You are completely right: on x86_64, a bit got lost in the move to
cmpxchg.h, here is the fix. It applies on 2.6.22-rc6-mm1.
x86_64 - Use non locked version for local_cmpxchg()
local_cmpxchg() should not use any LOCK prefix. This change probably got lost in
the move to cmpxchg.h.
Signed-off-by: Mathieu Desnoyers <[EMAIL PROTECTED]>
---
include/asm-x86_64/cmpxchg.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
Index: linux-2.6-lttng/include/asm-x86_64/cmpxchg.h
===================================================================
--- linux-2.6-lttng.orig/include/asm-x86_64/cmpxchg.h 2007-07-10
01:10:10.000000000 -0400
+++ linux-2.6-lttng/include/asm-x86_64/cmpxchg.h 2007-07-10
01:11:03.000000000 -0400
@@ -128,7 +128,7 @@
((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o),\
(unsigned long)(n),sizeof(*(ptr))))
#define cmpxchg_local(ptr,o,n)\
- ((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o),\
+ ((__typeof__(*(ptr)))__cmpxchg_local((ptr),(unsigned long)(o),\
(unsigned long)(n),sizeof(*(ptr))))
#endif
* Christoph Lameter ([EMAIL PROTECTED]) wrote:
> 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.
>
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
