linux-kernel - Re: [PATCH 1/5] trace: do not disable wake up tracer on output of trace

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Date:	Thu, 22 Jan 2009 09:41:05 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 1/5] trace: do not disable wake up tracer on output of
	trace


* Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <srostedt@...hat.com>
> 
> Impact: fix to erased trace output
> 
> To try not to have the outputing of a trace interfere with the wakeup 
> tracer, it would disable tracing while the output was printing. But if a 
> trace had started when it was disabled, it can show a partial trace. To 
> try to solve this, on closing of the tracer, it would clear the trace 
> buffer.
> 
> The latency tracers (wakeup and irqsoff) have two buffers. One for 
> recording and one for holding the max trace that is printed. The 
> clearing of the trace above should only affect the recording buffer. But 
> for some reason it would move the erased trace to the print buffer. 
> Probably due to a race with the closing of the trace and the saving ofhe 
> max race.

hm, that race needs to be fixed then.

> The above is all pretty useless, and if the user does not want the 
> printing of the trace to be traced itself, then the user can manual 
> disable tracing. This patch removes all the code that tries to keep the 
> output of the tracer from modifying the trace.

printing of the trace should not be traced. I cannot imagine any usage 
mode where that would be interesting - and i can imagine a ton of cases 
where users would be confused/distracted by the tracer in essence zapping 
their measurement by replacing it with some uninteresting 'the tracer 
itself has wakeup delays' data.

auto-disabling latency tracing while the trace is being output is 
essential. Measurement should never impact the workload that is being 
measured.

We should fix that race instead.

	Ingo
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