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Date: Thu, 15 Jan 2015 22:51:30 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
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        Jesper Dangaard Brouer <brouer@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by
 removing preemption on/off
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On Thu, 15 Jan 2015 21:27:14 -0600 (CST)
Christoph Lameter <cl@linux.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