[PATCH net-next v4] net: reduce RFS/ARFS flow updates by checking LLC affinity

[Chuang Wang <nashuiliang@gmail.com>]

The current implementation of rps_record_sock_flow() updates the flow
table every time a socket is processed on a different CPU. In high-load
scenarios, especially with Accelerated RFS (ARFS), this triggers
frequent flow steering updates via ndo_rx_flow_steer.

For drivers like mlx5 that implement hardware flow steering, these
constant updates lead to significant contention on internal driver locks
(e.g., arfs_lock). This contention often becomes a performance
bottleneck that outweighs the steering benefits.

This patch introduces a cache-aware update strategy: the flow record is
only updated if the flow migrates across Last Level Cache (LLC)
boundaries. This minimizes expensive hardware reconfigurations while
preserving cache locality for the application. A new sysctl,
net.core.rps_feat_llc_affinity, is added to toggle this feature.

Additionally, export sock_rps_record_flow_hash(). This resolves a symbol
visibility compilation error triggered by 'tun' using
sock_rps_record_flow_hash() in tun_flow_update() when CONFIG_TUN is
built as a module.

Performance Test Results:
The patch was tested in a K8s environment (AMD CPU 128*2, 16-core Pod
with CPU pinning, mlx5 NIC) using brpc[1] echo_server and rpc_press.

rpc_press Commands:

  for i in {1..8}; do
    ./rpc_press -proto=./echo.proto -method=example.EchoService.Echo
    -server=<IP>:8000 -input='{"message":"hello"}'
    -qps=0 -thread_num=512 -connection_type=pooled &
  done

Monitor mlx5e_rx_flow_steer frequency:

  /usr/share/bcc/tools/funccount -i 1 mlx5e_rx_flow_steer

Frequency of mlx5e_rx_flow_steer (via funccount[2]):

  Before: ~335,000 counts/sec
  After:   ~23,000 counts/sec (reduced by ~93%)

System Metrics (after enabling rps_feat_llc_affinity):

  CPU Utilization: 38% -> 32%
  CPU PSI (Pressure Stall Information): 20% -> 10%

These results demonstrate that filtering updates by LLC affinity
significantly reduces driver lock contention and improves overall
CPU efficiency under heavy network load.

[1] https://github.com/apache/brpc/
[2] https://github.com/iovisor/bcc/blob/master/tools/funccount.py

Signed-off-by: Chuang Wang <nashuiliang@gmail.com>
---
v3 -> v4: add rps_llc_check by Eric Dumazet
v2 -> v3: patch net -> net-next by Jakub Kicinski
v1 -> v2: add rps_feat_llc_affinity; add brpc tests

 include/net/rps.h          | 17 ++++------
 net/core/dev.c             | 65 ++++++++++++++++++++++++++++++++++++++
 net/core/sysctl_net_core.c | 35 ++++++++++++++++++++
 3 files changed, 107 insertions(+), 10 deletions(-)

diff --git a/include/net/rps.h b/include/net/rps.h
index e33c6a2fa8bb..65543354f6e6 100644
--- a/include/net/rps.h
+++ b/include/net/rps.h
@@ -12,6 +12,7 @@
 
 extern struct static_key_false rps_needed;
 extern struct static_key_false rfs_needed;
+extern struct static_key_false rps_feat_llc_affinity;
 
 /*
  * This structure holds an RPS map which can be of variable length.  The
@@ -55,11 +56,14 @@ struct rps_sock_flow_table {
 
 #define RPS_NO_CPU 0xffff
 
+bool rps_llc_check(u32 old_val, u32 new_val);
+
 static inline void rps_record_sock_flow(rps_tag_ptr tag_ptr, u32 hash)
 {
 	unsigned int index = hash & rps_tag_to_mask(tag_ptr);
 	u32 val = hash & ~net_hotdata.rps_cpu_mask;
 	struct rps_sock_flow_table *table;
+	u32 old_val;
 
 	/* We only give a hint, preemption can change CPU under us */
 	val |= raw_smp_processor_id();
@@ -68,7 +72,8 @@ static inline void rps_record_sock_flow(rps_tag_ptr tag_ptr, u32 hash)
 	/* The following WRITE_ONCE() is paired with the READ_ONCE()
 	 * here, and another one in get_rps_cpu().
 	 */
-	if (READ_ONCE(table[index].ent) != val)
+	old_val = READ_ONCE(table[index].ent);
+	if (old_val != val && rps_llc_check(old_val, val))
 		WRITE_ONCE(table[index].ent, val);
 }
 
@@ -136,15 +141,7 @@ static inline bool rfs_is_needed(void)
 #endif
 }
 
-static inline void sock_rps_record_flow_hash(__u32 hash)
-{
-#ifdef CONFIG_RPS
-	if (!rfs_is_needed())
-		return;
-
-	_sock_rps_record_flow_hash(hash);
-#endif
-}
+void sock_rps_record_flow_hash(__u32 hash);
 
 static inline void sock_rps_record_flow(const struct sock *sk)
 {
diff --git a/net/core/dev.c b/net/core/dev.c
index 26ac8eb9b259..df8d328447bd 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4997,6 +4997,8 @@ struct static_key_false rps_needed __read_mostly;
 EXPORT_SYMBOL(rps_needed);
 struct static_key_false rfs_needed __read_mostly;
 EXPORT_SYMBOL(rfs_needed);
+struct static_key_false rps_feat_llc_affinity __read_mostly;
+EXPORT_SYMBOL(rps_feat_llc_affinity);
 
 static u32 rfs_slot(u32 hash, rps_tag_ptr tag_ptr)
 {
@@ -5208,6 +5210,58 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
 	return cpu;
 }
 
+/**
+ * rps_llc_check - Determine if RPS flow table should be updated
+ * @old_val: Previous flow record value
+ * @new_val: Target flow record value
+ *
+ * Returns true if the record needs an update.
+ */
+bool rps_llc_check(u32 old_val, u32 new_val)
+{
+	u32 old_cpu = old_val & ~net_hotdata.rps_cpu_mask;
+	u32 new_cpu = new_val & ~net_hotdata.rps_cpu_mask;
+
+	if (old_val == new_val)
+		return false;
+
+	/*
+	 * RPS LLC Affinity Feature:
+	 * Reduce RFS/ARFS flow updates by checking LLC affinity.
+	 *
+	 * Frequent flow table updates can trigger constant hardware steering
+	 * reconfigurations (e.g., ndo_rx_flow_steer), leading to significant
+	 * contention on driver internal locks (like mlx5's arfs_lock).
+	 *
+	 * This strategy only updates the flow record if it migrates across LLC
+	 * boundaries. This minimizes expensive hardware updates while preserving
+	 * cache locality for the application.
+	 */
+	if (static_branch_unlikely(&rps_feat_llc_affinity)) {
+		/* Force update if the recorded CPU is invalid or has gone offline */
+		if (old_cpu >= nr_cpu_ids || !cpu_active(old_cpu))
+			return true;
+
+		/*
+		 * Force an update if the current task is no longer permitted
+		 * to run on the old_cpu.
+		 */
+		if (!cpumask_test_cpu(old_cpu, current->cpus_ptr))
+			return true;
+
+		/*
+		 * If CPUs do not share a cache, allow the update to prevent
+		 * expensive remote memory accesses and cache misses.
+		 */
+		if (!cpus_share_cache(old_cpu, new_cpu))
+			return true;
+
+		return false;
+	}
+
+	return true;
+}
+
 #ifdef CONFIG_RFS_ACCEL
 
 /**
@@ -5249,6 +5303,17 @@ bool rps_may_expire_flow(struct net_device *dev, u16 rxq_index,
 }
 EXPORT_SYMBOL(rps_may_expire_flow);
 
+void sock_rps_record_flow_hash(__u32 hash)
+{
+#ifdef CONFIG_RPS
+	if (!rfs_is_needed())
+		return;
+
+	_sock_rps_record_flow_hash(hash);
+#endif
+}
+EXPORT_SYMBOL(sock_rps_record_flow_hash);
+
 #endif /* CONFIG_RFS_ACCEL */
 
 /* Called from hardirq (IPI) context */
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index b508618bfc12..b6d4ebcbb6a6 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -210,6 +210,33 @@ static int rps_sock_flow_sysctl(const struct ctl_table *table, int write,
 	kvfree_rcu_mightsleep(tofree);
 	return ret;
 }
+
+static int rps_feat_llc_affinity_sysctl(const struct ctl_table *table, int write,
+					void *buffer, size_t *lenp, loff_t *ppos)
+{
+	u8 curr_state;
+	int ret;
+	const struct ctl_table tmp = {
+		.data = &curr_state,
+		.maxlen = sizeof(curr_state),
+		.mode = table->mode,
+		.extra1 = table->extra1,
+		.extra2 = table->extra2
+	};
+
+	curr_state = static_branch_unlikely(&rps_feat_llc_affinity) ? 1 : 0;
+
+	ret = proc_dou8vec_minmax(&tmp, write, buffer, lenp, ppos);
+	if (write && ret == 0) {
+		if (curr_state && !static_branch_unlikely(&rps_feat_llc_affinity))
+			static_branch_enable(&rps_feat_llc_affinity);
+		else if (!curr_state && static_branch_unlikely(&rps_feat_llc_affinity))
+			static_branch_disable(&rps_feat_llc_affinity);
+	}
+
+	return ret;
+}
+
 #endif /* CONFIG_RPS */
 
 #ifdef CONFIG_NET_FLOW_LIMIT
@@ -554,6 +581,14 @@ static struct ctl_table net_core_table[] = {
 		.mode		= 0644,
 		.proc_handler	= rps_sock_flow_sysctl
 	},
+	{
+		.procname   = "rps_feat_llc_affinity",
+		.maxlen     = sizeof(u8),
+		.mode       = 0644,
+		.proc_handler   = rps_feat_llc_affinity_sysctl,
+		.extra1     = SYSCTL_ZERO,
+		.extra2     = SYSCTL_ONE
+	},
 #endif
 #ifdef CONFIG_NET_FLOW_LIMIT
 	{
-- 
2.47.3