[PATCH bpf-next] bpf: make __htab_lookup_and_delete_batch faster when map is almost empty

[Brian Vazquez <brianvv@google.com>]

While running some experiments it was observed that map_lookup_batch was much
slower than get_next_key + lookup when the syscall overhead is minimal.
This was because the map_lookup_batch implementation was more expensive
traversing empty buckets, this can be really costly when the pre-allocated
map is too big.

This patch optimizes the case when the bucket is empty so we can move quickly
to next bucket.

The benchmark to exercise this is as follows:

-The map was populate with a single entry to make sure that the syscall overhead
is not helping the map_batch_lookup.
-The size of the preallocated map was increased to show the effect of
traversing empty buckets.

Results:

  Using get_next_key + lookup:

  Benchmark                Time(ns)        CPU(ns)     Iteration
  ---------------------------------------------------------------
  BM_DumpHashMap/1/1k          3593           3586         192680
  BM_DumpHashMap/1/4k          6004           5972         100000
  BM_DumpHashMap/1/16k        15755          15710          44341
  BM_DumpHashMap/1/64k        59525          59376          10000

  Using htab_lookup_batch before this patch:
  Benchmark                Time(ns)        CPU(ns)     Iterations
  ---------------------------------------------------------------
  BM_DumpHashMap/1/1k          3933           3927         177978
  BM_DumpHashMap/1/4k          9192           9177          73951
  BM_DumpHashMap/1/16k        42011          41970          16789
  BM_DumpHashMap/1/64k       117895         117661           6135

  Using htab_lookup_batch with this patch:
  Benchmark                Time(ns)        CPU(ns)     Iterations
  ---------------------------------------------------------------
  BM_DumpHashMap/1/1k          2809           2803         249212
  BM_DumpHashMap/1/4k          5318           5316         100000
  BM_DumpHashMap/1/16k        14925          14895          47448
  BM_DumpHashMap/1/64k        58870          58674          10000

Suggested-by: Luigi Rizzo <lrizzo@google.com>
Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Brian Vazquez <brianvv@google.com>
---
 kernel/bpf/hashtab.c | 23 ++++++++---------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 2137e2200d95..150015ea6737 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1351,7 +1351,6 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	struct hlist_nulls_head *head;
 	struct hlist_nulls_node *n;
 	unsigned long flags = 0;
-	bool locked = false;
 	struct htab_elem *l;
 	struct bucket *b;
 	int ret = 0;
@@ -1410,19 +1409,19 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	dst_val = values;
 	b = &htab->buckets[batch];
 	head = &b->head;
-	/* do not grab the lock unless need it (bucket_cnt > 0). */
-	if (locked)
-		flags = htab_lock_bucket(htab, b);
 
+	l = hlist_nulls_entry_safe(rcu_dereference_raw(hlist_nulls_first_rcu(head)),
+					struct htab_elem, hash_node);
+	if (!l && (batch + 1 < htab->n_buckets)) {
+		batch++;
+		goto again_nocopy;
+	}
+
+	flags = htab_lock_bucket(htab, b);
 	bucket_cnt = 0;
 	hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
 		bucket_cnt++;
 
-	if (bucket_cnt && !locked) {
-		locked = true;
-		goto again_nocopy;
-	}
-
 	if (bucket_cnt > (max_count - total)) {
 		if (total == 0)
 			ret = -ENOSPC;
@@ -1448,10 +1447,6 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 		goto alloc;
 	}
 
-	/* Next block is only safe to run if you have grabbed the lock */
-	if (!locked)
-		goto next_batch;
-
 	hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
 		memcpy(dst_key, l->key, key_size);
 
@@ -1494,7 +1489,6 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	}
 
 	htab_unlock_bucket(htab, b, flags);
-	locked = false;
 
 	while (node_to_free) {
 		l = node_to_free;
@@ -1502,7 +1496,6 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 		bpf_lru_push_free(&htab->lru, &l->lru_node);
 	}
 
-next_batch:
 	/* If we are not copying data, we can go to next bucket and avoid
 	 * unlocking the rcu.
 	 */
-- 
2.28.0.163.g6104cc2f0b6-goog