[patch 08/10] SLUB: Optional fast path using cmpxchg_local

[Christoph Lameter <clameter@sgi.com>]

Provide an alternate implementation of the SLUB fast paths for alloc
and free using cmpxchg_local. The cmpxchg_local fast path is selected
for arches that have CONFIG_FAST_CMPXCHG_LOCAL set. An arch should only
set CONFIG_FAST_CMPXCHG_LOCAL if the cmpxchg_local is faster than an
interrupt enable/disable sequence. This is known to be true for both
x86 platforms so set FAST_CMPXCHG_LOCAL for both arches.

Not all arches can support fast cmpxchg operations. Typically the
architecture must have an optimized cmpxchg instruction. The
cmpxchg fast path makes no sense on platforms whose cmpxchg is
slower than interrupt enable/disable (like f.e. IA64).

The advantages of a cmpxchg_local based fast path are:

1. Lower cycle count (30%-60% faster)

2. There is no need to disable and enable interrupts on the fast path.
   Currently interrupts have to be disabled and enabled on every
   slab operation. This is likely saving a significant percentage
   of interrupt off / on sequences in the kernel.

3. The disposal of freed slabs can occur with interrupts enabled.

The alternate path is realized using #ifdef's. Several attempts to do the
same with macros and in line functions resulted in a mess (in particular due
to the strange way that local_interrupt_save() handles its argument and due
to the need to define macros/functions that sometimes disable interrupts
and sometimes do something else. The macro based approaches made it also
difficult to preserve the optimizations for the non cmpxchg paths).

#ifdef seems to be the way to go here to have a readable source.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 arch/x86/Kconfig.i386   |    4 ++
 arch/x86/Kconfig.x86_64 |    4 ++
 mm/slub.c               |   71 ++++++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 77 insertions(+), 2 deletions(-)

Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c	2007-10-27 10:39:07.583665939 -0700
+++ linux-2.6/mm/slub.c	2007-10-27 10:40:19.710415861 -0700
@@ -1496,7 +1496,12 @@ static void *__slab_alloc(struct kmem_ca
 {
 	void **object;
 	struct page *new;
+#ifdef CONFIG_FAST_CMPXCHG_LOCAL
+	unsigned long flags;
 
+	local_irq_save(flags);
+	preempt_enable_no_resched();
+#endif
 	if (!c->page)
 		goto new_slab;
 
@@ -1518,6 +1523,10 @@ load_freelist:
 unlock_out:
 	slab_unlock(c->page);
 out:
+#ifdef CONFIG_FAST_CMPXCHG_LOCAL
+	preempt_disable();
+	local_irq_restore(flags);
+#endif
 	return object;
 
 another_slab:
@@ -1592,9 +1601,26 @@ static void __always_inline *slab_alloc(
 		gfp_t gfpflags, int node, void *addr)
 {
 	void **object;
-	unsigned long flags;
 	struct kmem_cache_cpu *c;
 
+#ifdef CONFIG_FAST_CMPXCHG_LOCAL
+	c = get_cpu_slab(s, get_cpu());
+	do {
+		object = c->freelist;
+		if (unlikely(is_end(object) || !node_match(c, node))) {
+			object = __slab_alloc(s, gfpflags, node, addr, c);
+			if (unlikely(!object)) {
+				put_cpu();
+				goto out;
+			}
+			break;
+		}
+	} while (cmpxchg_local(&c->freelist, object, object[c->offset])
+								!= object);
+	put_cpu();
+#else
+	unsigned long flags;
+
 	local_irq_save(flags);
 	c = get_cpu_slab(s, smp_processor_id());
 	if (unlikely((is_end(c->freelist)) || !node_match(c, node))) {
@@ -1609,6 +1635,7 @@ static void __always_inline *slab_alloc(
 		c->freelist = object[c->offset];
 	}
 	local_irq_restore(flags);
+#endif
 
 	if (unlikely((gfpflags & __GFP_ZERO)))
 		memset(object, 0, c->objsize);
@@ -1644,6 +1671,11 @@ static void __slab_free(struct kmem_cach
 	void *prior;
 	void **object = (void *)x;
 
+#ifdef CONFIG_FAST_CMPXCHG_LOCAL
+	unsigned long flags;
+
+	local_irq_save(flags);
+#endif
 	slab_lock(page);
 
 	if (unlikely(SlabDebug(page)))
@@ -1669,6 +1701,9 @@ checks_ok:
 
 out_unlock:
 	slab_unlock(page);
+#ifdef CONFIG_FAST_CMPXCHG_LOCAL
+	local_irq_restore(flags);
+#endif
 	return;
 
 slab_empty:
@@ -1679,6 +1714,9 @@ slab_empty:
 		remove_partial(s, page);
 
 	slab_unlock(page);
+#ifdef CONFIG_FAST_CMPXCHG_LOCAL
+	local_irq_restore(flags);
+#endif
 	discard_slab(s, page);
 	return;
 
@@ -1703,9 +1741,37 @@ static void __always_inline slab_free(st
 			struct page *page, void *x, void *addr)
 {
 	void **object = (void *)x;
-	unsigned long flags;
 	struct kmem_cache_cpu *c;
 
+#ifdef CONFIG_FAST_CMPXCHG_LOCAL
+	void **freelist;
+
+	c = get_cpu_slab(s, get_cpu());
+	debug_check_no_locks_freed(object, s->objsize);
+	do {
+		freelist = c->freelist;
+		barrier();
+		/*
+		 * If the compiler would reorder the retrieval of c->page to
+		 * come before c->freelist then an interrupt could
+		 * change the cpu slab before we retrieve c->freelist. We
+		 * could be matching on a page no longer active and put the
+		 * object onto the freelist of the wrong slab.
+		 *
+		 * On the other hand: If we already have the freelist pointer
+		 * then any change of cpu_slab will cause the cmpxchg to fail
+		 * since the freelist pointers are unique per slab.
+		 */
+		if (unlikely(page != c->page || c->node < 0)) {
+			__slab_free(s, page, x, addr, c->offset);
+			break;
+		}
+		object[c->offset] = freelist;
+	} while (cmpxchg_local(&c->freelist, freelist, object) != freelist);
+	put_cpu();
+#else
+	unsigned long flags;
+
 	local_irq_save(flags);
 	debug_check_no_locks_freed(object, s->objsize);
 	c = get_cpu_slab(s, smp_processor_id());
@@ -1716,6 +1782,7 @@ static void __always_inline slab_free(st
 		__slab_free(s, page, x, addr, c->offset);
 
 	local_irq_restore(flags);
+#endif
 }
 
 void kmem_cache_free(struct kmem_cache *s, void *x)
Index: linux-2.6/arch/x86/Kconfig.i386
===================================================================
--- linux-2.6.orig/arch/x86/Kconfig.i386	2007-10-27 10:38:33.630415778 -0700
+++ linux-2.6/arch/x86/Kconfig.i386	2007-10-27 10:40:19.710415861 -0700
@@ -51,6 +51,10 @@ config X86
 	bool
 	default y
 
+config FAST_CMPXCHG_LOCAL
+	bool
+	default y
+
 config MMU
 	bool
 	default y
Index: linux-2.6/arch/x86/Kconfig.x86_64
===================================================================
--- linux-2.6.orig/arch/x86/Kconfig.x86_64	2007-10-27 10:38:33.630415778 -0700
+++ linux-2.6/arch/x86/Kconfig.x86_64	2007-10-27 10:40:19.710415861 -0700
@@ -97,6 +97,10 @@ config X86_CMPXCHG
 	bool
 	default y
 
+config FAST_CMPXCHG_LOCAL
+	bool
+	default y
+
 config EARLY_PRINTK
 	bool
 	default y

-- 
-
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