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Subject[PATCH 4/4] slub: refactoring unfreeze_partials()
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Current implementation of unfreeze_partials() is so complicated,
but benefit from it is insignificant. In addition many code in
do {} while loop have a bad influence to a fail rate of cmpxchg_double_slab.
Under current implementation which test status of cpu partial slab
and acquire list_lock in do {} while loop,
we don't need to acquire a list_lock and gain a little benefit
when front of the cpu partial slab is to be discarded, but this is a rare case.
In case that add_partial is performed and cmpxchg_double_slab is failed,
remove_partial should be called case by case.
I think that these are disadvantages of current implementation,
so I do refactoring unfreeze_partials().

Minimizing code in do {} while loop introduce a reduced fail rate
of cmpxchg_double_slab. Below is output of 'slabinfo -r kmalloc-256'
when './perf stat -r 33 hackbench 50 process 4000 > /dev/null' is done.
** before **
Cmpxchg_double Looping
------------------------
Locked Cmpxchg Double redos 182685
Unlocked Cmpxchg Double redos 0
** after **
Cmpxchg_double Looping
------------------------
Locked Cmpxchg Double redos 177995
Unlocked Cmpxchg Double redos 1
We can see cmpxchg_double_slab fail rate is improved slightly.

Bolow is output of './perf stat -r 30 hackbench 50 process 4000 > /dev/null'.
** before **
Performance counter stats for './hackbench 50 process 4000' (30 runs):

108517.190463 task-clock # 7.926 CPUs utilized ( +- 0.24% )
2,919,550 context-switches # 0.027 M/sec ( +- 3.07% )
100,774 CPU-migrations # 0.929 K/sec ( +- 4.72% )
124,201 page-faults # 0.001 M/sec ( +- 0.15% )
401,500,234,387 cycles # 3.700 GHz ( +- 0.24% )
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
250,576,913,354 instructions # 0.62 insns per cycle ( +- 0.13% )
45,934,956,860 branches # 423.297 M/sec ( +- 0.14% )
188,219,787 branch-misses # 0.41% of all branches ( +- 0.56% )

13.691837307 seconds time elapsed ( +- 0.24% )

** after **
Performance counter stats for './hackbench 50 process 4000' (30 runs):

107784.479767 task-clock # 7.928 CPUs utilized ( +- 0.22% )
2,834,781 context-switches # 0.026 M/sec ( +- 2.33% )
93,083 CPU-migrations # 0.864 K/sec ( +- 3.45% )
123,967 page-faults # 0.001 M/sec ( +- 0.15% )
398,781,421,836 cycles # 3.700 GHz ( +- 0.22% )
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
250,189,160,419 instructions # 0.63 insns per cycle ( +- 0.09% )
45,855,370,128 branches # 425.436 M/sec ( +- 0.10% )
169,881,248 branch-misses # 0.37% of all branches ( +- 0.43% )

13.596272341 seconds time elapsed ( +- 0.22% )

No regression is found, but rather we can see slightly better result.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>

diff --git a/mm/slub.c b/mm/slub.c
index 69342fd..eebb6d0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1882,18 +1882,24 @@ redo:
/* Unfreeze all the cpu partial slabs */
static void unfreeze_partials(struct kmem_cache *s)
{
- struct kmem_cache_node *n = NULL;
+ struct kmem_cache_node *n = NULL, *n2 = NULL;
struct kmem_cache_cpu *c = this_cpu_ptr(s->cpu_slab);
struct page *page, *discard_page = NULL;

while ((page = c->partial)) {
- enum slab_modes { M_PARTIAL, M_FREE };
- enum slab_modes l, m;
struct page new;
struct page old;

c->partial = page->next;
- l = M_FREE;
+
+ n2 = get_node(s, page_to_nid(page));
+ if (n != n2) {
+ if (n)
+ spin_unlock(&n->list_lock);
+
+ n = n2;
+ spin_lock(&n->list_lock);
+ }

do {

@@ -1906,43 +1912,17 @@ static void unfreeze_partials(struct kmem_cache *s)

new.frozen = 0;

- if (!new.inuse && (!n || n->nr_partial > s->min_partial))
- m = M_FREE;
- else {
- struct kmem_cache_node *n2 = get_node(s,
- page_to_nid(page));
-
- m = M_PARTIAL;
- if (n != n2) {
- if (n)
- spin_unlock(&n->list_lock);
-
- n = n2;
- spin_lock(&n->list_lock);
- }
- }
-
- if (l != m) {
- if (l == M_PARTIAL) {
- remove_partial(n, page);
- stat(s, FREE_REMOVE_PARTIAL);
- } else {
- add_partial(n, page,
- DEACTIVATE_TO_TAIL);
- stat(s, FREE_ADD_PARTIAL);
- }
-
- l = m;
- }
-
} while (!__cmpxchg_double_slab(s, page,
old.freelist, old.counters,
new.freelist, new.counters,
"unfreezing slab"));

- if (m == M_FREE) {
+ if (unlikely(!new.inuse && n->nr_partial > s->min_partial)) {
page->next = discard_page;
discard_page = page;
+ } else {
+ add_partial(n, page, DEACTIVATE_TO_TAIL);
+ stat(s, FREE_ADD_PARTIAL);
}
}

--
1.7.9.5


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