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

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Message-ID: <20150115225130.00c0c99a@grimm.local.home>
Date:	Thu, 15 Jan 2015 22:51:30 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....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 21:27:14 -0600 (CST)
Christoph Lameter <cl@...ux.com> wrote:

> 
> The %gs register is not used since the address of the per cpu area is
> available as one of the first fields in the per cpu areas.

Have you disassembled your code?

Looking at put_cpu_partial() from 3.19-rc3 where it does:

		oldpage = this_cpu_read(s->cpu_slab->partial);

I get:

		mov    %gs:0x18(%rax),%rdx

Looks to me that %gs is used.


I haven't done benchmarks in a while, so perhaps accessing the %gs
segment isn't as expensive as I saw it before. I'll have to profile
function tracing on my i7 and see where things are slow again.


-- Steve
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