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

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Date:	Tue, 21 Aug 2007 21:18:05 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
Cc:	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 Tue, 21 Aug 2007, Mathieu Desnoyers wrote:
> 
> > As I am going back through the initial cmpxchg_local implementation, it
> > seems like it was executing __slab_alloc() with preemption disabled,
> > which is wrong. new_slab() is not designed for that.
> 
> The version I send you did not use preemption.
> 
> We need to make a decision if we want to go without preemption and cmpxchg 
> or with preemption and cmpxchg_local.
> 

I don't expect any performance improvements with cmpxchg() over irq
disable/restore. I think we'll have to use cmpxchg_local

Also, we may argue that locked cmpxchg will have more scalability impact
than cmpxchg_local. Actually, I expect the LOCK prefix to have a bigger
scalability impact than the irq save/restore pair.

> If we really want to do this then the implementation of all of these 
> components need to result in competitive performance on all platforms.
> 

The minor issue I see here is on architectures where we have to simulate
cmpxchg_local with irq save/restore. Depending on how we implement the
code, it may result in two irq save/restore pairs instead of one, which
could make the code slower. However, if we are clever enough in our
low-level primitive usage, I think we could make the code use
cmpxchg_local when available and fall back on only _one_ irq disabled
section surrounding the whole code for other architectures.

Mathieu

-- 
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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