netdev - [PATCH net-next 1/4] net: rps: change skb_flow

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Message-ID: <20250407163602.170356-2-edumazet@google.com>
Date: Mon,  7 Apr 2025 16:35:59 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 1/4] net: rps: change skb_flow_limit() hash function

As explained in commit f3483c8e1da6 ("net: rfs: hash function change"),
masking low order bits of skb_get_hash(skb) has low entropy.

A NIC with 32 RX queues uses the 5 low order bits of rss key
to select a queue. This means all packets landing to a given
queue share the same 5 low order bits.

Switch to hash_32() to reduce hash collisions.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/core/dev.c             | 2 +-
 net/core/dev.h             | 2 +-
 net/core/sysctl_net_core.c | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 0608605cfc24..f674236f34be 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5038,7 +5038,7 @@ static bool skb_flow_limit(struct sk_buff *skb, unsigned int qlen)
 	rcu_read_lock();
 	fl = rcu_dereference(sd->flow_limit);
 	if (fl) {
-		new_flow = skb_get_hash(skb) & (fl->num_buckets - 1);
+		new_flow = hash_32(skb_get_hash(skb), fl->log_buckets);
 		old_flow = fl->history[fl->history_head];
 		fl->history[fl->history_head] = new_flow;
 
diff --git a/net/core/dev.h b/net/core/dev.h
index 7ee203395d8e..b1cd44b5f009 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -16,7 +16,7 @@ struct cpumask;
 #define FLOW_LIMIT_HISTORY	(1 << 7)  /* must be ^2 and !overflow buckets */
 struct sd_flow_limit {
 	u64			count;
-	unsigned int		num_buckets;
+	u8			log_buckets;
 	unsigned int		history_head;
 	u16			history[FLOW_LIMIT_HISTORY];
 	u8			buckets[];
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index c7769ee0d9c5..5cfe76ede523 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -248,7 +248,7 @@ static int flow_limit_cpu_sysctl(const struct ctl_table *table, int write,
 					ret = -ENOMEM;
 					goto write_unlock;
 				}
-				cur->num_buckets = netdev_flow_limit_table_len;
+				cur->log_buckets = ilog2(netdev_flow_limit_table_len);
 				rcu_assign_pointer(sd->flow_limit, cur);
 			}
 		}
-- 
2.49.0.504.g3bcea36a83-goog