Message-ID: <20250520050205.2778391-1-krikku@gmail.com>
Date: Tue, 20 May 2025 10:32:05 +0530
From: Krishna Kumar <krikku@...il.com>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net,
	anthony.l.nguyen@...el.com,
	przemyslaw.kitszel@...el.com,
	edumazet@...gle.com,
	intel-wired-lan@...ts.osuosl.org,
	andrew+netdev@...n.ch,
	kuba@...nel.org,
	pabeni@...hat.com,
	sridhar.samudrala@...el.com,
	ahmed.zaki@...el.com,
	krishna.ku@...pkart.com
Subject: [PATCH] net: ice: Perform accurate aRFS flow match

This patch fixes an issue seen in a large-scale deployment under heavy
incoming pkts where the aRFS flow wrongly matches a flow and reprograms the
NIC with wrong settings. That mis-steering causes RX-path latency spikes
and noisy neighbor effects when many connections collide on the same has
(some of our production servers have 20-30K connections).

set_rps_cpu() calls ndo_rx_flow_steer() with flow_id that is calculated by
hashing the skb sized by the per rx-queue table size. This results in
multiple connections (even across different rx-queues) getting the same
hash value. The driver steer function modifies the wrong flow to use this
rx-queue, e.g.:
    Flow#1 is first added:
	    Flow#1:  <ip1, port1, ip2, port2>, Hash 'h', q#10
    Later when a new flow needs to be added:
	    Flow#2:  <ip3, port3, ip4, port4>, Hash 'h', q#20
The driver finds the hash 'h' from Flow#1 and updates it to use q#20. This
results in both flows getting un-optimized - packets for Flow#1 goes to
q#20, and then reprogrammed back to q#10 later and so on; and Flow #2
programming is never done as Flow#1 is matched first for all misses. Many
flows may wrongly share the same hash and reprogram rules of the original
flow each with their own q#.

Tested on two 144-core servers with 16K netperf sessions for 180s. Netperf
clients are pinned to cores 0-71 sequentially (so that wrong packets on q#s
72-143 can be measured). IRQs are set 1:1 for queues -> CPUs, enable XPS,
enable aRFS (global value is 144 * rps_flow_cnt).

Test notes about results from ice_rx_flow_steer():
---------------------------------------------------
1. "Skip:" counter increments here:
    if (fltr_info->q_index == rxq_idx ||
	arfs_entry->fltr_state != ICE_ARFS_ACTIVE)
	    goto out;
2. "Add:" counter increments here:
    ret = arfs_entry->fltr_info.fltr_id;
    INIT_HLIST_NODE(&arfs_entry->list_entry);
3. "Update:" counter increments here:
    /* update the queue to forward to on an already existing flow */

- **rps_flow_cnt=512**
  - Ratio of packets on good vs bad queues: 214 vs 822,392
  - Avoid updating wrong aRFS filter: 0 vs 310,826
  - CPU: User: 216 vs 183, System: 1441 vs 1171, Softirq: 1245 vs 920
         Total: 29.02 22.74
  - aRFS Add: 6,078,551 vs 6,126,286 Update: 533,973 vs 59
         Skip: 82,219,629 vs 77,088,191, Del: 6,078,409 vs 6,126,139

- **rps_flow_cnt=1024**
  - Ratio of packets on good vs bad queues: 854 vs 1,003,176
  - Avoid updating wrong aRFS filter: 0 vs 50,363
  - CPU: User: 220 vs 206, System: 1460 vs 1322 Softirq: 1216 vs 1027
         Total: 28.96 vs 25.55
  - aRFS Add: 7,000,757 vs 7,499,586 Update: 504,371 vs 33
         Skip: 27,455,269 vs 21,752,043, Del: 7,000,610 vs 7,499,438

- **rps_flow_cnt=2048**
  - Ratio of packets on good vs bad queues: 1,173,756 vs 981,892
  - Avoid updating wrong aRFS filter: 0 vs 30,145
  - CPU: User: 216 vs 206, System: 1447 vs 1320, Softirq: 1238 vs 961
         Total: 29.01 vs 24.87
  - aRFS Add: 7,226,598 vs 6,960,991, Update: 521,264 vs 32
         Skip: 7,236,716 vs 4,584,043, Del: 722,6430 vs 696,0798

A separate TCP_STREAM and TCP_RR with 1,4,8,16,64,128,256,512 connections
showed no performance degradation.

Some points on the patch/aRFS behavior:
1. Enabling full tuple matching ensures flows are always correctly matched,
   even with smaller hash sizes.
2. 5-6% drop in CPU utilization as the packets arrive at the correct CPUs
   and fewer calls to driver for programming on misses.
3. Larger hash tables reduces mis-steering due to more unique flow hashes,
   but still has clashes. However, with larger per-device rps_flow_cnt, old
   flows take more time to expire and new aRFS flows cannot be added if h/w
   limits are reached (rps_may_expire_flow() succeeds when 10*rps_flow_cnt
   pkts have been processed by this cpu that are not part of the flow).

Signed-off-by: Krishna Kumar <krikku@...il.com>
---
 drivers/net/ethernet/intel/ice/ice_arfs.c | 45 +++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/drivers/net/ethernet/intel/ice/ice_arfs.c b/drivers/net/ethernet/intel/ice/ice_arfs.c
index 2bc5c7f59844..b36bd189bd64 100644
--- a/drivers/net/ethernet/intel/ice/ice_arfs.c
+++ b/drivers/net/ethernet/intel/ice/ice_arfs.c
@@ -377,6 +377,47 @@ ice_arfs_is_perfect_flow_set(struct ice_hw *hw, __be16 l3_proto, u8 l4_proto)
 	return false;
 }
 
+/**
+ * ice_arfs_cmp - Check if aRFS filter matches this flow.
+ * @fltr_info: filter info of the saved ARFS entry.
+ * @fk: flow dissector keys.
+ * n_proto:  One of htons(IPv4) or htons(IPv6).
+ * ip_proto: One of IPPROTO_TCP or IPPROTO_UDP.
+ *
+ * Since this function assumes limited values for n_proto and ip_proto, it
+ * is meant to be called only from ice_rx_flow_steer().
+ */
+static bool
+ice_arfs_cmp(const struct ice_fdir_fltr *fltr_info, const struct flow_keys *fk,
+	     __be16 n_proto, u8 ip_proto)
+{
+	/*
+	 * Determine if the filter is for IPv4 or IPv6 based on flow_type,
+	 * which is one of ICE_FLTR_PTYPE_NONF_IPV{4,6}_{TCP,UDP}.
+	 */
+	bool is_v4 = fltr_info->flow_type == ICE_FLTR_PTYPE_NONF_IPV4_TCP ||
+		     fltr_info->flow_type == ICE_FLTR_PTYPE_NONF_IPV4_UDP;
+
+	/* Following checks are arranged in the quickest and most discriminative
+	 * fields first for early failure.
+	 */
+	if (is_v4)
+		return n_proto == htons(ETH_P_IP) &&
+			fltr_info->ip.v4.src_port == fk->ports.src &&
+			fltr_info->ip.v4.dst_port == fk->ports.dst &&
+			fltr_info->ip.v4.src_ip == fk->addrs.v4addrs.src &&
+			fltr_info->ip.v4.dst_ip == fk->addrs.v4addrs.dst &&
+			fltr_info->ip.v4.proto == ip_proto;
+
+	return fltr_info->ip.v6.src_port == fk->ports.src &&
+		fltr_info->ip.v6.dst_port == fk->ports.dst &&
+		fltr_info->ip.v6.proto == ip_proto &&
+		!memcmp(&fltr_info->ip.v6.src_ip, &fk->addrs.v6addrs.src,
+			sizeof(struct in6_addr)) &&
+		!memcmp(&fltr_info->ip.v6.dst_ip, &fk->addrs.v6addrs.dst,
+			sizeof(struct in6_addr));
+}
+
 /**
  * ice_rx_flow_steer - steer the Rx flow to where application is being run
  * @netdev: ptr to the netdev being adjusted
@@ -448,6 +489,10 @@ ice_rx_flow_steer(struct net_device *netdev, const struct sk_buff *skb,
 			continue;
 
 		fltr_info = &arfs_entry->fltr_info;
+
+		if (!ice_arfs_cmp(fltr_info, &fk, n_proto, ip_proto))
+			continue;
+
 		ret = fltr_info->fltr_id;
 
 		if (fltr_info->q_index == rxq_idx ||
-- 
2.39.5

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