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Subject[slubllv4 10/16] slub: Invert locking and avoid slab lock
Locking slabs is no longer necesary if the arch supports cmpxchg operations
and if no debuggin features are used on a slab. If the arch does not support
cmpxchg then we fallback to use the slab lock to do a cmpxchg like operation.

The patch also changes the lock order. Slab locks are subsumed to the node lock
now. With that approach slab_trylocking is no longer necessary.

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

---
mm/slub.c | 131 +++++++++++++++++++++++++-------------------------------------
1 file changed, 53 insertions(+), 78 deletions(-)
Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c 2011-05-06 12:54:35.000000000 -0500
+++ linux-2.6/mm/slub.c 2011-05-06 12:54:36.000000000 -0500
@@ -2,10 +2,11 @@
* SLUB: A slab allocator that limits cache line use instead of queuing
* objects in per cpu and per node lists.
*
- * The allocator synchronizes using per slab locks and only
- * uses a centralized lock to manage a pool of partial slabs.
+ * The allocator synchronizes using per slab locks or atomic operatios
+ * and only uses a centralized lock to manage a pool of partial slabs.
*
* (C) 2007 SGI, Christoph Lameter
+ * (C) 2011 Linux Foundation, Christoph Lameter
*/

#include <linux/mm.h>
@@ -32,15 +33,27 @@

/*
* Lock order:
- * 1. slab_lock(page)
- * 2. slab->list_lock
- *
- * The slab_lock protects operations on the object of a particular
- * slab and its metadata in the page struct. If the slab lock
- * has been taken then no allocations nor frees can be performed
- * on the objects in the slab nor can the slab be added or removed
- * from the partial or full lists since this would mean modifying
- * the page_struct of the slab.
+ * 1. slub_lock (Global Semaphore)
+ * 2. node->list_lock
+ * 3. slab_lock(page) (Only on some arches and for debugging)
+ *
+ * slub_lock
+ *
+ * The role of the slub_lock is to protect the list of all the slabs
+ * and to synchronize major metadata changes to slab cache structures.
+ *
+ * The slab_lock is only used for debugging and on arches that do not
+ * have the ability to do a cmpxchg_double. It only protects the second
+ * double word in the page struct. Meaning
+ * A. page->freelist -> List of object free in a page
+ * B. page->counters -> Counters of objects
+ * C. page->frozen -> frozen state
+ *
+ * If a slab is frozen then it is exempt from list management. It is not
+ * on any list. The processor that froze the slab is the one who can
+ * perform list operations on the page. Other processors may put objects
+ * onto the freelist but the processor that froze the slab is the only
+ * one that can retrieve the objects from the page's freelist.
*
* The list_lock protects the partial and full list on each node and
* the partial slab counter. If taken then no new slabs may be added or
@@ -53,20 +66,6 @@
* slabs, operations can continue without any centralized lock. F.e.
* allocating a long series of objects that fill up slabs does not require
* the list lock.
- *
- * The lock order is sometimes inverted when we are trying to get a slab
- * off a list. We take the list_lock and then look for a page on the list
- * to use. While we do that objects in the slabs may be freed. We can
- * only operate on the slab if we have also taken the slab_lock. So we use
- * a slab_trylock() on the slab. If trylock was successful then no frees
- * can occur anymore and we can use the slab for allocations etc. If the
- * slab_trylock() does not succeed then frees are in progress in the slab and
- * we must stay away from it for a while since we may cause a bouncing
- * cacheline if we try to acquire the lock. So go onto the next slab.
- * If all pages are busy then we may allocate a new slab instead of reusing
- * a partial slab. A new slab has no one operating on it and thus there is
- * no danger of cacheline contention.
- *
* Interrupts are disabled during allocation and deallocation in order to
* make the slab allocator safe to use in the context of an irq. In addition
* interrupts are disabled to ensure that the processor does not change
@@ -330,6 +329,19 @@ static inline int oo_objects(struct kmem
return x.x & OO_MASK;
}

+/*
+ * Per slab locking using the pagelock
+ */
+static __always_inline void slab_lock(struct page *page)
+{
+ bit_spin_lock(PG_locked, &page->flags);
+}
+
+static __always_inline void slab_unlock(struct page *page)
+{
+ __bit_spin_unlock(PG_locked, &page->flags);
+}
+
static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
void *freelist_old, unsigned long counters_old,
void *freelist_new, unsigned long counters_new,
@@ -344,11 +356,14 @@ static inline bool cmpxchg_double_slab(s
} else
#endif
{
+ slab_lock(page);
if (page->freelist == freelist_old && page->counters == counters_old) {
page->freelist = freelist_new;
page->counters = counters_new;
+ slab_unlock(page);
return 1;
}
+ slab_unlock(page);
}

cpu_relax();
@@ -364,7 +379,7 @@ static inline bool cmpxchg_double_slab(s
/*
* Determine a map of object in use on a page.
*
- * Slab lock or node listlock must be held to guarantee that the page does
+ * Node listlock must be held to guarantee that the page does
* not vanish from under us.
*/
static void get_map(struct kmem_cache *s, struct page *page, unsigned long *map)
@@ -796,10 +811,11 @@ static int check_slab(struct kmem_cache
static int on_freelist(struct kmem_cache *s, struct page *page, void *search)
{
int nr = 0;
- void *fp = page->freelist;
+ void *fp;
void *object = NULL;
unsigned long max_objects;

+ fp = page->freelist;
while (fp && nr <= page->objects) {
if (fp == search)
return 1;
@@ -1007,6 +1023,8 @@ bad:
static noinline int free_debug_processing(struct kmem_cache *s,
struct page *page, void *object, unsigned long addr)
{
+ slab_lock(page);
+
if (!check_slab(s, page))
goto fail;

@@ -1042,10 +1060,12 @@ static noinline int free_debug_processin
set_track(s, object, TRACK_FREE, addr);
trace(s, page, object, 0);
init_object(s, object, SLUB_RED_INACTIVE);
+ slab_unlock(page);
return 1;

fail:
slab_fix(s, "Object at 0x%p not freed", object);
+ slab_unlock(page);
return 0;
}

@@ -1365,27 +1385,6 @@ static void discard_slab(struct kmem_cac
}

/*
- * Per slab locking using the pagelock
- */
-static __always_inline void slab_lock(struct page *page)
-{
- bit_spin_lock(PG_locked, &page->flags);
-}
-
-static __always_inline void slab_unlock(struct page *page)
-{
- __bit_spin_unlock(PG_locked, &page->flags);
-}
-
-static __always_inline int slab_trylock(struct page *page)
-{
- int rc = 1;
-
- rc = bit_spin_trylock(PG_locked, &page->flags);
- return rc;
-}
-
-/*
* Management of partially allocated slabs
*/
static inline void add_partial(struct kmem_cache_node *n,
@@ -1411,17 +1410,13 @@ static inline void remove_partial(struct
*
* Must hold list_lock.
*/
-static inline int lock_and_freeze_slab(struct kmem_cache *s,
+static inline int acquire_slab(struct kmem_cache *s,
struct kmem_cache_node *n, struct page *page)
{
void *freelist;
unsigned long counters;
struct page new;

-
- if (!slab_trylock(page))
- return 0;
-
/*
* Zap the freelist and set the frozen bit.
* The old freelist is the list of objects for the
@@ -1457,7 +1452,6 @@ static inline int lock_and_freeze_slab(s
*/
printk(KERN_ERR "SLUB: %s : Page without available objects on"
" partial list\n", s->name);
- slab_unlock(page);
return 0;
}
}
@@ -1481,7 +1475,7 @@ static struct page *get_partial_node(str

spin_lock(&n->list_lock);
list_for_each_entry(page, &n->partial, lru)
- if (lock_and_freeze_slab(s, n, page))
+ if (acquire_slab(s, n, page))
goto out;
page = NULL;
out:
@@ -1770,8 +1764,6 @@ redo:
"unfreezing slab"))
goto redo;

- slab_unlock(page);
-
if (lock)
spin_unlock(&n->list_lock);

@@ -1784,7 +1776,6 @@ redo:
static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c)
{
stat(s, CPUSLAB_FLUSH);
- slab_lock(c->page);
deactivate_slab(s, c);
}

@@ -1931,7 +1922,6 @@ static void *__slab_alloc(struct kmem_ca
if (!page)
goto new_slab;

- slab_lock(page);
if (unlikely(!node_match(c, node)))
goto another_slab;

@@ -1960,8 +1950,6 @@ load_freelist:
if (unlikely(!object))
goto another_slab;

- slab_unlock(page);
-
c->freelist = get_freepointer(s, object);
c->tid = next_tid(c->tid);
local_irq_restore(flags);
@@ -2009,7 +1997,6 @@ load_from_page:
c->page = page;

stat(s, ALLOC_SLAB);
- slab_lock(page);

goto load_from_page;
}
@@ -2198,7 +2185,6 @@ static void __slab_free(struct kmem_cach
unsigned long flags;

local_irq_save(flags);
- slab_lock(page);
stat(s, FREE_SLOWPATH);

if (kmem_cache_debug(s) && !free_debug_processing(s, page, x, addr))
@@ -2264,7 +2250,6 @@ static void __slab_free(struct kmem_cach
spin_unlock(&n->list_lock);

out_unlock:
- slab_unlock(page);
local_irq_restore(flags);
return;

@@ -2278,7 +2263,6 @@ slab_empty:
}

spin_unlock(&n->list_lock);
- slab_unlock(page);
local_irq_restore(flags);
stat(s, FREE_SLAB);
discard_slab(s, page);
@@ -3199,14 +3183,8 @@ int kmem_cache_shrink(struct kmem_cache
* list_lock. page->inuse here is the upper limit.
*/
list_for_each_entry_safe(page, t, &n->partial, lru) {
- if (!page->inuse && slab_trylock(page)) {
- /*
- * Must hold slab lock here because slab_free
- * may have freed the last object and be
- * waiting to release the slab.
- */
+ if (!page->inuse) {
remove_partial(n, page);
- slab_unlock(page);
discard_slab(s, page);
} else {
list_move(&page->lru,
@@ -3794,12 +3772,9 @@ static int validate_slab(struct kmem_cac
static void validate_slab_slab(struct kmem_cache *s, struct page *page,
unsigned long *map)
{
- if (slab_trylock(page)) {
- validate_slab(s, page, map);
- slab_unlock(page);
- } else
- printk(KERN_INFO "SLUB %s: Skipped busy slab 0x%p\n",
- s->name, page);
+ slab_lock(page);
+ validate_slab(s, page, map);
+ slab_unlock(page);
}

static int validate_slab_node(struct kmem_cache *s,


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