Compute userspace timestamp after recvmsg in recvpacket

packet-exchange/src/recv_packet.c

@@ -39,9 +39,7 @@ static ingress_stat_t *stats;
 static uint64_t *kernel_latency_hist;
 static int use_histogram;
 
-static uint64_t timestamps_buffer[TIMESTAMP_BUFFER_SIZE];
-static int ts_buf_read_index = 0;
-static int ts_buf_write_index = 0;
+uint64_t post_kernel_timestamp;
 
 // Sets the interface
 static int set_if() {
@@ -130,12 +128,6 @@ void recv_udp_packet() {
 
   struct timespec ts;
 
-  if (params->use_timestamps) {
-    clock_gettime(CLOCK_REALTIME, &ts);
-    timestamps_buffer[ts_buf_write_index] = ts_to_uint(ts);
-    ts_buf_write_index = (ts_buf_write_index + 1) % TIMESTAMP_BUFFER_SIZE;
-  }
-
   iov.iov_base = &rx_buffer;
   iov.iov_len = MAX_BUFFER_SIZE - 1;
 
@@ -151,13 +143,19 @@ void recv_udp_packet() {
   if (recvmsgerr < 0)
     error(EXIT_FAILURE, errno, "recvmsg failed, ret value: %d\n", recvmsgerr);
 
+  if (params->use_timestamps) {
+    clock_gettime(CLOCK_REALTIME, &ts);
+    post_kernel_timestamp = ts_to_uint(ts);
+  }
+
+
   if (params->use_timestamps) {
     for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
       if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SO_TIMESTAMPING) {
 
         struct timespec *stamp = (struct timespec *)CMSG_DATA(cmsg);
 
-        uint64_t kernel_latency = timestamps_buffer[ts_buf_read_index] - ts_to_uint(*stamp);
+        uint64_t kernel_latency = post_kernel_timestamp - ts_to_uint(*stamp);
         ts_buf_read_index = (ts_buf_read_index + 1) % TIMESTAMP_BUFFER_SIZE;
         kernel_latency /= 1000u;