[PATCH net-next] 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.

Signed-off-by: Chuang Wang <nashuiliang@gmail.com>
---
 include/net/rps.h | 17 +--------------
 net/core/dev.c    | 54 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 16 deletions(-)

diff --git a/include/net/rps.h b/include/net/rps.h
index e33c6a2fa8bb..2cd8698a79d5 100644
--- a/include/net/rps.h
+++ b/include/net/rps.h
@@ -55,22 +55,7 @@ struct rps_sock_flow_table {
 
 #define RPS_NO_CPU 0xffff
 
-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;
-
-	/* We only give a hint, preemption can change CPU under us */
-	val |= raw_smp_processor_id();
-
-	table = rps_tag_to_table(tag_ptr);
-	/* 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)
-		WRITE_ONCE(table[index].ent, val);
-}
+void rps_record_sock_flow(rps_tag_ptr tag_ptr, u32 hash);
 
 static inline void _sock_rps_record_flow_hash(__u32 hash)
 {
diff --git a/net/core/dev.c b/net/core/dev.c
index 203dc36aaed5..770cfb6fe06b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5175,6 +5175,60 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
 	return cpu;
 }
 
+/**
+ * rps_record_cond - 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.
+ */
+static inline bool rps_record_cond(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;
+
+	/* 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;
+}
+
+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;
+
+	/* We only give a hint, preemption can change CPU under us */
+	val |= raw_smp_processor_id();
+
+	table = rps_tag_to_table(tag_ptr);
+	/* The following WRITE_ONCE() is paired with the READ_ONCE()
+	 * here, and another one in get_rps_cpu().
+	 */
+	if (rps_record_cond(READ_ONCE(table[index].ent), val))
+		WRITE_ONCE(table[index].ent, val);
+}
+EXPORT_SYMBOL(rps_record_sock_flow);
+
 #ifdef CONFIG_RFS_ACCEL
 
 /**
-- 
2.47.3