More assertive borrowing by exempt

Happy middle ground with what the KEP says?

Signed-off-by: Mike Spreitzer <mspreitz@us.ibm.com>

Kubernetes-commit: 56fc11f3bef9f6af16aa30731050168e732754a2

pkg/util/flowcontrol/apf_controller.go

@@ -204,7 +204,7 @@ type priorityLevelState struct {
 	// reached through this pointer is mutable.
 	pl *flowcontrol.PriorityLevelConfiguration
 
-	// qsCompleter holds the QueueSetCompleter derived from `config`
+	// qsCompleter holds the QueueSetCompleter derived from `pl`
 	// and `queues`.
 	qsCompleter fq.QueueSetCompleter
 
@@ -255,12 +255,12 @@ type priorityLevelState struct {
 type seatDemandStats struct {
 	avg           float64
 	stdDev        float64
-	highWatermark float64
+	highWatermark int
 	smoothed      float64
 }
 
 func (stats *seatDemandStats) update(obs fq.IntegratorResults) {
-	stats.highWatermark = obs.Max
+	stats.highWatermark = int(math.Round(obs.Max))
 	if obs.Duration <= 0 {
 		return
 	}
@@ -398,38 +398,63 @@ func (cfgCtlr *configController) updateBorrowing() {
 
 func (cfgCtlr *configController) updateBorrowingLocked(setCompleters bool, plStates map[string]*priorityLevelState) {
 	items := make([]allocProblemItem, 0, len(plStates))
-	plNames := make([]string, 0, len(plStates))
+	nonExemptPLNames := make([]string, 0, len(plStates))
+	idxOfNonExempt := map[string]int{}
+	cclOfExempt := map[string]int{}
+	var minCLSum, minCurrentCLSum int
+	remainingServerCL := cfgCtlr.nominalCLSum
 	for plName, plState := range plStates {
 		obs := plState.seatDemandIntegrator.Reset()
 		plState.seatDemandStats.update(obs)
-		// Lower bound on this priority level's adjusted concurreny limit is the lesser of:
+		var minCurrentCL int
-		// - its seat demamd high watermark over the last adjustment period, and
+		if plState.pl.Spec.Type == flowcontrol.PriorityLevelEnablementExempt {
-		// - its configured concurrency limit.
+			minCurrentCL = max(plState.minCL, plState.seatDemandStats.highWatermark)
-		// BUT: we do not want this to be lower than the lower bound from configuration.
+			cclOfExempt[plName] = minCurrentCL
-		// See KEP-1040 for a more detailed explanation.
+			remainingServerCL -= minCurrentCL
-		minCurrentCL := math.Max(float64(plState.minCL), math.Min(float64(plState.nominalCL), plState.seatDemandStats.highWatermark))
+		} else {
-		plNames = append(plNames, plName)
+			// Lower bound on this priority level's adjusted concurreny limit is the lesser of:
-		items = append(items, allocProblemItem{
+			// - its seat demamd high watermark over the last adjustment period, and
-			lowerBound: minCurrentCL,
+			// - its configured concurrency limit.
-			upperBound: float64(plState.maxCL),
+			// BUT: we do not want this to be lower than the lower bound from configuration.
-			target:     math.Max(minCurrentCL, plState.seatDemandStats.smoothed),
+			// See KEP-1040 for a more detailed explanation.
-		})
+			minCurrentCL = max(plState.minCL, min(plState.nominalCL, plState.seatDemandStats.highWatermark))
+			idxOfNonExempt[plName] = len(items)
+			nonExemptPLNames = append(nonExemptPLNames, plName)
+			items = append(items, allocProblemItem{
+				lowerBound: float64(minCurrentCL),
+				upperBound: float64(plState.maxCL),
+				target:     math.Max(float64(minCurrentCL), plState.seatDemandStats.smoothed),
+			})
+		}
+		minCLSum += plState.minCL
+		minCurrentCLSum += minCurrentCL
 	}
 	if len(items) == 0 && cfgCtlr.nominalCLSum > 0 {
 		klog.ErrorS(nil, "Impossible: no priority levels", "plStates", cfgCtlr.priorityLevelStates)
 		return
 	}
-	allocs, fairFrac, err := computeConcurrencyAllocation(cfgCtlr.nominalCLSum, items)
+	var allocs []float64
-	if err != nil {
+	var shareFrac, fairFrac float64
-		klog.ErrorS(err, "Unable to derive new concurrency limits", "plNames", plNames, "items", items)
+	var err error
-		allocs = make([]float64, len(items))
+	if remainingServerCL <= minCLSum {
-		for idx, plName := range plNames {
+		metrics.SetFairFrac(0)
-			plState := plStates[plName]
+	} else if remainingServerCL <= minCurrentCLSum {
-			allocs[idx] = float64(plState.currentCL)
+		shareFrac = float64(remainingServerCL-minCLSum) / float64(minCurrentCLSum-minCLSum)
+		metrics.SetFairFrac(0)
+	} else {
+		allocs, fairFrac, err = computeConcurrencyAllocation(cfgCtlr.nominalCLSum, items)
+		if err != nil {
+			klog.ErrorS(err, "Unable to derive new concurrency limits", "plNames", nonExemptPLNames, "items", items)
+			allocs = make([]float64, len(items))
+			for idx, plName := range nonExemptPLNames {
+				plState := plStates[plName]
+				allocs[idx] = float64(plState.currentCL)
+			}
 		}
+		metrics.SetFairFrac(float64(fairFrac))
 	}
-	for idx, plName := range plNames {
+	for plName, plState := range plStates {
-		plState := plStates[plName]
+		idx, isNonExempt := idxOfNonExempt[plName]
 		if setCompleters {
 			qsCompleter, err := queueSetCompleterForPL(cfgCtlr.queueSetFactory, plState.queues,
 				plState.pl, plState.reqsGaugePair, plState.execSeatsObs,
@@ -440,10 +465,20 @@ func (cfgCtlr *configController) updateBorrowingLocked(setCompleters bool, plSta
 			}
 			plState.qsCompleter = qsCompleter
 		}
-		currentCL := int(math.Round(float64(allocs[idx])))
+		var currentCL int
+		if !isNonExempt {
+			currentCL = cclOfExempt[plName]
+		} else if remainingServerCL <= minCLSum {
+			currentCL = plState.minCL
+		} else if remainingServerCL <= minCurrentCLSum {
+			minCurrentCL := max(plState.minCL, min(plState.nominalCL, plState.seatDemandStats.highWatermark))
+			currentCL = plState.minCL + int(math.Round(float64(minCurrentCL-plState.minCL)*shareFrac))
+		} else {
+			currentCL = int(math.Round(float64(allocs[idx])))
+		}
 		relChange := relDiff(float64(currentCL), float64(plState.currentCL))
 		plState.currentCL = currentCL
-		metrics.NotePriorityLevelConcurrencyAdjustment(plState.pl.Name, plState.seatDemandStats.highWatermark, plState.seatDemandStats.avg, plState.seatDemandStats.stdDev, plState.seatDemandStats.smoothed, float64(items[idx].target), currentCL)
+		metrics.NotePriorityLevelConcurrencyAdjustment(plState.pl.Name, float64(plState.seatDemandStats.highWatermark), plState.seatDemandStats.avg, plState.seatDemandStats.stdDev, plState.seatDemandStats.smoothed, float64(items[idx].target), currentCL)
 		logLevel := klog.Level(4)
 		if relChange >= 0.05 {
 			logLevel = 2
@@ -458,7 +493,6 @@ func (cfgCtlr *configController) updateBorrowingLocked(setCompleters bool, plSta
 		klog.V(logLevel).InfoS("Update CurrentCL", "plName", plName, "seatDemandHighWatermark", plState.seatDemandStats.highWatermark, "seatDemandAvg", plState.seatDemandStats.avg, "seatDemandStdev", plState.seatDemandStats.stdDev, "seatDemandSmoothed", plState.seatDemandStats.smoothed, "fairFrac", fairFrac, "currentCL", currentCL, "concurrencyDenominator", concurrencyDenominator, "backstop", err != nil)
 		plState.queues = plState.qsCompleter.Complete(fq.DispatchingConfig{ConcurrencyLimit: currentCL, ConcurrencyDenominator: concurrencyDenominator})
 	}
-	metrics.SetFairFrac(float64(fairFrac))
 }
 
 // runWorker is the logic of the one and only worker goroutine.  We