linux-kernel - Re: [PATCH 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off

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Date:	Thu, 8 Jan 2015 21:34:07 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
cc:	Davidlohr Bueso <dave@...olabs.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	rostedt@...dmis.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] mm/slub: optimize alloc/free fastpath by removing
 preemption on/off

On Thu, 8 Jan 2015, Joonsoo Kim wrote:

> > You'd need a smp_wmb() in between tid and c in the loop then, which
> > looks quite unpleasant. All in all disabling preemption isn't really
> > that expensive, and you should redo your performance number if you go
> > this way.
>
> This barrier() is not for read/write synchronization between cpus.
> All read/write operation to cpu_slab would happen on correct cpu in
> successful case. What I'd need to guarantee here is to prevent
> reordering between fetching operation for correctness of algorithm. In
> this case, barrier() seems enough to me. Am I wrong?

You are right.

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