* [PATCH] ext4: Use trace condition for ext4_mballoc tracepoints
@ 2013-11-15 4:08 Steven Rostedt
2013-11-18 21:09 ` Steven Rostedt
0 siblings, 1 reply; 2+ messages in thread
From: Steven Rostedt @ 2013-11-15 4:08 UTC (permalink / raw)
To: LKML; +Cc: Theodore Ts'o, Andreas Dilger, linux-ext4
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@goodmis.org>
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] ext4: Use trace condition for ext4_mballoc tracepoints
2013-11-15 4:08 [PATCH] ext4: Use trace condition for ext4_mballoc tracepoints Steven Rostedt
@ 2013-11-18 21:09 ` Steven Rostedt
0 siblings, 0 replies; 2+ messages in thread
From: Steven Rostedt @ 2013-11-18 21:09 UTC (permalink / raw)
To: Steven Rostedt; +Cc: LKML, Theodore Ts'o, Andreas Dilger, linux-ext4
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@goodmis.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@goodmis.org>
^ permalink raw reply [flat|nested] 2+ messages in thread
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