linux-kernel - Re: [cpuops cmpxchg V2 3/5] irq_work: Use per cpu atomics instead of regular atomics

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Date:	Wed, 15 Dec 2010 17:50:39 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Lameter <cl@...ux.com>, akpm@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [cpuops cmpxchg V2 3/5] irq_work: Use per cpu atomics instead
 of regular atomics

On Wed, 2010-12-15 at 17:32 +0100, Tejun Heo wrote:
> On 12/14/2010 05:28 PM, Christoph Lameter wrote:
> > The irq work queue is a per cpu object and it is sufficient for
> > synchronization if per cpu atomics are used. Doing so simplifies
> > the code and reduces the overhead of the code.
> > 
> > Before:
> > 
> > christoph@...ux-2.6$ size kernel/irq_work.o
> >    text	   data	    bss	    dec	    hex	filename
> >     451	      8	      1	    460	    1cc	kernel/irq_work.o
> > 
> > After:
> > 
> > christoph@...ux-2.6$ size kernel/irq_work.o 
> >    text	   data	    bss	    dec	    hex	filename
> >     438	      8	      1	    447	    1bf	kernel/irq_work.o
> > 
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> 
> Peter, can you please ack this one?

I guess so, I don't much like the bare preempt_disable/enable there, and
I'm wondering, aren't %fs prefixed insn slower than regular insn? Does
it really pay to avoid this one address computation if there's multiple
users in a function. %fs prefixes do take another byte, so it will also
result in larger code at some point.
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