[PATCH v2 bpf] bpf: Do not grab the bucket spinlock by default on htab batch ops

[Brian Vazquez <brianvv@xxxxxxxxxx>]

Grabbing the spinlock for every bucket even if it's empty, was causing
significant perfomance cost when traversing htab maps that have only a
few entries. This patch addresses the issue by checking first the
bucket_cnt, if the bucket has some entries then we go and grab the
spinlock and proceed with the batching.

Tested with a htab of size 50K and different value of populated entries.

Before:
  Benchmark             Time(ns)        CPU(ns)
  ---------------------------------------------
  BM_DumpHashMap/1       2759655        2752033
  BM_DumpHashMap/10      2933722        2930825
  BM_DumpHashMap/200     3171680        3170265
  BM_DumpHashMap/500     3639607        3635511
  BM_DumpHashMap/1000    4369008        4364981
  BM_DumpHashMap/5k     11171919       11134028
  BM_DumpHashMap/20k    69150080       69033496
  BM_DumpHashMap/39k   190501036      190226162

After:
  Benchmark             Time(ns)        CPU(ns)
  ---------------------------------------------
  BM_DumpHashMap/1        202707         200109
  BM_DumpHashMap/10       213441         210569
  BM_DumpHashMap/200      478641         472350
  BM_DumpHashMap/500      980061         967102
  BM_DumpHashMap/1000    1863835        1839575
  BM_DumpHashMap/5k      8961836        8902540
  BM_DumpHashMap/20k    69761497       69322756
  BM_DumpHashMap/39k   187437830      186551111

Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
Cc: Yonghong Song <yhs@xxxxxx>
Signed-off-by: Brian Vazquez <brianvv@xxxxxxxxxx>
---
v1 -> v2: Skip hlist_nulls_for_each_entry_safe if lock is not held

 kernel/bpf/hashtab.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 2d182c4ee9d99..ea3bf04a0a7b6 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1260,6 +1260,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	struct hlist_nulls_head *head;
 	struct hlist_nulls_node *n;
 	unsigned long flags;
+	bool locked = false;
 	struct htab_elem *l;
 	struct bucket *b;
 	int ret = 0;
@@ -1319,15 +1320,25 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	dst_val = values;
 	b = &htab->buckets[batch];
 	head = &b->head;
-	raw_spin_lock_irqsave(&b->lock, flags);
+	/* do not grab the lock unless need it (bucket_cnt > 0). */
+	if (locked)
+		raw_spin_lock_irqsave(&b->lock, flags);
 
 	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;
+		/* Note that since bucket_cnt > 0 here, it is implicit
+		 * that the locked was grabbed, so release it.
+		 */
 		raw_spin_unlock_irqrestore(&b->lock, flags);
 		rcu_read_unlock();
 		this_cpu_dec(bpf_prog_active);
@@ -1337,6 +1348,9 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 
 	if (bucket_cnt > bucket_size) {
 		bucket_size = bucket_cnt;
+		/* Note that since bucket_cnt > 0 here, it is implicit
+		 * that the locked was grabbed, so release it.
+		 */
 		raw_spin_unlock_irqrestore(&b->lock, flags);
 		rcu_read_unlock();
 		this_cpu_dec(bpf_prog_active);
@@ -1346,6 +1360,10 @@ __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);
 
@@ -1380,6 +1398,8 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	}
 
 	raw_spin_unlock_irqrestore(&b->lock, flags);
+	locked = false;
+next_batch:
 	/* If we are not copying data, we can go to next bucket and avoid
 	 * unlocking the rcu.
 	 */
-- 
2.25.0.265.gbab2e86ba0-goog