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

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Message-ID: <Pine.LNX.4.64.0708271324001.5884@schroedinger.engr.sgi.com>
Date:	Mon, 27 Aug 2007 13:26:11 -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
Subject: Re: [PATCH] SLUB use cmpxchg_local

On Mon, 27 Aug 2007, Mathieu Desnoyers wrote:

> * 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) ?

Not sure exactly what you are getting at?
This would mean running __alloc_pages tied to one processor even though 
waiting is possible?


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