Commit c3025e94 authored by Eric Dumazet's avatar Eric Dumazet Committed by Jakub Kicinski
Browse files

net: rps: change skb_flow_limit() hash function 



As explained in commit f3483c8e ("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: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarWillem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250407163602.170356-2-edumazet@google.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 34a07c5b

net/core/dev.c

+1 −1
Original line number Diff line number Diff line
@@ -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;

net/core/dev.h

+1 −1
Original line number Diff line number Diff line
@@ -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[];

net/core/sysctl_net_core.c

+1 −1
Original line number Diff line number Diff line
@@ -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);

			}

		}