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

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Date:	Thu, 19 Dec 2013 16:14:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	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>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow
 tracing units

On Thu, Dec 19, 2013 at 04:54:27PM +0200, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> 
> > On Thu, Dec 19, 2013 at 12:57:59PM +0100, Peter Zijlstra wrote:
> > So you're basically forced to stop the tracing on PMI anyhow; so your
> > continuous tracing argument goes out the window.
> 
> It's only stopped inside the PMI handler to set up another buffer, and
> is then started again, so no useful trace is lost. PMI handler is not
> traced. What you're proposing is stopping it for good till perf collects
> the previous data, which will lose us a lot of trace. So my argument
> stands.

That is not what I proposed at all.

The PMI will swizzle the pages and resume recording. If there is no
space in the output buffer, we'll simply re-use the existing pages and
overwrite data.



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