bpf: Optimize call_rcu in non-preallocated hash map.

Doing call_rcu() million times a second becomes a bottle neck. Convert non-preallocated hash map from call_rcu to SLAB_TYPESAFE_BY_RCU. The rcu critical section is no longer observed for one htab element which makes non-preallocated hash map behave just like preallocated hash map. The map elements are released back to kernel memory after observing rcu critical section. This improves 'map_perf_test 4' performance from 100k events per second to 250k events per second. bpf_mem_alloc + percpu_counter + typesafe_by_rcu provide 10x performance boost to non-preallocated hash map and make it within few % of preallocated map while consuming fraction of memory. Signed-off-by: Alexei Starovoitov <[email protected]> Signed-off-by: Daniel Borkmann <[email protected]> Acked-by: Kumar Kartikeya Dwivedi <[email protected]> Acked-by: Andrii Nakryiko <[email protected]> Link: https://lore.kernel.org/bpf/[email protected]
josf · Sep 5, 2022 · 0fd7c5d · 0fd7c5d
1 parent 86fe28f
commit 0fd7c5d
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Showing 3 changed files with 7 additions and 14 deletions.
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
@@ -953,8 +953,12 @@ static void free_htab_elem(struct bpf_htab *htab, struct htab_elem *l)
 		__pcpu_freelist_push(&htab->freelist, &l->fnode);
 	} else {
 		dec_elem_count(htab);
-		l->htab = htab;
-		call_rcu(&l->rcu, htab_elem_free_rcu);
+		if (htab->map.map_type == BPF_MAP_TYPE_PERCPU_HASH) {
+			l->htab = htab;
+			call_rcu(&l->rcu, htab_elem_free_rcu);
+		} else {
+			htab_elem_free(htab, l);
+		}
 	}
 }
 

diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
@@ -281,7 +281,7 @@ int bpf_mem_alloc_init(struct bpf_mem_alloc *ma, int size)
 			return -ENOMEM;
 		size += LLIST_NODE_SZ; /* room for llist_node */
 		snprintf(buf, sizeof(buf), "bpf-%u", size);
-		kmem_cache = kmem_cache_create(buf, size, 8, 0, NULL);
+		kmem_cache = kmem_cache_create(buf, size, 8, SLAB_TYPESAFE_BY_RCU, NULL);
 		if (!kmem_cache) {
 			free_percpu(pc);
 			return -ENOMEM;

diff --git a/tools/testing/selftests/bpf/progs/timer.c b/tools/testing/selftests/bpf/progs/timer.c
@@ -208,17 +208,6 @@ static int timer_cb2(void *map, int *key, struct hmap_elem *val)
 		 */
 		bpf_map_delete_elem(map, key);
 
-		/* in non-preallocated hashmap both 'key' and 'val' are RCU
-		 * protected and still valid though this element was deleted
-		 * from the map. Arm this timer for ~35 seconds. When callback
-		 * finishes the call_rcu will invoke:
-		 * htab_elem_free_rcu
-		 *   check_and_free_timer
-		 *     bpf_timer_cancel_and_free
-		 * to cancel this 35 second sleep and delete the timer for real.
-		 */
-		if (bpf_timer_start(&val->timer, 1ull << 35, 0) != 0)
-			err |= 256;
 		ok |= 4;
 	}
 	return 0;