linux-kernel - Re: [PATCH 0/7] slub: Fastpath optimization (especially for RT) V1

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Message-ID: <alpine.DEB.2.11.1412170953280.8347@gentwo.org>
Date:	Wed, 17 Dec 2014 10:10:43 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Joonsoo Kim <js1304@...il.com>
cc:	Joonsoo Kim <iamjoonsoo.kim@....com>, akpm@...uxfoundation.org,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...nel.org>,
	Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH 0/7] slub: Fastpath optimization (especially for RT) V1

On Wed, 17 Dec 2014, Joonsoo Kim wrote:

> +       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));


Assembly code produced is a bit weird. I think the compiler undoes what
you wanted to do:

 46fb:       49 8b 1e                mov    (%r14),%rbx				rbx = c =s->cpu_slab?
    46fe:       65 4c 8b 6b 08          mov    %gs:0x8(%rbx),%r13		r13 = tid
    4703:       e8 00 00 00 00          callq  4708 <kmem_cache_alloc+0x48>	??
    4708:       89 c0                   mov    %eax,%eax			??
    470a:       48 03 1c c5 00 00 00    add    0x0(,%rax,8),%rbx		??
    4711:       00
    4712:       4c 3b 6b 08             cmp    0x8(%rbx),%r13			tid == c->tid
    4716:       49 89 d8                mov    %rbx,%r8
    4719:       75 e0                   jne    46fb <kmem_cache_alloc+0x3b>

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