scx_bpfland: When reporting stats, use interval deltas

Three of the reported stats are cumulative. While they obviously can be
processed into delta values, that holds for the other direction too and the
cumulative values are difficult to make intutive sense of. Report interval
delta values instead.

Note that a stats client can reliably build back cumulative values even
under heavy system contention - the delta values reported between two
consecutive reads are guaranteed to be correct regardless of the duration of
the interval.

scheds/rust/scx_bpfland/src/stats.rs

@@ -47,15 +47,30 @@ impl Metrics {
         )?;
         Ok(())
     }
+
+    fn delta(&self, rhs: &Self) -> Self {
+        Self {
+            nr_direct_dispatches: self.nr_direct_dispatches - rhs.nr_direct_dispatches,
+            nr_prio_dispatches: self.nr_prio_dispatches - rhs.nr_prio_dispatches,
+            nr_shared_dispatches: self.nr_shared_dispatches - rhs.nr_shared_dispatches,
+            ..self.clone()
+        }
+    }
 }
 
 pub fn server_data() -> StatsServerData<(), Metrics> {
-    let open: Box<dyn StatsOpener<(), Metrics>> = Box::new(move |(_req_ch, _res_ch)| {
+    let open: Box<dyn StatsOpener<(), Metrics>> = Box::new(move |(req_ch, res_ch)| {
+        req_ch.send(())?;
+        let mut prev = res_ch.recv()?;
+
         let read: Box<dyn StatsReader<(), Metrics>> = Box::new(move |_args, (req_ch, res_ch)| {
             req_ch.send(())?;
-            let metrics = res_ch.recv()?;
-            metrics.to_json()
+            let cur = res_ch.recv()?;
+            let delta = cur.delta(&prev);
+            prev = cur;
+            delta.to_json()
         });
+
         Ok(read)
     });