date:	Tue, 7 Jan 2014 09:42:15 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow
 tracing units

On Tue, Jan 07, 2014 at 01:52:31AM +0100, Andi Kleen wrote:
> > > Also of course it requires disabling/enabling PT explicitly for 
> > > every perf message, which is slow. So you add at least 2*WRMSR cost
> > > (thousands of cycles).
> > 
> > That's just dumb, no flush the entire PT buffer into a few large
> > records.
> 
> How would that work?
> 
> You mean a separate buffer and then copy or map?
> 
> ------
> 
> Also here are some more problems with interleaving: 
> 
> A common PT config is to just run it as a ring buffer in the background
> and only take the data out when something happens (sample, crash etc.)
> 
> But the side band still needs to be logged and at arbitary times.
> 
> So the PT wrapping will happen much more often than the perf wrapping.

So create two events, one for the PT stuff and one to track the
side-band stuff. We have a NOP event for just this purpose.
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