linux-kernel - Re: [rfc 00/45] [RFC] CPU ops and a rework of per cpu data handling on x86

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Message-ID: <Pine.LNX.4.64.0711191758390.19954@schroedinger.engr.sgi.com>
Date:	Mon, 19 Nov 2007 17:59:33 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	David Miller <davem@...emloft.net>
cc:	ak@...e.de, akpm@...ux-foundation.org, travis@....com,
	mathieu.desnoyers@...ymtl.ca, linux-kernel@...r.kernel.org
Subject: Re: [rfc 00/45] [RFC] CPU ops and a rework of per cpu data handling
 on x86_64

On Mon, 19 Nov 2007, David Miller wrote:

> Although we have a per-cpu area base in a fixed global register
> for addressing, the above isn't beneficial on sparc64 because
> the atomic is much slower than doing a:
> 
> 	local_irq_disable();
> 	nonatomic_percpu_memory_op();
> 	local_irq_enable();
> 
> local_irq_{disable,enable}() together is about 18 cycles.
> Just the cmpxchg() part of the atomic sequence is at least
> 32 cycles and requires a loop:
> 
> 	while (1) {
> 		x = ld();
> 		if (cmpxchg(x, op(x)))
> 			break;
> 	}
> 
> which bloats up the atomic version even more.

In that case the generic fallbacks can just provide what you already have.

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