linux-kernel - Re: [PATCH v4 11/22] perf: add ITRACE_START record to indicate that tracing has started

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Date:	Tue, 9 Sep 2014 11:08:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Robert Richter <rric@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>, kan.liang@...el.com
Subject: Re: [PATCH v4 11/22] perf: add ITRACE_START record to indicate that
 tracing has started

On Wed, Aug 20, 2014 at 03:36:08PM +0300, Alexander Shishkin wrote:
> For counters such as instruction tracing, it is useful for the decoder
> to know which tasks are running when the event is first scheduled in,
> before the first sched_switch.
> 
> To single out such instruction tracing pmus, this patch alse introduces
> ITRACE PMU capability.

You forgot to tell why. Also explain why.

The next context switch event will also tell you the previous task. So
this seems like superfluous information. Also if you really want this
you could simply emit an empty AUX record I suppose, no need to create
yet another record type.

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