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

[Christoph Lameter]

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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