linux-kernel - Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs

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Date:	Wed, 4 May 2011 16:25:32 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Pekka Enberg <penberg@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	werner <w.landgraf@...ru>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs

Hello,

On Wed, May 04, 2011 at 04:10:29PM +0200, Thomas Gleixner wrote:
> > cases where the generic implementation is used.  Has anyone measured
> > the difference against before the whole this_cpu conversion?
> 
> Yes, that really wants to be done. The whole CMPXCHG_LOCAL ifdeffery
> should have been avoided in the first place. this_cpu_cmpxchg can
> really be implemented with preempt_enable/disable and the irqsafe
> variant in any case.

Yeah, slub code looks pretty scary with the #ifdefs.  IIUC, the
problem was that cmpxchg_double is an optimization for fast path which
was already very light weight and an extra locked op or irq on/off
would have made considerable difference.

The cmpxchg_double optimization made the fast path go quite faster
when CPU supports it but it may as well slow things down considerably
if CPU doesn't, due to extra irq on/off's.  Anyways, here's hoping
that the slow down is acceptable compared to the base code without
cmpxchg_double and the ugliness can be removed.

Thanks.

-- 
tejun
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