linux-kernel - Re: [PATCH UPDATED] percpu: use dynamic percpu allocator as the default percpu allocator

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Date:	Thu, 2 Apr 2009 04:13:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	rusty@...tcorp.com.au, tglx@...utronix.de, x86@...nel.org,
	linux-kernel@...r.kernel.org, hpa@...or.com,
	Paul Mundt <lethal@...ux-sh.org>, rmk@....linux.org.uk,
	starvik@...s.com, ralf@...ux-mips.org, davem@...emloft.net,
	cooloney@...nel.org, kyle@...artin.ca, grundler@...isc-linux.org,
	takata@...ux-m32r.org, benh@...nel.crashing.org, rth@...ddle.net,
	ink@...assic.park.msu.ru, heiko.carstens@...ibm.com,
	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH UPDATED] percpu: use dynamic percpu allocator as the
	default percpu allocator


* Matthew Wilcox <matthew@....cx> wrote:

> On Wed, Apr 01, 2009 at 12:39:46PM -0700, Linus Torvalds wrote:
>
> > The thing is, things like "read_mostly" or "access_rarely" may 
> > talk about how we access those individual variables, but you're 
> > missing a _huge_ chunk of the puzzle if you ignore the 
> > _correlations_ of those accesses with accesses to other 
> > variables.
> > 
> > The thign is, if you have variables 'a' and 'b', and they are 
> > always accessed together, then it's probably worth it to put 
> > them in the same cacheline.
> 
> If you've got two global variables that are generally accessed 
> together, they should probably be `annotated' as such by putting 
> them in a struct.

It is certainly done so in a number of cases (say the RCU core and 
the scheduler), but i dont think it should be forced or preferred in 
any way.

IMHO it's equally good and clean code to have the global variables 
separately at the top of a .c file. Sometimes it's cleaner.

	Ingo
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