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

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Message-ID: <alpine.DEB.1.10.0904012324580.20118@qirst.com>
Date:	Wed, 1 Apr 2009 23:28:44 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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

On Thu, 2 Apr 2009, Ingo Molnar wrote:

> You didnt really answer to my suggestion(s) though. You only stated
> that:
>
>    "problem XYZ is something normal that comes with the cpu caching
>     schemes. As long as there is no significant impact on
>     performance weare fine with it."
>
> Which i'd proffer is true for any given value of XYZ ;-)

Nope. If XYZ is a significant performance issue then we are not fine with
XYZ. __read_mostly was introduced to deal with signficant false aliasing
issues.

False aliasing can be set to occur anytime you place two variables in the
same cacheline. That is by design in the current cacheline schemes. One
would not place a variable in a separate cacheline without good
reason.





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