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

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Message-ID: <20140107005231.GZ20765@two.firstfloor.org>
Date:	Tue, 7 Jan 2014 01:52:31 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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

> > 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.

If you interleave you may actually end up with lots of small rings 
in a single buffer, unless you stop every time the buffer fills up
(which would add a lot more overhead)

I suppose it could be somehow parsed, but it would very different 
from what perf does today.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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