linux-kernel - Re: [GIT PULL] slab fixes for 3.2-rc4

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Message-ID: <CA+55aFyr69__vsorCXKZ7UezvXkCkZaot3=5b5xSHaYBEJ2ZKQ@mail.gmail.com>
Date:	Wed, 21 Dec 2011 18:19:34 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] slab fixes for 3.2-rc4

On Wed, Dec 21, 2011 at 9:05 AM, Tejun Heo <tj@...nel.org> wrote:
>
> machines.  (cc'ing arch) Does anyone have better insight here?  How
> much more expensive are local irq save/restore compared to inc/dec'ing
> preempt count on various archs?

I think powerpc does sw irq disable, so it's pretty much the same.

However, on *most* architectures, if the stupid generic "disable
interrupts and then do the op" is too expensive, they could easily
just do an architecture-specific "safe" version. Especially if they
only need to do cmpxchg and add.

Many architectures can do those safely with load-locked,
store-conditional (ie atomic_add), and do so quickly. Sure, there are
broken cases wher ethat is really slow (original alpha is an example
of that), but I think it's fairly rare.

So both arm (in v6+) and powerpc should be able to just use the
"atomic_add" version, with no real downside.

So I really suspect that we could just say: "make the irq-safe version
be the *only* version", and no architecture will really care. Sure, it
can be more expensive, but it usually isn't. Only when done badly and
stupidly is it nasty.

                  Linus
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