Merge pull request #1757 from prometheus/revert-121cas

Revert "exponential backoff for CAS operations on floats" and cut 1.21.1

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## Unreleased
 
+## 1.21.1 / 2025-03-04
+
+* [BUGFIX] prometheus: Revert of `Inc`, `Add` and `Observe` cumulative metric CAS optimizations (#1661), causing regressions on low contention cases.
+
 ## 1.21.0 / 2025-02-17
 
 :warning: This release contains potential breaking change if you upgrade `github.com/prometheus/common` to 0.62+ together with client_golang. :warning:

VERSION

@@ -1 +1 @@
-1.21.0
+1.21.1

prometheus/counter.go

@@ -134,9 +134,13 @@ func (c *counter) Add(v float64) {
 		return
 	}
 
-	atomicUpdateFloat(&c.valBits, func(oldVal float64) float64 {
-		return oldVal + v
-	})
+	for {
+		oldBits := atomic.LoadUint64(&c.valBits)
+		newBits := math.Float64bits(math.Float64frombits(oldBits) + v)
+		if atomic.CompareAndSwapUint64(&c.valBits, oldBits, newBits) {
+			return
+		}
+	}
 }
 
 func (c *counter) AddWithExemplar(v float64, e Labels) {

prometheus/gauge.go

@@ -120,9 +120,13 @@ func (g *gauge) Dec() {
 }
 
 func (g *gauge) Add(val float64) {
-	atomicUpdateFloat(&g.valBits, func(oldVal float64) float64 {
-		return oldVal + val
-	})
+	for {
+		oldBits := atomic.LoadUint64(&g.valBits)
+		newBits := math.Float64bits(math.Float64frombits(oldBits) + val)
+		if atomic.CompareAndSwapUint64(&g.valBits, oldBits, newBits) {
+			return
+		}
+	}
 }
 
 func (g *gauge) Sub(val float64) {

prometheus/histogram.go

@@ -1647,9 +1647,13 @@ func waitForCooldown(count uint64, counts *histogramCounts) {
 // atomicAddFloat adds the provided float atomically to another float
 // represented by the bit pattern the bits pointer is pointing to.
 func atomicAddFloat(bits *uint64, v float64) {
-	atomicUpdateFloat(bits, func(oldVal float64) float64 {
-		return oldVal + v
-	})
+	for {
+		loadedBits := atomic.LoadUint64(bits)
+		newBits := math.Float64bits(math.Float64frombits(loadedBits) + v)
+		if atomic.CompareAndSwapUint64(bits, loadedBits, newBits) {
+			break
+		}
+	}
 }
 
 // atomicDecUint32 atomically decrements the uint32 p points to.  See

prometheus/summary.go

@@ -471,9 +471,13 @@ func (s *noObjectivesSummary) Observe(v float64) {
 	n := atomic.AddUint64(&s.countAndHotIdx, 1)
 	hotCounts := s.counts[n>>63]
 
-	atomicUpdateFloat(&hotCounts.sumBits, func(oldVal float64) float64 {
-		return oldVal + v
-	})
+	for {
+		oldBits := atomic.LoadUint64(&hotCounts.sumBits)
+		newBits := math.Float64bits(math.Float64frombits(oldBits) + v)
+		if atomic.CompareAndSwapUint64(&hotCounts.sumBits, oldBits, newBits) {
+			break
+		}
+	}
 	// Increment count last as we take it as a signal that the observation
 	// is complete.
 	atomic.AddUint64(&hotCounts.count, 1)
@@ -515,13 +519,14 @@ func (s *noObjectivesSummary) Write(out *dto.Metric) error {
 	// Finally add all the cold counts to the new hot counts and reset the cold counts.
 	atomic.AddUint64(&hotCounts.count, count)
 	atomic.StoreUint64(&coldCounts.count, 0)
-
-	// Use atomicUpdateFloat to update hotCounts.sumBits atomically.
-	atomicUpdateFloat(&hotCounts.sumBits, func(oldVal float64) float64 {
-		return oldVal + sum.GetSampleSum()
-	})
-	atomic.StoreUint64(&coldCounts.sumBits, 0)
-
+	for {
+		oldBits := atomic.LoadUint64(&hotCounts.sumBits)
+		newBits := math.Float64bits(math.Float64frombits(oldBits) + sum.GetSampleSum())
+		if atomic.CompareAndSwapUint64(&hotCounts.sumBits, oldBits, newBits) {
+			atomic.StoreUint64(&coldCounts.sumBits, 0)
+			break
+		}
+	}
 	return nil
 }