linux-kernel - Re: [PATCH] ext4: Use trace condition for ext4

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Message-ID: <20131118160938.39f37e80@gandalf.local.home>
Date:	Mon, 18 Nov 2013 16:09:38 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Use trace condition for ext4_mballoc tracepoints


Ug,

Please ignore this for now. It wasn't suppose to go out yet, because we
didn't finish testing. I only wrote this up and put it in my claws-mail
"send-later" folder.

Well, what did I learn about claws mail today?

I learned that there's a "send" button next to the "compose" button
that when you hit it, it sends out all your "send later" messages :-p

Which I now have to look through and think about all the crap I wrote
about people just to feel good, and hit "send later" on, never
expecting it to ever go out.

Oh well, now everyone knows how much of an asshole I think they are ;-)

-- Steve


On Thu, 14 Nov 2013 23:08:11 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> Doing an if statement to test some condition to know if we should
> trigger a tracepoint is pointless when tracing is disabled. This just
> adds overhead and wastes a branch prediction. This is why the
> TRACE_EVENT_CONDITION() was created. It places the check inside the jump
> label so that the branch does not happen unless tracing is enabled.
> 
> That is, instead of doing:
> 
> 	if (em)
> 		trace_btrfs_get_extent(root, em);
> 
> Which is basically this:
> 
> 	if (em)
> 		if (static_key(trace_btrfs_get_extent)) {
> 
> 
> Using a TRACE_EVENT_CONDITION() we can just do:
> 
> 	trace_btrfs_get_extent(root, em);
> 
> And the condition trace event will do:
> 
> 	if (static_key(trace_btrfs_get_extent)) {
> 		if (em) {
> 			...
> 
> The static key is a non conditional jump (or nop) that is faster than
> having to check if em is NULL or not.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

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