linux-kernel - Re: [RFC][PATCH 1/2 v2] tracing: Add TRACE_EVENT

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Date:	Thu, 2 Dec 2010 23:54:02 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
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 <mathieu.desnoyers@...icios.com>
Subject: Re: [RFC][PATCH 1/2 v2] tracing: Add TRACE_EVENT_CONDITIONAL()

* Steven Rostedt (rostedt@...dmis.org) wrote:
[...]
> -#define __DO_TRACE(tp, proto, args)					\
> +#define __DO_TRACE(tp, proto, args, cond)				\
>  	do {								\
>  		struct tracepoint_func *it_func_ptr;			\
>  		void *it_func;						\
>  		void *__data;						\
>  									\
> +		if (!(cond))						\

One small documentation-related detail: my guess is that you are leaving
"cond" without likely/unlikely builtin expect purposefully so that we
can write, in TP_CONDITION:

  TP_CONDITION(unlikely(someparam)),

when we expect the condition to be usually false (and likely() for the
reverse). Maybe it could be worth documenting that expressions like the
following are valid :

  TP_CONDITION((likely(param1) && unlikely(param2)) || likely(param3))

It's fair to assume that kernel developers know this already, but given
we plan to re-use TRACE_EVENT() for the user-space tracer soon enough,
documenting this kind of use-case now can save us the trouble in the
future.

Other than that,

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

Thanks!

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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