mm/slub: optimize alloc/free fastpath by removing preemption on/off
We had to insert a preempt enable/disable in the fastpath a while ago in
order to guarantee that tid and kmem_cache_cpu are retrieved on the same
cpu. It is the problem only for CONFIG_PREEMPT in which scheduler can
move the process to other cpu during retrieving data.
Now, I reach the solution to remove preempt enable/disable in the
fastpath. If tid is matched with kmem_cache_cpu's tid after tid and
kmem_cache_cpu are retrieved by separate this_cpu operation, it means
that they are retrieved on the same cpu. If not matched, we just have
to retry it.
With this guarantee, preemption enable/disable isn't need at all even if
CONFIG_PREEMPT, so this patch removes it.
I saw roughly 5% win in a fast-path loop over kmem_cache_alloc/free in
CONFIG_PREEMPT. (14.821 ns -> 14.049 ns)
Below is the result of Christoph's slab_test reported by Jesper Dangaard
Brouer.
* Before
Single thread testing
=====================
1. Kmalloc: Repeatedly allocate then free test
10000 times kmalloc(8) -> 49 cycles kfree -> 62 cycles
10000 times kmalloc(16) -> 48 cycles kfree -> 64 cycles
10000 times kmalloc(32) -> 53 cycles kfree -> 70 cycles
10000 times kmalloc(64) -> 64 cycles kfree -> 77 cycles
10000 times kmalloc(128) -> 74 cycles kfree -> 84 cycles
10000 times kmalloc(256) -> 84 cycles kfree -> 114 cycles
10000 times kmalloc(512) -> 83 cycles kfree -> 116 cycles
10000 times kmalloc(1024) -> 81 cycles kfree -> 120 cycles
10000 times kmalloc(2048) -> 104 cycles kfree -> 136 cycles
10000 times kmalloc(4096) -> 142 cycles kfree -> 165 cycles
10000 times kmalloc(8192) -> 238 cycles kfree -> 226 cycles
10000 times kmalloc(16384) -> 403 cycles kfree -> 264 cycles
2. Kmalloc: alloc/free test
10000 times kmalloc(8)/kfree -> 68 cycles
10000 times kmalloc(16)/kfree -> 68 cycles
10000 times kmalloc(32)/kfree -> 69 cycles
10000 times kmalloc(64)/kfree -> 68 cycles
10000 times kmalloc(128)/kfree -> 68 cycles
10000 times kmalloc(256)/kfree -> 68 cycles
10000 times kmalloc(512)/kfree -> 74 cycles
10000 times kmalloc(1024)/kfree -> 75 cycles
10000 times kmalloc(2048)/kfree -> 74 cycles
10000 times kmalloc(4096)/kfree -> 74 cycles
10000 times kmalloc(8192)/kfree -> 75 cycles
10000 times kmalloc(16384)/kfree -> 510 cycles
* After
Single thread testing
=====================
1. Kmalloc: Repeatedly allocate then free test
10000 times kmalloc(8) -> 46 cycles kfree -> 61 cycles
10000 times kmalloc(16) -> 46 cycles kfree -> 63 cycles
10000 times kmalloc(32) -> 49 cycles kfree -> 69 cycles
10000 times kmalloc(64) -> 57 cycles kfree -> 76 cycles
10000 times kmalloc(128) -> 66 cycles kfree -> 83 cycles
10000 times kmalloc(256) -> 84 cycles kfree -> 110 cycles
10000 times kmalloc(512) -> 77 cycles kfree -> 114 cycles
10000 times kmalloc(1024) -> 80 cycles kfree -> 116 cycles
10000 times kmalloc(2048) -> 102 cycles kfree -> 131 cycles
10000 times kmalloc(4096) -> 135 cycles kfree -> 163 cycles
10000 times kmalloc(8192) -> 238 cycles kfree -> 218 cycles
10000 times kmalloc(16384) -> 399 cycles kfree -> 262 cycles
2. Kmalloc: alloc/free test
10000 times kmalloc(8)/kfree -> 65 cycles
10000 times kmalloc(16)/kfree -> 66 cycles
10000 times kmalloc(32)/kfree -> 65 cycles
10000 times kmalloc(64)/kfree -> 66 cycles
10000 times kmalloc(128)/kfree -> 66 cycles
10000 times kmalloc(256)/kfree -> 71 cycles
10000 times kmalloc(512)/kfree -> 72 cycles
10000 times kmalloc(1024)/kfree -> 71 cycles
10000 times kmalloc(2048)/kfree -> 71 cycles
10000 times kmalloc(4096)/kfree -> 71 cycles
10000 times kmalloc(8192)/kfree -> 65 cycles
10000 times kmalloc(16384)/kfree -> 511 cycles
Most of the results are better than before.
Note that this change slightly worses performance in !CONFIG_PREEMPT,
roughly 0.3%. Implementing each case separately would help performance,
but, since it's so marginal, I didn't do that. This would help
maintanance since we have same code for all cases.
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Christoph Lameter <cl@linux.com>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/slub.c b/mm/slub.c
index fe376fe..e7ed6f8 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2398,13 +2398,24 @@
* reading from one cpu area. That does not matter as long
* as we end up on the original cpu again when doing the cmpxchg.
*
- * Preemption is disabled for the retrieval of the tid because that
- * must occur from the current processor. We cannot allow rescheduling
- * on a different processor between the determination of the pointer
- * and the retrieval of the tid.
+ * We should guarantee that tid and kmem_cache are retrieved on
+ * the same cpu. It could be different if CONFIG_PREEMPT so we need
+ * to check if it is matched or not.
*/
- preempt_disable();
- c = this_cpu_ptr(s->cpu_slab);
+ do {
+ tid = this_cpu_read(s->cpu_slab->tid);
+ c = raw_cpu_ptr(s->cpu_slab);
+ } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
+
+ /*
+ * Irqless object alloc/free algorithm used here depends on sequence
+ * of fetching cpu_slab's data. tid should be fetched before anything
+ * on c to guarantee that object and page associated with previous tid
+ * won't be used with current tid. If we fetch tid first, object and
+ * page could be one associated with next tid and our alloc/free
+ * request will be failed. In this case, we will retry. So, no problem.
+ */
+ barrier();
/*
* The transaction ids are globally unique per cpu and per operation on
@@ -2412,8 +2423,6 @@
* occurs on the right processor and that there was no operation on the
* linked list in between.
*/
- tid = c->tid;
- preempt_enable();
object = c->freelist;
page = c->page;
@@ -2659,11 +2668,13 @@
* data is retrieved via this pointer. If we are on the same cpu
* during the cmpxchg then the free will succedd.
*/
- preempt_disable();
- c = this_cpu_ptr(s->cpu_slab);
+ do {
+ tid = this_cpu_read(s->cpu_slab->tid);
+ c = raw_cpu_ptr(s->cpu_slab);
+ } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
- tid = c->tid;
- preempt_enable();
+ /* Same with comment on barrier() in slab_alloc_node() */
+ barrier();
if (likely(page == c->page)) {
set_freepointer(s, object, c->freelist);