linux-kernel - Re: Should we be using unlikely() around tests of GFP

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Date:	Mon, 03 Jan 2011 09:26:36 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Minchan Kim <minchan.kim@...il.com>, Theodore Ts'o <tytso@....edu>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, David Rientjes <rientjes@...gle.com>,
	npiggin@...nel.dk
Subject: Re: Should we be using unlikely() around tests of GFP_ZERO?

On Mon, 2011-01-03 at 16:10 +0200, Pekka Enberg wrote:

> >  correct incorrect  %        Function                  File              Line
> >  ------- ---------  -        --------                  ----              ----
> >  6890998  2784830  28        slab_alloc                slub.c            1719
> >
> > That's incorrect 28% of the time.
> 
> Thanks! AFAICT, that number is high enough to justify removing the
> unlikely() annotations, no?

Personally, I think anything that is incorrect more that 5% of the time
should not have any annotation.

My rule is to use the annotation when a branch goes one way 95% or more.
With the exception of times when we want a particular path to be the
faster path, because we know its in a more critical position (as there
are cases in the scheduler and the tracing infrastructure itself).

But here, I think removing it is the right decision.

-- Steve


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