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

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Date:	Thu, 19 Dec 2013 11:26:25 +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 09:53:44AM +0200, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> > The thing is; why can't you zero-copy whatever buffer the hardware
> > writes into, into the normal buffer?
> 
> I'm not sure I understand. You mean, have the buffer split between perf
> data and trace data?

Yep, I don't see any reason why this wouldn't work.

When the hardware thing sends an interrupt to notify us its buffer is
'full', stop the recorder, try to create a single record in the buffer
that's big enough + 1 page, then swizzle the hardware pages and the
buffer pages for that record, using the +1 page to page align the actual
data. Then (re)start the hardware on the 'new' pages.


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