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

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Message-ID: <20070827202203.GB5653@Krystal>
Date:	Mon, 27 Aug 2007 16:22:03 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [PATCH] SLUB use cmpxchg_local

* Christoph Lameter (clameter@....com) wrote:
> On Mon, 27 Aug 2007, Peter Zijlstra wrote:
> 
> > So, if the fast path can be done with a preempt off, it might be doable
> > to suffer the slow path with a per cpu lock like that.
> 
> Sadly the cmpxchg_local requires local per cpu data access. Isnt there 
> some way to make this less expensive on RT? Acessing cpu local memory is 
> really good for performance on NUMA since the data is optimally placed and 
> one can avoid/reduce locking if the process stays tied to the processor.
> 

On the slow path, in slab_new, we already have to reenable interrupts
because we can sleep. If we make sure that whenever we return to an irq
disable code path we take the current per-cpu data structure again, can
we make the preempt-disable/irq-disabled code paths O(1) ?

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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