net: rfs: hash function change

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>

diff --git a/include/net/rps.h b/include/net/rps.h
index a93401d23d66e45210acc73f0326087813b69d59..e358e9711f27523534fdf4cbf57729cbdf629b8a 100644 (file)
--- a/include/net/rps.h
+++ b/include/net/rps.h
@@ -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[];
 };

diff --git a/net/core/dev.c b/net/core/dev.c
index bcf81c3ff6a32d56cf0613a4165677492987dd4d..b6b1c7898281b5966f90ea9504d3d9b0266b538e 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -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();

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index abaa1c919b984cb3340e99bf1af26864a0cc3405..b6fbe629ccee1ea874a796c6146e4a6e8296c5dc 100644 (file)
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -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 {