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

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Date:	Mon, 27 Aug 2007 17:38:45 -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, Mathieu Desnoyers wrote:
> 
> > > a clean solution source code wise. It also minimizes the interrupt holdoff 
> > > for the non-cmpxchg_local arches. However, it means that we will have to 
> > > disable interrupts twice for the slow path. If that is too expensive then 
> > > we need a different solution.
> > > 
> > 
> > cmpxchg_local is not used on the slow path... ?
> 
> Right.
> 
> > Did you meant:
> > 
> > it means that we will have to disable preemption _and_ interrupts on the
> > fast path for non-cmpxchg_local arches ?
> 
> We would have to disable preemption and interrupts once on the fast path. 
> The interrupt holdoff would just be a couple of instructions.
> 

Right.

> The slow path would require disable preemption and two interrupt disables.
> 

If the slow path have to call new_slab, then yes. But it seems that not
every slow path must call it, so for the other slow paths, only one
interrupt disable would be required.

> Question is if this makes sense performance wise. If not then we may have 
> to look at more complicated schemes.
> 

Yep, such as the arch_have_cmpxchg() macro that I proposed, but it
really hurts my eyes... :(

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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