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Subject[slubllv3 17/21] slub: Avoid disabling interrupts in free slowpath
Disabling interrupts can be avoided now. However, list operation still require
disabling interrupts since allocations can occur from interrupt
contexts and there is no way to perform atomic list operations. So
acquire the list lock opportunistically if there is a chance
that list operations would be needed. This may result in
needless synchronizations but allows the avoidance of synchronization
in the majority of the cases.

Dropping interrupt handling significantly simplifies the slowpath.

Signed-off-by: Christoph Lameter <cl@linux.com>

---
mm/slub.c | 23 ++++++++++++-----------
1 file changed, 12 insertions(+), 11 deletions(-)

Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c 2011-04-15 14:30:05.000000000 -0500
+++ linux-2.6/mm/slub.c 2011-04-15 14:30:06.000000000 -0500
@@ -2225,13 +2225,11 @@ static void __slab_free(struct kmem_cach
struct kmem_cache_node *n = NULL;
#ifdef CONFIG_CMPXCHG_LOCAL
unsigned long flags;
-
- local_irq_save(flags);
#endif
stat(s, FREE_SLOWPATH);

if (kmem_cache_debug(s) && !free_debug_processing(s, page, x, addr))
- goto out_unlock;
+ return;

do {
prior = page->freelist;
@@ -2250,7 +2248,11 @@ static void __slab_free(struct kmem_cach
* Otherwise the list_lock will synchronize with
* other processors updating the list of slabs.
*/
+#ifdef CONFIG_CMPXCHG_LOCAL
+ spin_lock_irqsave(&n->list_lock, flags);
+#else
spin_lock(&n->list_lock);
+#endif
}
inuse = new.inuse;

@@ -2266,7 +2268,7 @@ static void __slab_free(struct kmem_cach
*/
if (was_frozen)
stat(s, FREE_FROZEN);
- goto out_unlock;
+ return;
}

/*
@@ -2289,12 +2291,10 @@ static void __slab_free(struct kmem_cach
stat(s, FREE_ADD_PARTIAL);
}
}
-
- spin_unlock(&n->list_lock);
-
-out_unlock:
#ifdef CONFIG_CMPXCHG_LOCAL
- local_irq_restore(flags);
+ spin_unlock_irqrestore(&n->list_lock, flags);
+#else
+ spin_unlock(&n->list_lock);
#endif
return;

@@ -2307,9 +2307,10 @@ slab_empty:
stat(s, FREE_REMOVE_PARTIAL);
}

- spin_unlock(&n->list_lock);
#ifdef CONFIG_CMPXCHG_LOCAL
- local_irq_restore(flags);
+ spin_unlock_irqrestore(&n->list_lock, flags);
+#else
+ spin_unlock(&n->list_lock);
#endif
stat(s, FREE_SLAB);
discard_slab(s, page);


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