linux-kernel - Re: [RFC][PATCH 1/2] [PATCH 1/2] tracing: Add TRACE_EVENT

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Date:	Thu, 02 Dec 2010 14:58:48 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Arjan van de Ven <arjan@...radead.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Subject: Re: [RFC][PATCH 1/2] [PATCH 1/2] tracing: Add TRACE_EVENT_CONDITIONAL()

On 12/02/2010 02:36 PM, Steven Rostedt wrote:
> From: Steven Rostedt<srostedt@...hat.com>
>
> There are instances in the kernel that we only want to trace
> a tracepoint when a certain condition is set. But we do not
> want to test for that condition in the core kernel.
> If we test for that condition before calling the tracepoin, then
> we will be performing that test even when tracing is not enabled.
> This is 99.99% of the time.
>
> We currently can just filter out on that condition, but that happens
> after we write to the trace buffer. We just wasted time writing to
> the ring buffer for an event we never cared about.
>
> This patch adds:
>
>     TRACE_EVENT_CONDITION() and DECLARE_TRACE_CLASS_CONDITION()
>
> These have a new TP_CONDITION() argument that comes right after
> the TP_ARGS().  This condition can use the parameters of the
> TRACE_EVENT() to determine if the tracepoint should be traced
> or not. The TP_CONDITION() will be placed in a if (cond) trace;
>
> For example, for the tracepoint sched_wakeup, it is useless to
> trace an wakeup event where the caller never actually work
> anything up (success = 0). So adding:
                       ^^^
                    s/=/==/

As much as I hate to be a pedant, I would suggest changing this change 
log for the sake of clarity.

David Daney
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