linux-kernel - Re: [RFC][PATCH 0/2 v2] tracing: Add conditional to tracepoints

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Date:	Fri, 3 Dec 2010 15:47:23 +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 v2] tracing: Add conditional to tracepoints

On Thu, Dec 02, 2010 at 11:03:58PM -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 DEFINE_EVENT_CONDITIONAL()) that has a "cond" argument.
> This argument is encapsulated with "TP_CONDITIONAL()" which turns into:
> 
>         if (!cond)
>                 return;
> 
> What's new with this version?
> 
> This version adds this if statement to __DO_TRACE(). As it is
> in a static inline function, the return still applies.
> 
> Also, since the shortcut is made before the callbacks, the test
> is at the call site. This means that you can have a DECLARE_EVENT_CLASS()
> where one of its DEFINE_EVENTS() is normal and the other is a
> DEFINE_EVENT_CONDITION().
> 
> The following patches are in:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
> 
>     branch: rfc/trace-conditional-v2
> 
> 
> Steven Rostedt (2):
>       tracing: Add TRACE_EVENT_CONDITIONAL()
>       tracing: Only trace sched_wakeup if it actually work something up
> 
> ----
>  include/linux/tracepoint.h   |   29 +++++++++++++++++++++++------
>  include/trace/define_trace.h |   15 +++++++++++++++
>  include/trace/events/sched.h |   16 ++++++++--------
>  3 files changed, 46 insertions(+), 14 deletions(-)


Looks good

Acked-by: Frederic Weisbecker <fweisbec@...il.com>

I suspect we'll need to fix some scripts/subperf commands in perf. Those that do:

	if (success)

tests after reading a trace from a wakeup event.
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