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

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Date:	Wed, 18 Dec 2013 16:22:36 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
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

Peter Zijlstra <peterz@...radead.org> writes:

> On Wed, Dec 18, 2013 at 04:01:04PM +0200, Alexander Shishkin wrote:
>> > Why don't you start by explaining _why_ you need a second stream to
>> > begin with?
>> 
>> Oh, I'm sure I've explained it earlier ([1], [2])
>
> See, I didn't read 0 because that information gets lost and patches
> should be self explanatory, and i didn't get to the Intel driver yet
> because well, I got stuck in the generic code.

Sure. The general concept is more important than the actual driver at
this point anyway.

>> but why not. The data
>> in the second stream is generated at a rate which is hundreds of
>> megabytes per second per core. Decoding this data is ~1000 times slower
>> than generating it. Ergo, can't be done in kernel, needs to be exported
>> as-is to userspace for later retreival and decoding. Doing it via perf
>> stream means an extra copy, which at these rates is a waste. Ergo, a
>> second buffer.
>
> Still confused, if you cannot copy it into one buffer, then why can you
> copy it into a second buffer?

It's not copied, hardware writes directly into that second buffer.

I've done the same with BTS now (as Ingo suggested) and it also benefits
from this approach.

Regards,
--
Alex
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