linux-kernel - Re: [PATCH 2/5] mm: remove unlikly NULL from kfree

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Date:	Wed, 25 Mar 2009 17:24:06 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Matt Mackall <mpm@...enic.com>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Hua Zhong <hzhong@...il.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Steven Rostedt <srostedt@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/5] mm: remove unlikly NULL from kfree


On Wed, 25 Mar 2009, Matt Mackall wrote:
> 
> The cost is an unconditional branch; the ret already exists. There's a
> slightly larger cache footprint for the small branch and a slightly
> smaller footprint for the large branch. If p is close to .5 and
> calls/sec is high, the cache footprint is the sum of the footprint of
> both branches. But if calls/sec is close to low, cache footprint is also
> low.
> 
> So, yeah, I think this is a good additional argument to err on the side
> of not adding these things at all. And I certainly wasn't intending to
> defend the ones in kfree. 
> 
> But I'm also skeptical of whether it's worth spending much time actively
> routing out the moderately incorrect instances. It's going to be nearly
> immune to performance benchmarking. We should instead just actively
> discourage using unlikely in new code.
> 

Sure. OK, actually I'd say they are valid for 100% hits. These are for 
error conditions and trace point like data. Where, we want the least 
amount of overhead when the condition is false.

But I can see, we've been brought up (well some of us) that branches are 
horrible for pipelines, and any help in deciding the choice is always 
tempting.

-- Steve

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