Immediately scale up autoscaler for cold-start

When the autoscaler gets a stat to scale a service up from 0 we should
act on this immediately in order to decrease cold-start time.

Closes knative#2961

pkg/autoscaler/multiscaler.go

@@ -76,6 +76,7 @@ type UniScalerFactory func(*Metric, *DynamicConfig) (UniScaler, error)
 type scalerRunner struct {
 	scaler UniScaler
 	stopCh chan struct{}
+	pokeCh chan struct{}
 
 	// mux guards access to metric
 	mux    sync.RWMutex
@@ -147,6 +148,17 @@ func (m *MultiScaler) Get(ctx context.Context, namespace, name string) (*Metric,
 	return (&scaler.metric).DeepCopy(), nil
 }
 
+// SetScale Directly sets the scale for a given metric key. This does not perform any ticking
+// or updating of other sacler components.
+func (m *MultiScaler) SetScale(metricKey string, scale int32) bool {
+	scaler, exists := m.scalers[metricKey]
+	if !exists {
+		return false
+	}
+	scaler.updateLatestScale(scale)
+	return true
+}
+
 // Create instantiates the desired Metric.
 func (m *MultiScaler) Create(ctx context.Context, metric *Metric) (*Metric, error) {
 	m.scalersMutex.Lock()
@@ -229,6 +241,7 @@ func (m *MultiScaler) createScaler(ctx context.Context, metric *Metric) (*scaler
 		scaler: scaler,
 		stopCh: stopCh,
 		metric: *metric,
+		pokeCh: make(chan struct{}),
 	}
 	runner.metric.Status.DesiredScale = -1
 
@@ -247,6 +260,8 @@ func (m *MultiScaler) createScaler(ctx context.Context, metric *Metric) (*scaler
 				return
 			case <-ticker.C:
 				m.tickScaler(ctx, scaler, scaleChan)
+			case <-runner.pokeCh:
+				m.tickScaler(ctx, scaler, scaleChan)
 			}
 		}
 	}()
@@ -300,6 +315,10 @@ func (m *MultiScaler) RecordStat(key string, stat Stat) {
 	if exists {
 		logger := m.logger.With(zap.String(logkey.Key, key))
 		ctx := logging.WithLogger(context.TODO(), logger)
+
 		scaler.scaler.Record(ctx, stat)
+		if scaler.getLatestScale() == 0 && stat.AverageConcurrentRequests != 0 {
+			scaler.pokeCh <- struct{}{}
+		}
 	}
 }

pkg/autoscaler/multiscaler_test.go

@@ -174,6 +174,64 @@ func TestMultiScalerScaleToZero(t *testing.T) {
 	}
 }
 
+func TestMultiScalerScaleFromZero(t *testing.T) {
+	ctx := context.TODO()
+	ms, stopCh, uniScaler := createMultiScaler(t, &autoscaler.Config{
+		TickInterval:      time.Second * 60,
+		EnableScaleToZero: true,
+	})
+	defer close(stopCh)
+
+	metric := newMetric()
+	metricKey := fmt.Sprintf("%s/%s", metric.Namespace, metric.Name)
+
+	uniScaler.setScaleResult(1, true)
+
+	done := make(chan struct{})
+	defer close(done)
+	ms.Watch(func(key string) {
+		// When we return, let the main process know.
+		defer func() {
+			done <- struct{}{}
+		}()
+		if key != metricKey {
+			t.Errorf("Watch() = %v, wanted %v", key, metricKey)
+		}
+		m, err := ms.Get(ctx, metric.Namespace, metric.Name)
+		if err != nil {
+			t.Errorf("Get() = %v", err)
+		}
+		if got, want := m.Status.DesiredScale, int32(1); got != want {
+			t.Errorf("Get() = %v, wanted %v", got, want)
+		}
+	})
+
+	_, err := ms.Create(ctx, metric)
+	if err != nil {
+		t.Errorf("Create() = %v", err)
+	}
+	if ok := ms.SetScale(autoscaler.NewMetricKey(metric.Namespace, metric.Name), 0); !ok {
+		t.Error("Failed to set scale for metric to 0")
+	}
+
+	now := time.Now()
+	testStat := autoscaler.Stat{
+		Time:                      &now,
+		PodName:                   "test-pod",
+		AverageConcurrentRequests: 1,
+		RequestCount:              1,
+	}
+	ms.RecordStat(testKPAKey, testStat)
+
+	// Verify that we see a "tick" on scale from zero
+	select {
+	case <-done:
+		// We got the signal!
+	case <-time.After(30 * time.Millisecond):
+		t.Fatalf("timed out waiting for Watch()")
+	}
+}
+
 func TestMultiScalerWithoutScaleToZero(t *testing.T) {
 	ctx := context.TODO()
 	ms, stopCh, uniScaler := createMultiScaler(t, &autoscaler.Config{