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net: rfs: hash function change

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RFS is using two kinds of hash tables.

First one is controlled by /proc/sys/net/core/rps_sock_flow_entries = 2^N
and using the N low order bits of the l4 hash is good enough.

Then each RX queue has its own hash table, controlled by
/sys/class/net/eth1/queues/rx-$q/rps_flow_cnt = 2^X

Current hash function, using the X low order bits is suboptimal,
because RSS is usually using Func(hash) = (hash % power_of_two);

For example, with 32 RX queues, 6 low order bits have no entropy
for a given queue.

Switch this hash function to hash_32(hash, log) to increase
chances to use all possible slots and reduce collisions.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <tom@herbertland.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250321171309.634100-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Eric Dumazet 2025-03-21 17:13:09 +00:00 committed by Jakub Kicinski
parent 1952e19c02
commit f3483c8e1d
3 changed files with 12 additions and 7 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

2
include/net/rps.h
View file

@ -39,7 +39,7 @@ struct rps_dev_flow {
* The rps_dev_flow_table structure contains a table of flow mappings.
*/
struct rps_dev_flow_table {
unsigned int mask;
u8 log;
struct rcu_head rcu;
struct rps_dev_flow flows[];
};

13
net/core/dev.c
View file

@ -4751,6 +4751,11 @@ EXPORT_SYMBOL(rps_needed);
struct static_key_false rfs_needed __read_mostly;
EXPORT_SYMBOL(rfs_needed);
static u32 rfs_slot(u32 hash, const struct rps_dev_flow_table *flow_table)
{
return hash_32(hash, flow_table->log);
}
static struct rps_dev_flow *
set_rps_cpu(struct net_device *dev, struct sk_buff *skb,
struct rps_dev_flow *rflow, u16 next_cpu)
@ -4777,7 +4782,7 @@ set_rps_cpu(struct net_device *dev, struct sk_buff *skb,
flow_table = rcu_dereference(rxqueue->rps_flow_table);
if (!flow_table)
goto out;
flow_id = skb_get_hash(skb) & flow_table->mask;
flow_id = rfs_slot(skb_get_hash(skb), flow_table);
rc = dev->netdev_ops->ndo_rx_flow_steer(dev, skb,
rxq_index, flow_id);
if (rc < 0)
@ -4856,7 +4861,7 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
/* OK, now we know there is a match,
* we can look at the local (per receive queue) flow table
*/
rflow = &flow_table->flows[hash & flow_table->mask];
rflow = &flow_table->flows[rfs_slot(hash, flow_table)];
tcpu = rflow->cpu;
/*
@ -4923,13 +4928,13 @@ bool rps_may_expire_flow(struct net_device *dev, u16 rxq_index,
rcu_read_lock();
flow_table = rcu_dereference(rxqueue->rps_flow_table);
if (flow_table && flow_id <= flow_table->mask) {
if (flow_table && flow_id < (1UL << flow_table->log)) {
rflow = &flow_table->flows[flow_id];
cpu = READ_ONCE(rflow->cpu);
if (READ_ONCE(rflow->filter) == filter_id && cpu < nr_cpu_ids &&
((int)(READ_ONCE(per_cpu(softnet_data, cpu).input_queue_head) -
READ_ONCE(rflow->last_qtail)) <
(int)(10 * flow_table->mask)))
(int)(10 << flow_table->log)))
expire = false;
}
rcu_read_unlock();

4
net/core/net-sysfs.c
View file

@ -1056,7 +1056,7 @@ static ssize_t show_rps_dev_flow_table_cnt(struct netdev_rx_queue *queue,
rcu_read_lock();
flow_table = rcu_dereference(queue->rps_flow_table);
if (flow_table)
val = (unsigned long)flow_table->mask + 1;
val = 1UL << flow_table->log;
rcu_read_unlock();
return sysfs_emit(buf, "%lu\n", val);
@ -1109,7 +1109,7 @@ static ssize_t store_rps_dev_flow_table_cnt(struct netdev_rx_queue *queue,
if (!table)
return -ENOMEM;
table->mask = mask;
table->log = ilog2(mask) + 1;
for (count = 0; count <= mask; count++)
table->flows[count].cpu = RPS_NO_CPU;
} else {