feat(metrics/net): add tx backlog metrics

This metric accumulates the number of pending
requests sitting in the Tx virtio queue while a request
is being processed.
This may give an insight at how sensible it would
be to think of processing ingress traffic parallelly.

(cherry picked from commit 3753f35)

Signed-off-by: Nikita Kalyazin <[email protected]>

Author: kalyazin

src/vmm/src/devices/virtio/net/device.rs

@@ -592,6 +592,9 @@ impl Net {
         let tx_queue = &mut self.queues[TX_INDEX];
 
         while let Some(head) = tx_queue.pop_or_enable_notification(mem) {
+            self.metrics
+                .tx_remaining_reqs_count
+                .add(tx_queue.len(mem).into());
             let head_index = head.index;
             // Parse IoVecBuffer from descriptor head
             let buffer = match IoVecBuffer::from_descriptor_chain(head) {

src/vmm/src/devices/virtio/net/metrics.rs

@@ -206,6 +206,8 @@ pub struct NetDeviceMetrics {
     pub tx_rate_limiter_throttled: SharedIncMetric,
     /// Number of packets with a spoofed mac, sent by the guest.
     pub tx_spoofed_mac_count: SharedIncMetric,
+    /// Number of remaining requests in the TX queue.
+    pub tx_remaining_reqs_count: SharedIncMetric,
 }
 
 impl NetDeviceMetrics {
@@ -269,6 +271,8 @@ impl NetDeviceMetrics {
             .add(other.tx_rate_limiter_throttled.fetch_diff());
         self.tx_spoofed_mac_count
             .add(other.tx_spoofed_mac_count.fetch_diff());
+        self.tx_remaining_reqs_count
+            .add(other.tx_remaining_reqs_count.fetch_diff());
     }
 }
 

tests/host_tools/fcmetrics.py

@@ -138,6 +138,7 @@ def validate_fc_metrics(metrics):
             "tx_rate_limiter_event_count",
             "tx_rate_limiter_throttled",
             "tx_spoofed_mac_count",
+            "tx_remaining_reqs_count",
         ],
         "patch_api_requests": [
             "drive_count",