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

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Message-ID: <alpine.DEB.1.10.0904020948570.31756@qirst.com>
Date:	Thu, 2 Apr 2009 09:53:31 -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:

> to quote an earlier part of my mail:
>
> > > We encourage kfree(NULL) and it triggers commonly in the kernel
> > > today [on distro kernels we checked it can trigger once per
> > > syscall!] - so i think we should consider free_percpu(NULL) a
> > > possibly common pattern too. (even though today it's likely
> > > _not_ common at all.)
>
> I specifically mentioned that it is not at all common now.

What is this? Nonsense day? Consider it a common pattern although its
likely not common at all? April fools day?

> But there's no reason why an object shutdown fastpath with an
> optional percpu buffer (say for debug statistics, not enabled by
> default) couldnt look like this:
>
> 	percpu_free(NULL);
>
> We actually have such patterns of kfree(ptr) use, where the _common_
> case in a fastpath is kfree(NULL).

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.


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