Re: [PATCH 3/4] tracing: Add TRACE_EVENT_CONDITIONAL()

[KOSAKI Motohiro]

> From: Steven Rostedt <srostedt@xxxxxxxxxx>
> 
> 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 DEFINE_EVENT_CONDITION()
> 
> These have a new TP_CONDITION() argument that comes right after
> the TP_ARGS().  This condition can use the parameters of TP_ARGS()
> in 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 a wakeup event where the caller never actually wakes
> anything up (where success == 0). So adding:
> 
> 	TP_CONDITION(success),
> 
> which uses the "success" parameter of the wakeup tracepoint
> will have it only trace when we have successfully woken up a
> task.
> 
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Acked-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

I love this patch. thanks, Steven :)



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