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

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Date:	Mon, 13 Apr 2009 14:18:08 -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 Wed, 8 Apr 2009, Ingo Molnar wrote:

> Dude, this is a new facility freshly modernized and freshly made
> usable. What did you expect, for a thousand usecases pop up in the
> kernel overnight? _None_ of this code is "common" today per se. (the
> networking folks are working on making it more and more common
> though)

?? kfree(NULL) has been allowed for years. None of this is new.

> > Speculation. A shutdown fastpath? The percpu allocation and free
> > operations are expensive and deal with teardown and setup of
> > virtual mappings. Those paths are *not* optimized for fastpath
> > use. kfree is different.
>
> Of course a lot of this is speculation, dynamic percpu so far has
> been a rarely used facility compared to kmalloc()/kfree(). If you
> dont accept my analogy that's fine - but that is opinion against
> opinion - while you state you opinion as truism.


Please look at the kernel source for the use of percpu_free and
percpu_alloc.

> So my point remains: your patch had effects you clearly did not
> anticipate, and the cacheline alignment management situation is not
> nearly as clear-cut as you imagine it to be.

There was no effect that I did not anticipate. Just imagination on your
part that percpu_free is used like kfree.

Again: The frequent insertion of __read_mostly will
destroy the purpose and function of the read mostly areas!

> Unfortunately you failed to answer my detailed mail that made very
> specific points though, you only got into generalities and flames
> about my summary mail - so it's hard to judge what your opinion
> about those specific facts is - you have not stated one.

I was pretty clear on those points. Not sure what material question I did
not answer. What you say here fits your posts. Generalizing from kfree to
percpu_free etc...
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