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

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Date:	Mon, 05 Jan 2015 19:03:12 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	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 Mon, 2015-01-05 at 10:36 +0900, Joonsoo Kim wrote:
> -	preempt_disable();
> -	c = this_cpu_ptr(s->cpu_slab);
> +	do {
> +		tid = this_cpu_read(s->cpu_slab->tid);
> +		c = this_cpu_ptr(s->cpu_slab);
> +	} while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
> +	barrier();

I don't see the compiler reodering the object/page stores below, since c
is updated in the loop anyway. Is this really necessary (same goes for
slab_free)? The generated code by gcc 4.8 looks correct without it.
Additionally, the implied barriers for preemption control aren't really
the same semantics used here (if that is actually the reason why you are
using them).

Thanks,
Davidlohr

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