linux-kernel - Re: [RFC PATCH] tracing: Don't call wakeup() when committing the event

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Date:	Tue, 3 May 2011 16:27:22 -0700
From:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Michael Rubin <mrubin@...gle.com>,
	David Sharp <dhsharp@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] tracing: Don't call wakeup() when committing the event

On Tue, May 3, 2011 at 4:08 PM, Frederic Weisbecker <fweisbec@...il.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

Vaibhav Nagarnaik
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