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. Change from v1: add comment about barrier() usage Acked-by: Christoph Lameter <cl@xxxxxxxxx> Tested-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> --- mm/slub.c | 35 +++++++++++++++++++++++------------ 1 file changed, 23 insertions(+), 12 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index fe376fe..ceee1d7 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2398,13 +2398,24 @@ redo: * 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 = this_cpu_ptr(s->cpu_slab); + } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid)); + + /* + * Irqless object alloc/free alogorithm 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 @@ redo: * 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 @@ redo: * 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 = this_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); -- 1.7.9.5 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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