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Date: Wed, 4 May 2011 01:35:06 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Vaibhav Nagarnaik <vnagarnaik@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        Michael Rubin <mrubin@google.com>, David Sharp <dhsharp@google.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] tracing: Don't call wakeup() when committing the
 event
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On Tue, May 03, 2011 at 04:27:22PM -0700, Vaibhav Nagarnaik wrote:
> On Tue, May 3, 2011 at 4:08 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > On Tue, May 03, 2011 at 06:47:32PM -0400, Steven Rostedt wrote:
> >> On Wed, 2011-05-04 at 00:09 +0200, Frederic Weisbecker wrote:
> >>
> >> > I wonder if we should have a lite version of wake_up() that checks
> >> > if the list of waiters is empty before locking the queue.
> >> > After all we don't care much about tight races for tracing.
> >>
> >> Since tracing is a special case here, we probably could get away to just
> >> do it ourselves. That is, have our own flag that determines if we should
> >> wakeup or not.
> >
> > Yep, agreed.
> >
> How about this? Check if the waitqueue is active and if true, operate on the
> trace_wait queue.
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 06f4458..e03ee24 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -353,7 +353,7 @@ void trace_wake_up(void)
>  {
>  	int cpu;
> 
> -	if (trace_flags & TRACE_ITER_BLOCK)
> +	if (trace_flags & TRACE_ITER_BLOCK || !waitqueue_active(&trace_wait))
>  		return;
>  	/*
>  	 * The runqueue_is_locked() can fail, but this is the best we
> 

Hehe, seems it should work :)

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