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Subject[RFC/PATCH] slub: Drop slab lock for partial list handling
From: Pekka Enberg <penberg@kernel.org>

There's no need to hold 'page' slab lock for partial list handling functions. A
page is bound to a node so 'page->lru' is always protected by n->list_lock.

Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
---
David, assuming this actually works, does it help with netperf TCP_RR? The
patch is on top of my slab/next branch.
mm/slub.c | 26 +++++++++++++++-----------
1 files changed, 15 insertions(+), 11 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index 8fd5401..f41126f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1452,8 +1452,11 @@ static void unfreeze_slab(struct kmem_cache *s, struct page *page, int tail)

__ClearPageSlubFrozen(page);
if (page->inuse) {
+ void *prior = page->freelist;

- if (page->freelist) {
+ slab_unlock(page);
+
+ if (prior) {
add_partial(n, page, tail);
stat(s, tail ? DEACTIVATE_TO_TAIL : DEACTIVATE_TO_HEAD);
} else {
@@ -1461,8 +1464,8 @@ static void unfreeze_slab(struct kmem_cache *s, struct page *page, int tail)
if (kmem_cache_debug(s) && (s->flags & SLAB_STORE_USER))
add_full(n, page);
}
- slab_unlock(page);
} else {
+ slab_unlock(page);
stat(s, DEACTIVATE_EMPTY);
if (n->nr_partial < s->min_partial) {
/*
@@ -1476,9 +1479,7 @@ static void unfreeze_slab(struct kmem_cache *s, struct page *page, int tail)
* the partial list.
*/
add_partial(n, page, 1);
- slab_unlock(page);
} else {
- slab_unlock(page);
stat(s, FREE_SLAB);
discard_slab(s, page);
}
@@ -1831,13 +1832,16 @@ checks_ok:
page->inuse--;

if (unlikely(PageSlubFrozen(page))) {
+ slab_unlock(page);
stat(s, FREE_FROZEN);
- goto out_unlock;
+ goto out;
}

if (unlikely(!page->inuse))
goto slab_empty;

+ slab_unlock(page);
+
/*
* Objects left in the slab. If it was not on the partial list before
* then add it.
@@ -1846,12 +1850,11 @@ checks_ok:
add_partial(get_node(s, page_to_nid(page)), page, 1);
stat(s, FREE_ADD_PARTIAL);
}
-
-out_unlock:
- slab_unlock(page);
+out:
return;

slab_empty:
+ slab_unlock(page);
if (prior) {
/*
* Slab still on the partial list.
@@ -1859,14 +1862,15 @@ slab_empty:
remove_partial(s, page);
stat(s, FREE_REMOVE_PARTIAL);
}
- slab_unlock(page);
stat(s, FREE_SLAB);
discard_slab(s, page);
return;

debug:
- if (!free_debug_processing(s, page, x, addr))
- goto out_unlock;
+ if (!free_debug_processing(s, page, x, addr)) {
+ slab_unlock(page);
+ goto out;
+ }
goto checks_ok;
}

--
1.7.1


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