linux-kernel - Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off

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Date:	Thu, 15 Jan 2015 20:30:45 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by
 removing preemption on/off

On Thu, 15 Jan 2015 17:16:34 -0800
Andrew Morton <akpm@...ux-foundation.org> 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?

profiling function tracing I discovered that accessing preempt_count
was actually quite expensive, even just to read. But it may not be as
bad since Peter Zijlstra converted preempt_count to a per_cpu variable.
Although, IIRC, the perf profiling showed the access to the %gs
register was where the time consuming was happening, which is what
I believe per_cpu variables still use.

-- Steve
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