[PATCH net v5 0/3] Avoid TCP resets when using ECMP for load-balancing between multiple servers.

[PATCH net v5 0/3] Avoid TCP resets when using ECMP for load-balancing between multiple servers.

[Sriram Yagnaraman]

All packets in the same flow (L3/L4 depending on multipath hash policy)
should be directed to the same target, but after [0]/[1] we see stray
packets directed towards other targets. This, for instance, causes RST
to be sent on TCP connections.

The first two patches solve the problem by ignoring route hints for
destinations that are part of multipath group, by using new SKB flags
for IPv4 and IPv6. The third patch is a selftest that tests the
scenario.

Thanks to Ido, for reviewing and suggesting a way forward in [2] and
also suggesting how to write a selftest for this.

v4->v5:
- Fixed review comments from Ido
v3->v4:
- Remove single path test
- Rebase to latest
v2->v3:
- Add NULL check for skb in fib6_select_path (Ido Schimmel)
- Use fib_tests.sh for selftest instead of the forwarding suite (Ido
  Schimmel)
v1->v2:
- Update to commit messages describing the solution (Ido Schimmel)
- Use perf stat to count fib table lookups in selftest (Ido Schimmel)


Sriram Yagnaraman (3):
  ipv4: ignore dst hint for multipath routes
  ipv6: ignore dst hint for multipath routes
  selftests: fib_tests: Add multipath list receive tests

 include/linux/ipv6.h                     |   1 +
 include/net/ip.h                         |   1 +
 net/ipv4/ip_input.c                      |   3 +-
 net/ipv4/route.c                         |   1 +
 net/ipv6/ip6_input.c                     |   3 +-
 net/ipv6/route.c                         |   3 +
 tools/testing/selftests/net/fib_tests.sh | 155 ++++++++++++++++++++++-
 7 files changed, 164 insertions(+), 3 deletions(-)

-- 
2.34.1

[PATCH net v5 1/3] ipv4: ignore dst hint for multipath routes

[Sriram Yagnaraman]

Route hints when the nexthop is part of a multipath group causes packets
in the same receive batch to be sent to the same nexthop irrespective of
the multipath hash of the packet. So, do not extract route hint for
packets whose destination is part of a multipath group.

A new SKB flag IPSKB_MULTIPATH is introduced for this purpose, set the
flag when route is looked up in ip_mkroute_input() and use it in
ip_extract_route_hint() to check for the existence of the flag.

Fixes: 02b24941619f ("ipv4: use dst hint for ipv4 list receive")
Signed-off-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
---
 include/net/ip.h    | 1 +
 net/ipv4/ip_input.c | 3 ++-
 net/ipv4/route.c    | 1 +
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/net/ip.h b/include/net/ip.h
index 19adacd5ece0..464176a88f86 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -57,6 +57,7 @@ struct inet_skb_parm {
 #define IPSKB_FRAG_PMTU		BIT(6)
 #define IPSKB_L3SLAVE		BIT(7)
 #define IPSKB_NOPOLICY		BIT(8)
+#define IPSKB_MULTIPATH		BIT(9)
 
 	u16			frag_max_size;
 };
diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index fe9ead9ee863..5e9c8156656a 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -584,7 +584,8 @@ static void ip_sublist_rcv_finish(struct list_head *head)
 static struct sk_buff *ip_extract_route_hint(const struct net *net,
 					     struct sk_buff *skb, int rt_type)
 {
-	if (fib4_has_custom_rules(net) || rt_type == RTN_BROADCAST)
+	if (fib4_has_custom_rules(net) || rt_type == RTN_BROADCAST ||
+	    IPCB(skb)->flags & IPSKB_MULTIPATH)
 		return NULL;
 
 	return skb;
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index a4e153dd615b..6a3f57a3fa41 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2144,6 +2144,7 @@ static int ip_mkroute_input(struct sk_buff *skb,
 		int h = fib_multipath_hash(res->fi->fib_net, NULL, skb, hkeys);
 
 		fib_select_multipath(res, h);
+		IPCB(skb)->flags |= IPSKB_MULTIPATH;
 	}
 #endif
 
-- 
2.34.1

[PATCH net v5 2/3] ipv6: ignore dst hint for multipath routes

[Sriram Yagnaraman]

Route hints when the nexthop is part of a multipath group causes packets
in the same receive batch to be sent to the same nexthop irrespective of
the multipath hash of the packet. So, do not extract route hint for
packets whose destination is part of a multipath group.

A new SKB flag IP6SKB_MULTIPATH is introduced for this purpose, set the
flag when route is looked up in fib6_select_path() and use it in
ip6_can_use_hint() to check for the existence of the flag.

Fixes: 197dbf24e360 ("ipv6: introduce and uses route look hints for list input.")
Signed-off-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
---
 include/linux/ipv6.h | 1 +
 net/ipv6/ip6_input.c | 3 ++-
 net/ipv6/route.c     | 3 +++
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 5883551b1ee8..af8a771a053c 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -147,6 +147,7 @@ struct inet6_skb_parm {
 #define IP6SKB_JUMBOGRAM      128
 #define IP6SKB_SEG6	      256
 #define IP6SKB_FAKEJUMBO      512
+#define IP6SKB_MULTIPATH      1024
 };
 
 #if defined(CONFIG_NET_L3_MASTER_DEV)
diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
index d94041bb4287..b8378814532c 100644
--- a/net/ipv6/ip6_input.c
+++ b/net/ipv6/ip6_input.c
@@ -99,7 +99,8 @@ static bool ip6_can_use_hint(const struct sk_buff *skb,
 static struct sk_buff *ip6_extract_route_hint(const struct net *net,
 					      struct sk_buff *skb)
 {
-	if (fib6_routes_require_src(net) || fib6_has_custom_rules(net))
+	if (fib6_routes_require_src(net) || fib6_has_custom_rules(net) ||
+	    IP6CB(skb)->flags & IP6SKB_MULTIPATH)
 		return NULL;
 
 	return skb;
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 846aec8e0093..01d6d352850a 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -423,6 +423,9 @@ void fib6_select_path(const struct net *net, struct fib6_result *res,
 	if (match->nh && have_oif_match && res->nh)
 		return;
 
+	if (skb)
+		IP6CB(skb)->flags |= IP6SKB_MULTIPATH;
+
 	/* We might have already computed the hash for ICMPv6 errors. In such
 	 * case it will always be non-zero. Otherwise now is the time to do it.
 	 */
-- 
2.34.1

[PATCH net v5 3/3] selftests: fib_tests: Add multipath list receive tests

[Sriram Yagnaraman]

The test uses perf stat to count the number of fib:fib_table_lookup
tracepoint hits for IPv4 and the number of fib6:fib6_table_lookup for
IPv6. The measured count is checked to be within 5% of the total number
of packets sent via veth1.

Signed-off-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
---
 tools/testing/selftests/net/fib_tests.sh | 155 ++++++++++++++++++++++-
 1 file changed, 154 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/net/fib_tests.sh b/tools/testing/selftests/net/fib_tests.sh
index d328af4a149c..e7d2a530618a 100755
--- a/tools/testing/selftests/net/fib_tests.sh
+++ b/tools/testing/selftests/net/fib_tests.sh
@@ -12,7 +12,8 @@ ksft_skip=4
 TESTS="unregister down carrier nexthop suppress ipv6_notify ipv4_notify \
        ipv6_rt ipv4_rt ipv6_addr_metric ipv4_addr_metric ipv6_route_metrics \
        ipv4_route_metrics ipv4_route_v6_gw rp_filter ipv4_del_addr \
-       ipv6_del_addr ipv4_mangle ipv6_mangle ipv4_bcast_neigh fib6_gc_test"
+       ipv6_del_addr ipv4_mangle ipv6_mangle ipv4_bcast_neigh fib6_gc_test \
+       ipv4_mpath_list ipv6_mpath_list"
 
 VERBOSE=0
 PAUSE_ON_FAIL=no
@@ -2352,6 +2353,156 @@ ipv4_bcast_neigh_test()
 	cleanup
 }
 
+mpath_dep_check()
+{
+	if [ ! -x "$(command -v mausezahn)" ]; then
+		echo "mausezahn command not found. Skipping test"
+		return 1
+	fi
+
+	if [ ! -x "$(command -v jq)" ]; then
+		echo "jq command not found. Skipping test"
+		return 1
+	fi
+
+	if [ ! -x "$(command -v bc)" ]; then
+		echo "bc command not found. Skipping test"
+		return 1
+	fi
+
+	if [ ! -x "$(command -v perf)" ]; then
+		echo "perf command not found. Skipping test"
+		return 1
+	fi
+
+	perf list fib:* | grep -q fib_table_lookup
+	if [ $? -ne 0 ]; then
+		echo "IPv4 FIB tracepoint not found. Skipping test"
+		return 1
+	fi
+
+	perf list fib6:* | grep -q fib6_table_lookup
+	if [ $? -ne 0 ]; then
+		echo "IPv6 FIB tracepoint not found. Skipping test"
+		return 1
+	fi
+
+	return 0
+}
+
+link_stats_get()
+{
+	local ns=$1; shift
+	local dev=$1; shift
+	local dir=$1; shift
+	local stat=$1; shift
+
+	ip -n $ns -j -s link show dev $dev \
+		| jq '.[]["stats64"]["'$dir'"]["'$stat'"]'
+}
+
+list_rcv_eval()
+{
+	local file=$1; shift
+	local expected=$1; shift
+
+	local count=$(tail -n 1 $file | jq '.["counter-value"] | tonumber | floor')
+	local ratio=$(echo "scale=2; $count / $expected" | bc -l)
+	local res=$(echo "$ratio >= 0.95" | bc)
+	[[ $res -eq 1 ]]
+	log_test $? 0 "Multipath route hit ratio ($ratio)"
+}
+
+ipv4_mpath_list_test()
+{
+	echo
+	echo "IPv4 multipath list receive tests"
+
+	mpath_dep_check || return 1
+
+	route_setup
+
+	set -e
+	run_cmd "ip netns exec ns1 ethtool -K veth1 tcp-segmentation-offload off"
+
+	run_cmd "ip netns exec ns2 bash -c \"echo 20000 > /sys/class/net/veth2/gro_flush_timeout\""
+	run_cmd "ip netns exec ns2 bash -c \"echo 1 > /sys/class/net/veth2/napi_defer_hard_irqs\""
+	run_cmd "ip netns exec ns2 ethtool -K veth2 generic-receive-offload on"
+	run_cmd "ip -n ns2 link add name nh1 up type dummy"
+	run_cmd "ip -n ns2 link add name nh2 up type dummy"
+	run_cmd "ip -n ns2 address add 172.16.201.1/24 dev nh1"
+	run_cmd "ip -n ns2 address add 172.16.202.1/24 dev nh2"
+	run_cmd "ip -n ns2 neigh add 172.16.201.2 lladdr 00:11:22:33:44:55 nud perm dev nh1"
+	run_cmd "ip -n ns2 neigh add 172.16.202.2 lladdr 00:aa:bb:cc:dd:ee nud perm dev nh2"
+	run_cmd "ip -n ns2 route add 203.0.113.0/24
+		nexthop via 172.16.201.2 nexthop via 172.16.202.2"
+	run_cmd "ip netns exec ns2 sysctl -qw net.ipv4.fib_multipath_hash_policy=1"
+	set +e
+
+	local dmac=$(ip -n ns2 -j link show dev veth2 | jq -r '.[]["address"]')
+	local tmp_file=$(mktemp)
+	local cmd="ip netns exec ns1 mausezahn veth1 -a own -b $dmac
+		-A 172.16.101.1 -B 203.0.113.1 -t udp 'sp=12345,dp=0-65535' -q"
+
+	# Packets forwarded in a list using a multipath route must not reuse a
+	# cached result so that a flow always hits the same nexthop. In other
+	# words, the FIB lookup tracepoint needs to be triggered for every
+	# packet.
+	local t0_rx_pkts=$(link_stats_get ns2 veth2 rx packets)
+	run_cmd "perf stat -e fib:fib_table_lookup --filter 'err == 0' -j -o $tmp_file -- $cmd"
+	local t1_rx_pkts=$(link_stats_get ns2 veth2 rx packets)
+	local diff=$(echo $t1_rx_pkts - $t0_rx_pkts | bc -l)
+	list_rcv_eval $tmp_file $diff
+
+	rm $tmp_file
+	route_cleanup
+}
+
+ipv6_mpath_list_test()
+{
+	echo
+	echo "IPv6 multipath list receive tests"
+
+	mpath_dep_check || return 1
+
+	route_setup
+
+	set -e
+	run_cmd "ip netns exec ns1 ethtool -K veth1 tcp-segmentation-offload off"
+
+	run_cmd "ip netns exec ns2 bash -c \"echo 20000 > /sys/class/net/veth2/gro_flush_timeout\""
+	run_cmd "ip netns exec ns2 bash -c \"echo 1 > /sys/class/net/veth2/napi_defer_hard_irqs\""
+	run_cmd "ip netns exec ns2 ethtool -K veth2 generic-receive-offload on"
+	run_cmd "ip -n ns2 link add name nh1 up type dummy"
+	run_cmd "ip -n ns2 link add name nh2 up type dummy"
+	run_cmd "ip -n ns2 -6 address add 2001:db8:201::1/64 dev nh1"
+	run_cmd "ip -n ns2 -6 address add 2001:db8:202::1/64 dev nh2"
+	run_cmd "ip -n ns2 -6 neigh add 2001:db8:201::2 lladdr 00:11:22:33:44:55 nud perm dev nh1"
+	run_cmd "ip -n ns2 -6 neigh add 2001:db8:202::2 lladdr 00:aa:bb:cc:dd:ee nud perm dev nh2"
+	run_cmd "ip -n ns2 -6 route add 2001:db8:301::/64
+		nexthop via 2001:db8:201::2 nexthop via 2001:db8:202::2"
+	run_cmd "ip netns exec ns2 sysctl -qw net.ipv6.fib_multipath_hash_policy=1"
+	set +e
+
+	local dmac=$(ip -n ns2 -j link show dev veth2 | jq -r '.[]["address"]')
+	local tmp_file=$(mktemp)
+	local cmd="ip netns exec ns1 mausezahn -6 veth1 -a own -b $dmac
+		-A 2001:db8:101::1 -B 2001:db8:301::1 -t udp 'sp=12345,dp=0-65535' -q"
+
+	# Packets forwarded in a list using a multipath route must not reuse a
+	# cached result so that a flow always hits the same nexthop. In other
+	# words, the FIB lookup tracepoint needs to be triggered for every
+	# packet.
+	local t0_rx_pkts=$(link_stats_get ns2 veth2 rx packets)
+	run_cmd "perf stat -e fib6:fib6_table_lookup --filter 'err == 0' -j -o $tmp_file -- $cmd"
+	local t1_rx_pkts=$(link_stats_get ns2 veth2 rx packets)
+	local diff=$(echo $t1_rx_pkts - $t0_rx_pkts | bc -l)
+	list_rcv_eval $tmp_file $diff
+
+	rm $tmp_file
+	route_cleanup
+}
+
 ################################################################################
 # usage
 
@@ -2433,6 +2584,8 @@ do
 	ipv6_mangle)			ipv6_mangle_test;;
 	ipv4_bcast_neigh)		ipv4_bcast_neigh_test;;
 	fib6_gc_test|ipv6_gc)		fib6_gc_test;;
+	ipv4_mpath_list)		ipv4_mpath_list_test;;
+	ipv6_mpath_list)		ipv6_mpath_list_test;;
 
 	help) echo "Test names: $TESTS"; exit 0;;
 	esac
-- 
2.34.1

Re: [PATCH net v5 0/3] Avoid TCP resets when using ECMP for load-balancing between multiple servers.

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by David S. Miller <davem@davemloft.net>:

On Thu, 31 Aug 2023 10:03:29 +0200 you wrote:
> All packets in the same flow (L3/L4 depending on multipath hash policy)
> should be directed to the same target, but after [0]/[1] we see stray
> packets directed towards other targets. This, for instance, causes RST
> to be sent on TCP connections.
> 
> The first two patches solve the problem by ignoring route hints for
> destinations that are part of multipath group, by using new SKB flags
> for IPv4 and IPv6. The third patch is a selftest that tests the
> scenario.
> 
> [...]

Here is the summary with links:
  - [net,v5,1/3] ipv4: ignore dst hint for multipath routes
    https://git.kernel.org/netdev/net/c/6ac66cb03ae3
  - [net,v5,2/3] ipv6: ignore dst hint for multipath routes
    https://git.kernel.org/netdev/net/c/8423be8926aa
  - [net,v5,3/3] selftests: fib_tests: Add multipath list receive tests
    https://git.kernel.org/netdev/net/c/8ae9efb859c0

You are awesome, thank you!
-- 
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