histograms: Add timer to reset ASAP after bucket limiting has happened (#1367)

Fixes #1248. See issue description for all the details.

Signed-off-by: beorn7 <beorn@grafana.com>

prometheus/histogram.go

@@ -475,6 +475,9 @@ type HistogramOpts struct {
 
 	// now is for testing purposes, by default it's time.Now.
 	now func() time.Time
+
+	// afterFunc is for testing purposes, by default it's time.AfterFunc.
+	afterFunc func(time.Duration, func()) *time.Timer
 }
 
 // HistogramVecOpts bundles the options to create a HistogramVec metric.
@@ -526,7 +529,9 @@ func newHistogram(desc *Desc, opts HistogramOpts, labelValues ...string) Histogr
 	if opts.now == nil {
 		opts.now = time.Now
 	}
-
+	if opts.afterFunc == nil {
+		opts.afterFunc = time.AfterFunc
+	}
 	h := &histogram{
 		desc:                            desc,
 		upperBounds:                     opts.Buckets,
@@ -536,6 +541,7 @@ func newHistogram(desc *Desc, opts HistogramOpts, labelValues ...string) Histogr
 		nativeHistogramMinResetDuration: opts.NativeHistogramMinResetDuration,
 		lastResetTime:                   opts.now(),
 		now:                             opts.now,
+		afterFunc:                       opts.afterFunc,
 	}
 	if len(h.upperBounds) == 0 && opts.NativeHistogramBucketFactor <= 1 {
 		h.upperBounds = DefBuckets
@@ -716,9 +722,16 @@ type histogram struct {
 	nativeHistogramMinResetDuration time.Duration
 	// lastResetTime is protected by mtx. It is also used as created timestamp.
 	lastResetTime time.Time
+	// resetScheduled is protected by mtx. It is true if a reset is
+	// scheduled for a later time (when nativeHistogramMinResetDuration has
+	// passed).
+	resetScheduled bool
 
 	// now is for testing purposes, by default it's time.Now.
 	now func() time.Time
+
+	// afterFunc is for testing purposes, by default it's time.AfterFunc.
+	afterFunc func(time.Duration, func()) *time.Timer
 }
 
 func (h *histogram) Desc() *Desc {
@@ -874,21 +887,31 @@ func (h *histogram) limitBuckets(counts *histogramCounts, value float64, bucket
 	if h.maybeReset(hotCounts, coldCounts, coldIdx, value, bucket) {
 		return
 	}
+	// One of the other strategies will happen. To undo what they will do as
+	// soon as enough time has passed to satisfy
+	// h.nativeHistogramMinResetDuration, schedule a reset at the right time
+	// if we haven't done so already.
+	if h.nativeHistogramMinResetDuration > 0 && !h.resetScheduled {
+		h.resetScheduled = true
+		h.afterFunc(h.nativeHistogramMinResetDuration-h.now().Sub(h.lastResetTime), h.reset)
+	}
+
 	if h.maybeWidenZeroBucket(hotCounts, coldCounts) {
 		return
 	}
 	h.doubleBucketWidth(hotCounts, coldCounts)
 }
 
-// maybeReset resets the whole histogram if at least h.nativeHistogramMinResetDuration
+// maybeReset resets the whole histogram if at least
-// has been passed. It returns true if the histogram has been reset. The caller
+// h.nativeHistogramMinResetDuration has been passed. It returns true if the
-// must have locked h.mtx.
+// histogram has been reset. The caller must have locked h.mtx.
 func (h *histogram) maybeReset(
 	hot, cold *histogramCounts, coldIdx uint64, value float64, bucket int,
 ) bool {
 	// We are using the possibly mocked h.now() rather than
 	// time.Since(h.lastResetTime) to enable testing.
-	if h.nativeHistogramMinResetDuration == 0 ||
+	if h.nativeHistogramMinResetDuration == 0 || // No reset configured.
+		h.resetScheduled || // Do not interefere if a reset is already scheduled.
 		h.now().Sub(h.lastResetTime) < h.nativeHistogramMinResetDuration {
 		return false
 	}
@@ -906,6 +929,29 @@ func (h *histogram) maybeReset(
 	return true
 }
 
+// reset resets the whole histogram. It locks h.mtx itself, i.e. it has to be
+// called without having locked h.mtx.
+func (h *histogram) reset() {
+	h.mtx.Lock()
+	defer h.mtx.Unlock()
+
+	n := atomic.LoadUint64(&h.countAndHotIdx)
+	hotIdx := n >> 63
+	coldIdx := (^n) >> 63
+	hot := h.counts[hotIdx]
+	cold := h.counts[coldIdx]
+	// Completely reset coldCounts.
+	h.resetCounts(cold)
+	// Make coldCounts the new hot counts while resetting countAndHotIdx.
+	n = atomic.SwapUint64(&h.countAndHotIdx, coldIdx<<63)
+	count := n & ((1 << 63) - 1)
+	waitForCooldown(count, hot)
+	// Finally, reset the formerly hot counts, too.
+	h.resetCounts(hot)
+	h.lastResetTime = h.now()
+	h.resetScheduled = false
+}
+
 // maybeWidenZeroBucket widens the zero bucket until it includes the existing
 // buckets closest to the zero bucket (which could be two, if an equidistant
 // negative and a positive bucket exists, but usually it's only one bucket to be

prometheus/histogram_test.go

@@ -925,16 +925,16 @@ func TestNativeHistogram(t *testing.T) {
 			maxBuckets:       4,
 			minResetDuration: 9 * time.Minute,
 			want: &dto.Histogram{
-				SampleCount:   proto.Uint64(2),
+				SampleCount:   proto.Uint64(3),
-				SampleSum:     proto.Float64(7),
+				SampleSum:     proto.Float64(12.1),
 				Schema:        proto.Int32(2),
 				ZeroThreshold: proto.Float64(2.938735877055719e-39),
 				ZeroCount:     proto.Uint64(0),
 				PositiveSpan: []*dto.BucketSpan{
-					{Offset: proto.Int32(7), Length: proto.Uint32(2)},
+					{Offset: proto.Int32(7), Length: proto.Uint32(4)},
 				},
-				PositiveDelta:    []int64{1, 0},
+				PositiveDelta:    []int64{1, 0, -1, 1},
-				CreatedTimestamp: timestamppb.New(now.Add(10 * time.Minute)), // We expect reset to happen after 9 minutes.
+				CreatedTimestamp: timestamppb.New(now.Add(9 * time.Minute)), // We expect reset to happen after 8 minutes.
 			},
 		},
 		{
@@ -945,23 +945,27 @@ func TestNativeHistogram(t *testing.T) {
 			maxZeroThreshold: 1.2,
 			minResetDuration: 9 * time.Minute,
 			want: &dto.Histogram{
-				SampleCount:   proto.Uint64(2),
+				SampleCount:   proto.Uint64(3),
-				SampleSum:     proto.Float64(7),
+				SampleSum:     proto.Float64(12.1),
 				Schema:        proto.Int32(2),
 				ZeroThreshold: proto.Float64(2.938735877055719e-39),
 				ZeroCount:     proto.Uint64(0),
 				PositiveSpan: []*dto.BucketSpan{
-					{Offset: proto.Int32(7), Length: proto.Uint32(2)},
+					{Offset: proto.Int32(7), Length: proto.Uint32(4)},
 				},
-				PositiveDelta:    []int64{1, 0},
+				PositiveDelta:    []int64{1, 0, -1, 1},
-				CreatedTimestamp: timestamppb.New(now.Add(10 * time.Minute)), // We expect reset to happen after 9 minutes.
+				CreatedTimestamp: timestamppb.New(now.Add(9 * time.Minute)), // We expect reset to happen after 8 minutes.
 			},
 		},
 	}
 
 	for _, s := range scenarios {
 		t.Run(s.name, func(t *testing.T) {
-			ts := now
+			var (
+				ts         = now
+				funcToCall func()
+				whenToCall time.Duration
+			)
 
 			his := NewHistogram(HistogramOpts{
 				Name:                            "name",
@@ -972,12 +976,22 @@ func TestNativeHistogram(t *testing.T) {
 				NativeHistogramMinResetDuration: s.minResetDuration,
 				NativeHistogramMaxZeroThreshold: s.maxZeroThreshold,
 				now:                             func() time.Time { return ts },
+				afterFunc: func(d time.Duration, f func()) *time.Timer {
+					funcToCall = f
+					whenToCall = d
+					return nil
+				},
 			})
 
 			ts = ts.Add(time.Minute)
 			for _, o := range s.observations {
 				his.Observe(o)
 				ts = ts.Add(time.Minute)
+				whenToCall -= time.Minute
+				if funcToCall != nil && whenToCall <= 0 {
+					funcToCall()
+					funcToCall = nil
+				}
 			}
 			m := &dto.Metric{}
 			if err := his.Write(m); err != nil {