[PATCH net-next 4/4] gro: improve flow distribution across GRO buckets in dev_gro_receive()

[Alexander Lobakin]

Most of the functions that "convert" hash value into an index
(when RPS is configured / XPS is not configured / etc.) set
reciprocal_scale() on it. Its logics is simple, but fair enough and
accounts the entire input value.
On the opposite side, 'hash & (GRO_HASH_BUCKETS - 1)' expression uses
only 3 least significant bits of the value, which is far from
optimal (especially for XOR RSS hashers, where the hashes of two
different flows may differ only by 1 bit somewhere in the middle).

Use reciprocal_scale() here too to take the entire hash value into
account and improve flow dispersion between GRO hash buckets.

Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
---
 net/core/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 65d9e7d9d1e8..bd7c9ba54623 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5952,7 +5952,7 @@ static void gro_flush_oldest(struct napi_struct *napi, struct list_head *head)

 static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
 {
-	u32 bucket = skb_get_hash_raw(skb) & (GRO_HASH_BUCKETS - 1);
+	u32 bucket = reciprocal_scale(skb_get_hash_raw(skb), GRO_HASH_BUCKETS);
 	struct gro_list *gro_list = &napi->gro_hash[bucket];
 	struct list_head *gro_head = &gro_list->list;
 	struct list_head *head = &offload_base;
--
2.30.2