linux-kernel - Re: [RFC][PATCH 0/2] tracing: Add conditional to tracepoints

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Date:	Fri, 3 Dec 2010 00:19:59 +0100
From:	Frederic Weisbecker <fweisbec@...il.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>,
	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 0/2] tracing: Add conditional to tracepoints

On Thu, Dec 02, 2010 at 05:36:54PM -0500, Steven Rostedt wrote:
> This is an RFC that adds the TP_CONDITION() to the TRACE_EVENT()
> code.
> 
> There are certain cases that a tracepoint only makes sense if
> a specific condition is met. But because we do not want to dirty
> the fast path (the non tracing case) with if statements that are
> there only to avoid tracing, we just pass the condition variables
> to the tracepoint, and let the user filter them out if needed.
> 
> A perfect example is the tracepoint sched_wakeup. It traces all
> calls to try_to_wake_up() even if it fails to wake up. But if we add:
> 
> 	if (success)
> 		trace_sched_wakeup(p);
> 
> We have that "if (success)" tested for every time we call try_to_wake_up().
> Even when tracing is not (or never will be) enabled.
> 
> This patch set adds a variant TRACE_EVENT_CONDITIONAL()
> (and DECLARE_EVENT_CLASS_CONDITIONAL()) that has a "cond" argument.
> This argument is encapsulated with "TP_CONDITIONAL()" which turns into:
> 
> 	if (!cond)
> 		return;
> 
> This is placed inside the called tracepoint routine, and is only tested
> when the trace is enabled. Otherwise it is a nop as tracepoints normally
> are when disabled.
> 
> Note, another variant of this, is to move the test directly into the
> _DO_TRACE() macro, and not call any registered event callbacks. This would
> even speed it up faster when tracing is enabled. I did not do this
> orginially because I just thought of it now as I wrote this change log ;-)

Hehe :)

Yeah indeed. And that looks fairly possible.


> I'm posting this version now just in case people prefer it instead.
> 
> The following patches are in:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
> 
>     branch: rfc/trace-conditional
> 
> 
> Steven Rostedt (2):
>       tracing: Add TRACE_EVENT_CONDITIONAL()
>       tracing: Only trace sched_wakeup if it actually work something up
> 
> ----
>  include/linux/tracepoint.h   |    3 ++
>  include/trace/define_trace.h |    6 +++
>  include/trace/events/sched.h |   12 +++---
>  include/trace/ftrace.h       |   70 +++++++++++++++++++++++++++++++----------
>  4 files changed, 68 insertions(+), 23 deletions(-)
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