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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