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Date: Thu, 15 Jan 2015 21:28:26 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Andrew Morton <akpm@linux-foundation.org>
cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>, linux-mm@kvack.org,
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        Jesper Dangaard Brouer <brouer@redhat.com>, rostedt@goodmis.org,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by removing
 preemption on/off
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References: <1421307633-24045-1-git-send-email-iamjoonsoo.kim@lge.com> <20150115171634.685237a4.akpm@linux-foundation.org>
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On Thu, 15 Jan 2015, Andrew Morton wrote:

> > I saw roughly 5% win in a fast-path loop over kmem_cache_alloc/free
> > in CONFIG_PREEMPT. (14.821 ns -> 14.049 ns)
>
> I'm surprised.  preempt_disable/enable are pretty fast.  I wonder why
> this makes a measurable difference.  Perhaps preempt_enable()'s call to
> preempt_schedule() added pain?

The rest of the fastpath is already highly optimized. That is why
something like preempt enable/disable has such a disproportionate effect.