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

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Date:	Mon, 5 Jan 2015 21:25:02 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Hillf Danton <hillf.zj@...baba-inc.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	'Christoph Lameter' <cl@...ux.com>,
	'Pekka Enberg' <penberg@...nel.org>,
	'David Rientjes' <rientjes@...gle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	'Jesper Dangaard Brouer' <brouer@...hat.com>
Subject: Re: [PATCH 1/2] mm/slub: optimize alloc/free fastpath by removing
 preemption on/off

On Tue, 6 Jan 2015 10:32:47 +0900
Joonsoo Kim <iamjoonsoo.kim@....com> wrote:


> > > +++ b/mm/slub.c
> > > @@ -2398,13 +2398,15 @@ redo:
> > >  	 * reading from one cpu area. That does not matter as long
> > >  	 * as we end up on the original cpu again when doing the cmpxchg.
> > >  	 *
> > > -	 * Preemption is disabled for the retrieval of the tid because that
> > > -	 * must occur from the current processor. We cannot allow rescheduling
> > > -	 * on a different processor between the determination of the pointer
> > > -	 * and the retrieval of the tid.
> > > +	 * We should guarantee that tid and kmem_cache are retrieved on
> > > +	 * the same cpu. It could be different if CONFIG_PREEMPT so we need
> > > +	 * to check if it is matched or not.
> > >  	 */
> > > -	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();
> > 
> > Help maintenance more if barrier is documented in commit message.
> 
> Hello,
> 
> Okay. Will add some information about this barrier in commit message.

A comment in the commit message is useless. Adding a small comment
above the barrier() call itself would be much more useful.

-- Steve
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