add the GetStats D-Bus method

README.md

@@ -205,6 +205,36 @@ instance's thread count. Setting `sticky=true` suppresses automatic scaling for
 that instance until a later call clears it. This debug-only override is kept in
 memory and is lost when the daemon restarts.
 
+`GetStats()` returns a JSON fleet snapshot. For example:
+
+```json
+{
+  "tick": 42,
+  "vms": [{
+    "vm": "vm-1",
+    "thread_count": 3,
+    "manual_scaling_sticky": false,
+    "scaling_allowed": true,
+    "vcpu_count": 8,
+    "per_thread_util": 0.72,
+    "read_io_count": 1000,
+    "write_io_count": 250,
+    "other_io_count": 0
+  }]
+}
+```
+
+```sh
+busctl --system call \
+  com.nutanix.io_thread_controller1 \
+  /com/nutanix/io_thread_controller1 \
+  com.nutanix.io_thread_controller1 \
+  GetStats
+```
+
+`GetStats` is available in release builds and world-readable under the shipped
+D-Bus policy.
+
 ```sh
 busctl --system call \
   com.nutanix.io_thread_controller1 \

src/controller.rs

@@ -282,6 +282,36 @@ impl Controller {
                 };
                 let _ = reply.send(result);
             }
+            DbusRequest::GetStats { reply } => {
+                let mut vms = Vec::with_capacity(self.instances.len());
+                for (id, instance) in &self.instances {
+                    let status = instance.status.read().await;
+                    let (read_io_count, write_io_count, other_io_count) = match status.perf {
+                        Some(perf) => {
+                            (perf.read_io_count, perf.write_io_count, perf.other_io_count)
+                        }
+                        None => (0, 0, 0),
+                    };
+                    vms.push(serde_json::json!({
+                        "vm": id,
+                        "thread_count": status.thread_count,
+                        "manual_scaling_sticky": status.manual_scaling_sticky,
+                        "scaling_allowed": status.scaling_allowed,
+                        "vcpu_count": status.vcpu_count,
+                        "per_thread_util": status.per_thread_util,
+                        // FIXME omit if status.perf.is_none()?
+                        "read_io_count": read_io_count,
+                        "write_io_count": write_io_count,
+                        "other_io_count": other_io_count,
+                    }));
+                }
+                let snapshot = serde_json::json!({
+                    "tick": self.tick_index,
+                    "vms": vms,
+                })
+                .to_string();
+                let _ = reply.send(snapshot);
+            }
             DbusRequest::AddIoThread {
                 vm,
                 id,

src/dbus.rs

@@ -11,6 +11,7 @@
 //!
 //! Verbs currently exposed:
 //!   * debug-only `SetThreadCount(vm, threads, sticky)`;
+//!   * read-only `GetStats()`;
 //!   * named-IOThread and virtqueue-mapping operations for clients that support
 //!     them.
 
@@ -50,6 +51,11 @@ pub enum DbusRequest {
     /// Structured snapshot of every tracked instance. The reply is a JSON
     /// string so the wire signature stays a bare `s` and the payload shape can
     /// evolve without D-Bus IDL churn.  Field contract is documented on
+    /// JSON snapshot of all tracked VMs.
+    GetStats {
+        /// Reply channel carrying the serialized snapshot.
+        reply: oneshot::Sender<String>,
+    },
     /// [`crate::controller::SnapshotPayload`].
     GetSnapshot {
         /// Reply channel; JSON-encoded
@@ -152,6 +158,17 @@ impl Service {
             Err(e) => Err(zbus::fdo::Error::Failed(e)),
         }
     }
+
+    /// Return the controller's machine-readable fleet snapshot.
+    async fn get_stats(&self) -> zbus::fdo::Result<String> {
+        let (tx, rx) = oneshot::channel();
+        self.tx
+            .send(DbusRequest::GetStats { reply: tx })
+            .await
+            .map_err(|_| zbus::fdo::Error::Failed("controller channel closed".into()))?;
+        await_with_timeout(rx).await
+    }
+
     /// D-Bus wire signature: `GetSnapshot() -> s`.
     ///
     /// The `s` return is a JSON payload matching