Re: [patch 00/10] [RFC] SLUB patches for more functionality, performance and maintenance

[Christoph Lameter <clameter@sgi.com>]

On Mon, 9 Jul 2007, Mathieu Desnoyers wrote:

> > > Yep, I volountarily used the variant without lock prefix because the
> > > data is per cpu and I disable preemption.
> > 
> > local_cmpxchg generates this?
> > 
> 
> Yes.

Does not work here. If I use

static void __always_inline *slab_alloc(struct kmem_cache *s,
                gfp_t gfpflags, int node, void *addr)
{
        void **object;
        struct kmem_cache_cpu *c;

        preempt_disable();
        c = get_cpu_slab(s, smp_processor_id());
redo:
        object = c->freelist;
        if (unlikely(!object || !node_match(c, node)))
                return __slab_alloc(s, gfpflags, node, addr, c);

        if (cmpxchg_local(&c->freelist, object, object[c->offset]) != object)
                goto redo;

        preempt_enable();
        if (unlikely((gfpflags & __GFP_ZERO)))
                memset(object, 0, c->objsize);

        return object;
}

Then the code will include a lock prefix:

    3270:       48 8b 1a                mov    (%rdx),%rbx
    3273:       48 85 db                test   %rbx,%rbx
    3276:       74 23                   je     329b <kmem_cache_alloc+0x4b>
    3278:       8b 42 14                mov    0x14(%rdx),%eax
    327b:       4c 8b 0c c3             mov    (%rbx,%rax,8),%r9
    327f:       48 89 d8                mov    %rbx,%rax
    3282:       f0 4c 0f b1 0a          lock cmpxchg %r9,(%rdx)
    3287:       48 39 c3                cmp    %rax,%rbx
    328a:       75 e4                   jne    3270 <kmem_cache_alloc+0x20>
    328c:       66 85 f6                test   %si,%si
    328f:       78 19                   js     32aa <kmem_cache_alloc+0x5a>
    3291:       48 89 d8                mov    %rbx,%rax
    3294:       48 83 c4 08             add    $0x8,%rsp
    3298:       5b                      pop    %rbx
    3299:       c9                      leaveq
    329a:       c3                      retq


> What applies to local_inc, given as example in the local_ops.txt
> document, applies integrally to local_cmpxchg. And I would say that
> local_cmpxchg is by far the cheapest locking mechanism I have found, and
> use today, for my kernel tracer. The idea emerged from my need to trace
> every execution context, including NMIs, while still providing good
> performances. local_cmpxchg was the perfect fit; that's why I deployed
> it in local.h in each and every architecture.

Great idea. The SLUB allocator may be able to use your idea to improve 
both the alloc and free path.

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