[PATCH net-next] ipvs: allow connection reuse for unconfirmed conntrack

[Julian Anastasov <ja@xxxxxx>]

YangYuxi is reporting that connection reuse
is causing one-second delay when SYN hits
existing connection in TIME_WAIT state.
Such delay was added to give time to expire
both the IPVS connection and the corresponding
conntrack. This was considered a rare case
at that time but it is causing problem for
some environments such as Kubernetes.

As nf_conntrack_tcp_packet() can decide to
release the conntrack in TIME_WAIT state and
to replace it with a fresh NEW conntrack, we
can use this to allow rescheduling just by
tuning our check: if the conntrack is
confirmed we can not schedule it to different
real server and the one-second delay still
applies but if new conntrack was created,
we are free to select new real server without
any delays.

YangYuxi lists some of the problem reports:

- One second connection delay in masquerading mode:
https://marc.info/?t=151683118100004&r=1&w=2

- IPVS low throughput #70747
https://github.com/kubernetes/kubernetes/issues/70747

- Apache Bench can fill up ipvs service proxy in seconds #544
https://github.com/cloudnativelabs/kube-router/issues/544

- Additional 1s latency in `host -> service IP -> pod`
https://github.com/kubernetes/kubernetes/issues/90854

Fixes: f719e3754ee2 ("ipvs: drop first packet to redirect conntrack")
Co-developed-by: YangYuxi <yx.atom1@xxxxxxxxx>
Signed-off-by: YangYuxi <yx.atom1@xxxxxxxxx>
Signed-off-by: Julian Anastasov <ja@xxxxxx>
---
 include/net/ip_vs.h             | 10 ++++------
 net/netfilter/ipvs/ip_vs_core.c | 12 +++++++-----
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/include/net/ip_vs.h b/include/net/ip_vs.h
index 83be2d93b407..fe96aa462d05 100644
--- a/include/net/ip_vs.h
+++ b/include/net/ip_vs.h
@@ -1624,18 +1624,16 @@ static inline void ip_vs_conn_drop_conntrack(struct 
ip_vs_conn *cp)
 }
 #endif /* CONFIG_IP_VS_NFCT */
 
-/* Really using conntrack? */
-static inline bool ip_vs_conn_uses_conntrack(struct ip_vs_conn *cp,
-                                            struct sk_buff *skb)
+/* Using old conntrack that can not be redirected to another real server? */
+static inline bool ip_vs_conn_uses_old_conntrack(struct ip_vs_conn *cp,
+                                                struct sk_buff *skb)
 {
 #ifdef CONFIG_IP_VS_NFCT
        enum ip_conntrack_info ctinfo;
        struct nf_conn *ct;
 
-       if (!(cp->flags & IP_VS_CONN_F_NFCT))
-               return false;
        ct = nf_ct_get(skb, &ctinfo);
-       if (ct)
+       if (ct && nf_ct_is_confirmed(ct))
                return true;
 #endif
        return false;
diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
index aa6a603a2425..517f6a2ac15a 100644
--- a/net/netfilter/ipvs/ip_vs_core.c
+++ b/net/netfilter/ipvs/ip_vs_core.c
@@ -2066,14 +2066,14 @@ ip_vs_in(struct netns_ipvs *ipvs, unsigned int hooknum, 
struct sk_buff *skb, int
 
        conn_reuse_mode = sysctl_conn_reuse_mode(ipvs);
        if (conn_reuse_mode && !iph.fragoffs && is_new_conn(skb, &iph) && cp) {
-               bool uses_ct = false, resched = false;
+               bool old_ct = false, resched = false;
 
                if (unlikely(sysctl_expire_nodest_conn(ipvs)) && cp->dest &&
                    unlikely(!atomic_read(&cp->dest->weight))) {
                        resched = true;
-                       uses_ct = ip_vs_conn_uses_conntrack(cp, skb);
+                       old_ct = ip_vs_conn_uses_old_conntrack(cp, skb);
                } else if (is_new_conn_expected(cp, conn_reuse_mode)) {
-                       uses_ct = ip_vs_conn_uses_conntrack(cp, skb);
+                       old_ct = ip_vs_conn_uses_old_conntrack(cp, skb);
                        if (!atomic_read(&cp->n_control)) {
                                resched = true;
                        } else {
@@ -2081,15 +2081,17 @@ ip_vs_in(struct netns_ipvs *ipvs, unsigned int hooknum, 
struct sk_buff *skb, int
                                 * that uses conntrack while it is still
                                 * referenced by controlled connection(s).
                                 */
-                               resched = !uses_ct;
+                               resched = !old_ct;
                        }
                }
 
                if (resched) {
+                       if (!old_ct)
+                               cp->flags &= ~IP_VS_CONN_F_NFCT;
                        if (!atomic_read(&cp->n_control))
                                ip_vs_conn_expire_now(cp);
                        __ip_vs_conn_put(cp);
-                       if (uses_ct)
+                       if (old_ct)
                                return NF_DROP;
                        cp = NULL;
                }
-- 
2.26.2