Make RTT values show up on both flow directions

bpf/flows.c

@@ -60,15 +60,17 @@ static inline int flow_monitor(struct __sk_buff *skb, u8 direction) {
         return TC_ACT_OK;
     }
 
-    if (enable_rtt) {
-        // This is currently gated as its not to be enabled by default.
-        calculate_flow_rtt(&pkt, direction, data_end);
-    }
-
     //Set extra fields
     id.if_index = skb->ifindex;
     id.direction = direction;
 
+    // We calculate the RTT before looking up aggregated_flows map because we want
+    // to keep the critical section between map lookup and update consume minimum time.
+    if (enable_rtt) {
+        // This is currently not to be enabled by default.
+        calculate_flow_rtt(&pkt, direction, data_end);
+    }
+
     // TODO: we need to add spinlock here when we deprecate versions prior to 5.1, or provide
     // a spinlocked alternative version and use it selectively https://lwn.net/Articles/779120/
     flow_metrics *aggregate_flow = (flow_metrics *)bpf_map_lookup_elem(&aggregated_flows, &id);

bpf/rtt_tracker.h

@@ -26,6 +26,41 @@ static __always_inline void fill_flow_seq_id(flow_seq_id *seq_id, pkt_info *pkt,
     seq_id->seq_id = seq;
 }
 
+static __always_inline void reverse_flow_id(flow_id *o, flow_id *r) {
+    /* eth_protocol, transport_protocol and if_index remains same */
+    r->eth_protocol = o->eth_protocol;
+    r->transport_protocol = o->transport_protocol;
+    r->if_index = o->if_index;
+    /* reverse the direction */
+    r->direction = (o->direction == INGRESS) ? EGRESS : INGRESS;
+    /* src mac and dst mac gets reversed */
+    __builtin_memcpy(r->src_mac, o->dst_mac, ETH_ALEN);
+    __builtin_memcpy(r->dst_mac, o->src_mac, ETH_ALEN);
+    /* src ip and dst ip gets reversed */
+    __builtin_memcpy(r->src_ip, o->dst_ip, IP_MAX_LEN);
+    __builtin_memcpy(r->dst_ip, o->src_ip, IP_MAX_LEN);
+    /* src port and dst port gets reversed */
+    r->src_port = o->dst_port;
+    r->dst_port = o->src_port;
+    /* ICMP type can be ignore for now. We only deal with TCP packets for now.*/
+}
+
+static __always_inline void update_reverse_flow_rtt(pkt_info *pkt) {
+    flow_id rev_flow_id;
+    __builtin_memset(&rev_flow_id, 0, sizeof(rev_flow_id));
+
+    reverse_flow_id(pkt->id, &rev_flow_id);
+
+    flow_metrics *reverse_flow = (flow_metrics *)bpf_map_lookup_elem(&aggregated_flows, &rev_flow_id);
+    if (reverse_flow != NULL) {
+        reverse_flow->flow_rtt = pkt->rtt;
+        long ret = bpf_map_update_elem(&aggregated_flows, &rev_flow_id, reverse_flow, BPF_ANY);
+        if (trace_messages && ret != 0) {
+            bpf_printk("error updating rtt value in flow %d\n", ret);
+        }
+    }
+}
+
 static __always_inline void calculate_flow_rtt_tcp(pkt_info *pkt, u8 direction, void *data_end, flow_seq_id *seq_id) {
     struct tcphdr *tcp = (struct tcphdr *) pkt->l4_hdr;
     if ( !tcp || ((void *)tcp + sizeof(*tcp) > data_end) ) {
@@ -62,6 +97,9 @@ static __always_inline void calculate_flow_rtt_tcp(pkt_info *pkt, u8 direction,
                 if (trace_messages && ret != 0) {
                     bpf_printk("error evicting flow sequence: %d", ret);
                 }
+                // This is an ACK packet with valid sequence id so a SYN must
+                // have been sent. We can safely update the reverse flow RTT here.
+                update_reverse_flow_rtt(pkt);
             }
         }
         break;