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

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Message-ID: <alpine.DEB.1.10.0904012216370.32656@qirst.com>
Date:	Wed, 1 Apr 2009 22:18:59 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Ingo Molnar <mingo@...e.hu>
cc:	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, matthew@....cx,
	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:

> So ... we regressed the performance of percpu_free(NULL) with a
> potential cross-CPU cacheline bounce. Without the patch,
> percpu_free(NULL) would never do such a bounce. So i dont think the
> patch is a good idea.


But percpu_free is not an operation typical for hot code paths. Per cpu
variables are allocated and freed in rarely used code paths. Typically
the use of the per cpu variables occurs in hot code paths, not their
allocations.

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