linux-kernel - Re: [PATCH] SLUB use cmpxchg

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Message-ID: <Pine.LNX.4.64.0708281240260.16473@schroedinger.engr.sgi.com>
Date:	Tue, 28 Aug 2007 12:42:45 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, mingo@...hat.com,
	linux-ia64@...r.kernel.org
Subject: Re: [PATCH] SLUB use cmpxchg_local

On Tue, 28 Aug 2007, Mathieu Desnoyers wrote:

> Ok, I just had a look at ia64 instruction set, and I fear that cmpxchg
> must always come with the acquire or release semantic. Is there any
> cmpxchg equivalent on ia64 that would be acquire and release semantic
> free ? This implicit memory ordering in the instruction seems to be
> responsible for the slowdown.

No. There is no cmpxchg used in the patches that I tested. The slowdown 
seem to come from the need to serialize at barriers. Adding an interrupt
enable/disable in the middle of the hot path creates another serialization 
point.

> If such primitive does not exist, then we should think about an irq
> disable fallback for this local atomic operation. However, I would
> prefer to let the cmpxchg_local primitive be bound to the "slow"
> cmpxchg_acq and create something like _cmpxchg_local that would be
> interrupt-safe, but not reentrant wrt NMIs.

Ummm... That is what I did. See the included patch that you quoted. The 
measurements show that such a fallback is not preserving the performance 
on IA64.
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