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Date:   Fri, 12 Mar 2021 16:22:09 +0000
To:     "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andriin@fb.com>,
        Alexander Lobakin <alobakin@pm.me>,
        Eric Dumazet <edumazet@google.com>,
        Wei Wang <weiwan@google.com>,
        Cong Wang <cong.wang@bytedance.com>,
        Taehee Yoo <ap420073@gmail.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: [PATCH net-next 4/4] gro: improve flow distribution across GRO buckets in dev_gro_receive()
Message-ID: <20210312162127.239795-5-alobakin@pm.me>
In-Reply-To: <20210312162127.239795-1-alobakin@pm.me>
References: <20210312162127.239795-1-alobakin@pm.me>
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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@pm.me>
---
 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)
 {
-=09u32 bucket =3D skb_get_hash_raw(skb) & (GRO_HASH_BUCKETS - 1);
+=09u32 bucket =3D reciprocal_scale(skb_get_hash_raw(skb), GRO_HASH_BUCKETS=
);
 =09struct gro_list *gro_list =3D &napi->gro_hash[bucket];
 =09struct list_head *gro_head =3D &gro_list->list;
 =09struct list_head *head =3D &offload_base;
--
2.30.2