[PATCH net-next v4 0/2] seg6: skip route refcounts in seg6local lookups

[PATCH net-next v4 0/2] seg6: skip route refcounts in seg6local lookups

[Yuya Kusakabe]

seg6local looks up the nexthop of its IPv6 and IPv4 behaviors from the
RCU read-side section of the receive path, yet takes and drops a route
reference for every packet.  Skip the reference, as ip6_route_input()
and ip_rcv_finish() do.

An LLM coding assistant helped write and test this series.

---
Changes in v4:
- Take no reference on ip6_blk_hole_entry after a noref lookup,
  including for a missing table.  An End whose next segment is a local
  address took one for every packet (Andrea Mayer).
- Comment that the noref helpers need RCU held until dst_input()
  (Andrea Mayer, Sashiko).
- Link to v3: https://patch.msgid.link/20261015-seg6-lookup-noref-v3-0-6d51a60a67d3@gmail.com

Changes in v3:
- Keep the refcounted lookup in seg6_lookup_nexthop() and give
  seg6_lookup_any_nexthop() a noref argument instead, so the BPF LWT
  helpers see no change while a route is released (Andrea Mayer,
  Gabriel Goller, Sashiko).
- Mention NFQUEUE reinjection and the release race in the commit
  messages (Andrea Mayer).
- Link to v2: https://patch.msgid.link/20261010-seg6-lookup-noref-v2-0-51c09dd98197@gmail.com

Changes in v2:
- Split the lookup into a noref primitive for the seg6local input
  actions and a refcounted seg6_lookup_nexthop() wrapper, which the BPF
  LWT helpers also call (Andrea Mayer, Sashiko).
- Rebase onto net-next and re-measure.
- Link to v1: https://patch.msgid.link/20261005-seg6-lookup-noref-v1-0-092e84774e80@gmail.com

---
Yuya Kusakabe (2):
      seg6: skip the route refcount in seg6local IPv6 lookups
      seg6: skip the route refcount in seg6local IPv4 lookups

 net/ipv6/seg6_local.c | 59 +++++++++++++++++++++++++++++++++++----------------
 1 file changed, 41 insertions(+), 18 deletions(-)
---
base-commit: 758fe10f83a655bed0f7b98fcfacdfc3556fb9a1
change-id: 20261004-seg6-lookup-noref-e016495b60ed

Best regards,
--  
Yuya Kusakabe <yuya.kusakabe@gmail.com>

[PATCH net-next v4 1/2] seg6: skip the route refcount in seg6local IPv6 lookups

[Yuya Kusakabe]

ip6_route_input() attaches the input route to the skb without taking a
reference, relying on the RCU read-side section the lookup runs in.
The seg6local input actions also look the route up and hand the skb to
dst_input() within one RCU read-side section: the one of the receive
path, or that of the nfnetlink verdict handler for a packet reinjected
from NFQUEUE.  Yet seg6_lookup_any_nexthop() takes and drops a route
reference for every packet.

Give seg6_lookup_any_nexthop() a noref argument, and set it for the
input and table lookups of those actions, as ip6_route_input() does.
A packet that races with the release of its route can then be dropped
without an ICMPv6 error, where ip6_hold_safe() used to send it on to
the next rule or to ip6_null_entry; ip6_route_input() has the same
window.  seg6_lookup_nexthop(), which the BPF LWT helpers also call,
keeps the refcounted lookup: BPF_PROG_TEST_RUN runs them repeatedly on
the same skb and may leave the RCU read-side section between runs.

Throughput at 0.5% packet loss of 94-byte End packets (two-segment
SRH, no payload), spread by RSS on an ixgbe 82599ES across two 2.30 GHz
Xeon E5-2650 v3 with 10 cores each, offered by TRex and binary-searched
over 10 runs of 10 s, in Mpps:

  queues  cores                   before   after
  1       1                        0.740   0.756  (+2.2%)
  10      10 on the NIC's socket   6.404   6.950  (+8.5%)
  16      10 + 6 on two sockets    6.550  10.124  (+54.6%)

With the default rules, fib6_rule_lookup() tries the local table
first, and the miss takes and drops a reference on ip6_null_entry,
one route shared by every CPU.  At 16 queues that cache line bounces
between the sockets, and ip6_hold_safe() and dst_release() together
take about a third of the cycles.

Signed-off-by: Yuya Kusakabe <yuya.kusakabe@gmail.com>
---
 net/ipv6/seg6_local.c | 55 ++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 39 insertions(+), 16 deletions(-)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 0c3d1be7f129..7fbcf590297d 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c
@@ -319,7 +319,7 @@ static struct ipv6_sr_hdr *advance_nextseg(struct sk_buff *skb,
 
 static int
 seg6_lookup_any_nexthop(struct sk_buff *skb, struct in6_addr *nhaddr,
-			u32 tbl_id, bool local_delivery, int oif)
+			u32 tbl_id, bool local_delivery, int oif, bool noref)
 {
 	struct net *net = dev_net(skb->dev);
 	struct ipv6hdr *hdr = ipv6_hdr(skb);
@@ -342,10 +342,14 @@ seg6_lookup_any_nexthop(struct sk_buff *skb, struct in6_addr *nhaddr,
 		fl6.flowi6_flags = FLOWI_FLAG_KNOWN_NH;
 
 	if (!tbl_id && !oif) {
+		if (noref)
+			flags |= RT6_LOOKUP_F_DST_NOREF;
 		dst = ip6_route_input_lookup(net, skb->dev, &fl6, skb, flags);
 	} else if (tbl_id) {
 		struct fib6_table *table;
 
+		if (noref)
+			flags |= RT6_LOOKUP_F_DST_NOREF;
 		table = fib6_get_table(net, tbl_id);
 		if (!table)
 			goto out;
@@ -363,26 +367,45 @@ seg6_lookup_any_nexthop(struct sk_buff *skb, struct in6_addr *nhaddr,
 		dev_flags |= IFF_LOOPBACK;
 
 	if (dst && (dst_dev(dst)->flags & dev_flags) && !dst->error) {
-		dst_release(dst);
+		ip6_rt_put_flags(dst_rt6_info(dst), flags);
 		dst = NULL;
 	}
 
 out:
 	if (!dst) {
-		rt = net->ipv6.ip6_blk_hole_entry;
-		dst = &rt->dst;
-		dst_hold(dst);
+		dst = &net->ipv6.ip6_blk_hole_entry->dst;
+		if (!(flags & RT6_LOOKUP_F_DST_NOREF))
+			dst_hold(dst);
 	}
 
 	skb_dst_drop(skb);
-	skb_dst_set(skb, dst);
+	if ((flags & RT6_LOOKUP_F_DST_NOREF) && !dst->rt_uncached_list)
+		skb_dst_set_noref(skb, dst);
+	else
+		skb_dst_set(skb, dst);
 	return dst->error;
 }
 
+/* rcu must be held from the lookup until dst_input() */
+static int
+seg6_lookup_any_nexthop_noref(struct sk_buff *skb, struct in6_addr *nhaddr,
+			      u32 tbl_id, bool local_delivery, int oif)
+{
+	return seg6_lookup_any_nexthop(skb, nhaddr, tbl_id, local_delivery, oif,
+				       true);
+}
+
+/* rcu must be held from the lookup until dst_input() */
+static int seg6_lookup_nexthop_noref(struct sk_buff *skb,
+				     struct in6_addr *nhaddr, u32 tbl_id)
+{
+	return seg6_lookup_any_nexthop_noref(skb, nhaddr, tbl_id, false, 0);
+}
+
 int seg6_lookup_nexthop(struct sk_buff *skb,
 			struct in6_addr *nhaddr, u32 tbl_id)
 {
-	return seg6_lookup_any_nexthop(skb, nhaddr, tbl_id, false, 0);
+	return seg6_lookup_any_nexthop(skb, nhaddr, tbl_id, false, 0, false);
 }
 
 static __u8 seg6_flv_lcblock_octects(const struct seg6_flavors_info *finfo)
@@ -446,7 +469,7 @@ seg6_next_csid_advance_arg(struct sk_buff *skb,
 static int input_action_end_finish(struct sk_buff *skb,
 				   struct seg6_local_lwt *slwt)
 {
-	seg6_lookup_nexthop(skb, NULL, 0);
+	seg6_lookup_nexthop_noref(skb, NULL, 0);
 
 	return dst_input(skb);
 }
@@ -497,7 +520,7 @@ static int end_next_csid_core(struct sk_buff *skb, struct seg6_local_lwt *slwt)
 static int input_action_end_x_finish(struct sk_buff *skb,
 				     struct seg6_local_lwt *slwt)
 {
-	seg6_lookup_any_nexthop(skb, &slwt->nh6, 0, false, slwt->oif);
+	seg6_lookup_any_nexthop_noref(skb, &slwt->nh6, 0, false, slwt->oif);
 
 	return dst_input(skb);
 }
@@ -939,7 +962,7 @@ static int input_action_end_t(struct sk_buff *skb, struct seg6_local_lwt *slwt)
 		goto drop;
 	}
 
-	seg6_lookup_nexthop(skb, NULL, slwt->table);
+	seg6_lookup_nexthop_noref(skb, NULL, slwt->table);
 
 	return dst_input(skb);
 
@@ -1023,7 +1046,7 @@ static int input_action_end_dx6_finish(struct net *net, struct sock *sk,
 	if (!ipv6_addr_any(&slwt->nh6))
 		nhaddr = &slwt->nh6;
 
-	seg6_lookup_nexthop(skb, nhaddr, 0);
+	seg6_lookup_nexthop_noref(skb, nhaddr, 0);
 
 	return dst_input(skb);
 }
@@ -1383,7 +1406,7 @@ static int input_action_end_dt6(struct sk_buff *skb,
 	/* note: this time we do not need to specify the table because the VRF
 	 * takes care of selecting the correct table.
 	 */
-	seg6_lookup_any_nexthop(skb, NULL, 0, true, 0);
+	seg6_lookup_any_nexthop_noref(skb, NULL, 0, true, 0);
 
 	return dst_input(skb);
 
@@ -1391,7 +1414,7 @@ static int input_action_end_dt6(struct sk_buff *skb,
 #endif
 	skb_set_transport_header(skb, sizeof(struct ipv6hdr));
 
-	seg6_lookup_any_nexthop(skb, NULL, slwt->table, true, 0);
+	seg6_lookup_any_nexthop_noref(skb, NULL, slwt->table, true, 0);
 
 	return dst_input(skb);
 
@@ -1446,7 +1469,7 @@ static int input_action_end_b6(struct sk_buff *skb, struct seg6_local_lwt *slwt)
 
 	skb_set_transport_header(skb, sizeof(struct ipv6hdr));
 
-	seg6_lookup_nexthop(skb, NULL, 0);
+	seg6_lookup_nexthop_noref(skb, NULL, 0);
 
 	return dst_input(skb);
 
@@ -1482,7 +1505,7 @@ static int input_action_end_b6_encap(struct sk_buff *skb,
 
 	skb_set_transport_header(skb, sizeof(struct ipv6hdr));
 
-	seg6_lookup_nexthop(skb, NULL, 0);
+	seg6_lookup_nexthop_noref(skb, NULL, 0);
 
 	return dst_input(skb);
 
@@ -1571,7 +1594,7 @@ static int input_action_end_bpf(struct sk_buff *skb,
 	local_unlock_nested_bh(&seg6_bpf_srh_states.bh_lock);
 
 	if (ret != BPF_REDIRECT)
-		seg6_lookup_nexthop(skb, NULL, 0);
+		seg6_lookup_nexthop_noref(skb, NULL, 0);
 
 	return dst_input(skb);
 

-- 
2.50.1

[PATCH net-next v4 2/2] seg6: skip the route refcount in seg6local IPv4 lookups

[Yuya Kusakabe]

ip_route_input() looks the input route up with ip_route_input_noref()
and then forces a reference on it, which is dropped again when the
packet is transmitted.  End.DX4 and End.DT4 call it and hand the skb
to dst_input() within one RCU read-side section: the one of the
receive path, or that of the nfnetlink verdict handler for a packet
reinjected from NFQUEUE.

Use ip_route_input_noref(), as ip_rcv_finish() does.

Throughput at 0.5% packet loss of 82-byte IPv4-in-IPv6 UDP packets
(no SRH), spread by RSS on an ixgbe 82599ES across two 10-core Xeon
E5-2650 v3, offered by TRex over 10 runs of 10 s, in Mpps:

  End.DX4
  queues  cores                   before   after
  1       1                        0.791   0.811  (+2.5%)
  10      10 on the NIC's socket   6.118   7.394  (+20.9%)
  16      10 + 6 on two sockets    3.305  10.205  (+208.8%)

  End.DT4
  queues  cores                   before   after
  1       1                        0.481   0.489  (+1.6%)
  10      10 on the NIC's socket   4.328   4.588  (+6.0%)
  16      10 + 6 on two sockets    3.351   7.153  (+113.5%)

Every CPU takes and drops a reference on the same input route, the
one cached on the nexthop.  At 16 queues that cache line bounces
between the sockets, and skb_dst_force() and dst_release() together
take more than half of the cycles for End.DX4 and over 40% for End.DT4.

Signed-off-by: Yuya Kusakabe <yuya.kusakabe@gmail.com>
---
 net/ipv6/seg6_local.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 7fbcf590297d..209e31c21e33 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c
@@ -1096,7 +1096,7 @@ static int input_action_end_dx4_finish(struct net *net, struct sock *sk,
 
 	skb_dst_drop(skb);
 
-	reason = ip_route_input(skb, nhaddr, iph->saddr, 0, skb->dev);
+	reason = ip_route_input_noref(skb, nhaddr, iph->saddr, 0, skb->dev);
 	if (reason) {
 		kfree_skb_reason(skb, reason);
 		return -EINVAL;
@@ -1322,7 +1322,7 @@ static int input_action_end_dt4(struct sk_buff *skb,
 
 	iph = ip_hdr(skb);
 
-	reason = ip_route_input(skb, iph->daddr, iph->saddr, 0, skb->dev);
+	reason = ip_route_input_noref(skb, iph->daddr, iph->saddr, 0, skb->dev);
 	if (unlikely(reason))
 		goto drop;
 

-- 
2.50.1

Re: [PATCH net-next v4 0/2] seg6: skip route refcounts in seg6local lookups

[Jakub Kicinski]

On Fri, 23 Oct 2026 22:47:55 +0900 Yuya Kusakabe wrote:
> seg6local looks up the nexthop of its IPv6 and IPv4 behaviors from the
> RCU read-side section of the receive path, yet takes and drops a route
> reference for every packet.  Skip the reference, as ip6_route_input()
> and ip_rcv_finish() do.

## Form letter - net-next-closed

The merge window for Linux v7.4 has begun, and the net-next
tree is now closed for new drivers, features, code refactoring
and optimizations. We are currently accepting bug fixes only.

Please repost when net-next reopens. There is some uncertainty
about how long this merge window will last, so please check
https://netdev.bots.linux.dev/net-next.html
before reposting. net-next _may_ reopen on Nov 2nd.

See: https://www.kernel.org/doc/html/next/process/maintainer-netdev.html#development-cycle
-- 
pw-bot: defer
pv-bot: closed