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Subject[PATCH v3 2/2] mm/slub: refactor deactivate_slab()
Date
Simplify deactivate_slab() by unlocking n->list_lock and retrying
cmpxchg_double() when cmpxchg_double() fails, and perform
add_{partial,full} only when it succeed.

Releasing and taking n->list_lock again here is not harmful as SLUB
avoids deactivating slabs as much as possible.

[ vbabka@suse.cz: perform add_{partial,full} when cmpxchg_double()
succeed.

count deactivating full slabs even if debugging flag is not set. ]

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
---
mm/slub.c | 91 +++++++++++++++++++++++--------------------------------
1 file changed, 38 insertions(+), 53 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 1ce09b0347ad..f0cb9d0443ac 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2348,10 +2348,10 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
void *freelist)
{
- enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE };
+ enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE, M_FULL_NOLIST };
struct kmem_cache_node *n = get_node(s, slab_nid(slab));
- int lock = 0, free_delta = 0;
- enum slab_modes l = M_NONE, m = M_NONE;
+ int free_delta = 0;
+ enum slab_modes mode = M_NONE;
void *nextfree, *freelist_iter, *freelist_tail;
int tail = DEACTIVATE_TO_HEAD;
unsigned long flags = 0;
@@ -2393,14 +2393,10 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
* Ensure that the slab is unfrozen while the list presence
* reflects the actual number of objects during unfreeze.
*
- * We setup the list membership and then perform a cmpxchg
- * with the count. If there is a mismatch then the slab
- * is not unfrozen but the slab is on the wrong list.
- *
- * Then we restart the process which may have to remove
- * the slab from the list that we just put it on again
- * because the number of objects in the slab may have
- * changed.
+ * We first perform cmpxchg holding lock and insert to list
+ * when it succeed. If there is mismatch then the slab is not
+ * unfrozen and number of objects in the slab may have changed.
+ * Then release lock and retry cmpxchg again.
*/
redo:

@@ -2420,61 +2416,50 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
new.frozen = 0;

if (!new.inuse && n->nr_partial >= s->min_partial)
- m = M_FREE;
+ mode = M_FREE;
else if (new.freelist) {
- m = M_PARTIAL;
- if (!lock) {
- lock = 1;
- /*
- * Taking the spinlock removes the possibility that
- * acquire_slab() will see a slab that is frozen
- */
- spin_lock_irqsave(&n->list_lock, flags);
- }
- } else {
- m = M_FULL;
- if (kmem_cache_debug_flags(s, SLAB_STORE_USER) && !lock) {
- lock = 1;
- /*
- * This also ensures that the scanning of full
- * slabs from diagnostic functions will not see
- * any frozen slabs.
- */
- spin_lock_irqsave(&n->list_lock, flags);
- }
- }
-
- if (l != m) {
- if (l == M_PARTIAL)
- remove_partial(n, slab);
- else if (l == M_FULL)
- remove_full(s, n, slab);
+ mode = M_PARTIAL;
+ /*
+ * Taking the spinlock removes the possibility that
+ * acquire_slab() will see a slab that is frozen
+ */
+ spin_lock_irqsave(&n->list_lock, flags);
+ } else if (kmem_cache_debug_flags(s, SLAB_STORE_USER)) {
+ mode = M_FULL;
+ /*
+ * This also ensures that the scanning of full
+ * slabs from diagnostic functions will not see
+ * any frozen slabs.
+ */
+ spin_lock_irqsave(&n->list_lock, flags);
+ } else
+ mode = M_FULL_NOLIST;

- if (m == M_PARTIAL)
- add_partial(n, slab, tail);
- else if (m == M_FULL)
- add_full(s, n, slab);
- }

- l = m;
if (!cmpxchg_double_slab(s, slab,
old.freelist, old.counters,
new.freelist, new.counters,
- "unfreezing slab"))
+ "unfreezing slab")) {
+ if (mode == M_PARTIAL || mode == M_FULL)
+ spin_unlock_irqrestore(&n->list_lock, flags);
goto redo;
+ }

- if (lock)
- spin_unlock_irqrestore(&n->list_lock, flags);

- if (m == M_PARTIAL)
+ if (mode == M_PARTIAL) {
+ add_partial(n, slab, tail);
+ spin_unlock_irqrestore(&n->list_lock, flags);
stat(s, tail);
- else if (m == M_FULL)
- stat(s, DEACTIVATE_FULL);
- else if (m == M_FREE) {
+ } else if (mode == M_FREE) {
stat(s, DEACTIVATE_EMPTY);
discard_slab(s, slab);
stat(s, FREE_SLAB);
- }
+ } else if (mode == M_FULL) {
+ add_full(s, n, slab);
+ spin_unlock_irqrestore(&n->list_lock, flags);
+ stat(s, DEACTIVATE_FULL);
+ } else if (mode == M_FULL_NOLIST)
+ stat(s, DEACTIVATE_FULL);
}

#ifdef CONFIG_SLUB_CPU_PARTIAL
--
2.33.1
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