vm-monitor: Rate-limit upscale requests (#5263)

Some VMs, when already scaled up as much as possible, end up spamming
the autoscaler-agent with upscale requests that will never be fulfilled.
If postgres is using memory greater than the cgroup's memory.high, it
can emit new memory.high events 1000 times per second, which... just
means unnecessary load on the rest of the system.

This changes the vm-monitor so that we skip sending upscale requests if
we already sent one within the last second, to avoid spamming the
autoscaler-agent. This matches previous behavior that the vm-informant
hand.

libs/vm_monitor/src/runner.rs

@@ -5,6 +5,7 @@
 //! all functionality.
 
 use std::sync::Arc;
+use std::time::{Duration, Instant};
 use std::{fmt::Debug, mem};
 
 use anyhow::{bail, Context};
@@ -36,6 +37,8 @@ pub struct Runner {
     /// by us vs the autoscaler-agent.
     counter: usize,
 
+    last_upscale_request_at: Option<Instant>,
+
     /// A signal to kill the main thread produced by `self.run()`. This is triggered
     /// when the server receives a new connection. When the thread receives the
     /// signal off this channel, it will gracefully shutdown.
@@ -99,6 +102,7 @@ impl Runner {
             cgroup: None,
             dispatcher,
             counter: 1, // NB: must be odd, see the comment about the field for more.
+            last_upscale_request_at: None,
             kill,
         };
 
@@ -397,6 +401,20 @@ impl Runner {
                     if request.is_none() {
                         bail!("failed to listen for upscale event from cgroup")
                     }
+
+                    // If it's been less than 1 second since the last time we requested upscaling,
+                    // ignore the event, to avoid spamming the agent (otherwise, this can happen
+                    // ~1k times per second).
+                    if let Some(t) = self.last_upscale_request_at {
+                        let elapsed = t.elapsed();
+                        if elapsed < Duration::from_secs(1) {
+                            info!(elapsed_millis = elapsed.as_millis(), "cgroup asked for upscale but too soon to forward the request, ignoring");
+                            continue;
+                        }
+                    }
+
+                    self.last_upscale_request_at = Some(Instant::now());
+
                     info!("cgroup asking for upscale; forwarding request");
                     self.counter += 2; // Increment, preserving parity (i.e. keep the
                                        // counter odd). See the field comment for more.