More granular locking for scrapeLoop. (#8104)

Don't lock for all of Sync/stop/reload as that holds up /metrics and the
UI when they want a list of active/dropped targets. Instead take
advantage of the fact that Sync/stop/reload cannot be called
concurrently by the scrape Manager and lock just on the targets
themselves.

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

scrape/scrape.go

@@ -192,7 +192,13 @@ type scrapePool struct {
 	appendable storage.Appendable
 	logger     log.Logger
 
-	mtx    sync.Mutex
+	// targetMtx protects activeTargets and droppedTargets from concurrent reads
+	// and writes. Only one of Sync/stop/reload may be called at once due to
+	// manager.mtxScrape so we only need to protect from concurrent reads from
+	// the ActiveTargets and DroppedTargets methods. This allows those two
+	// methods to always complete without having to wait on scrape loops to gracefull stop.
+	targetMtx sync.Mutex
+
 	config *config.ScrapeConfig
 	client *http.Client
 	// Targets and loops must always be synchronized to have the same
@@ -273,8 +279,8 @@ func newScrapePool(cfg *config.ScrapeConfig, app storage.Appendable, jitterSeed
 }
 
 func (sp *scrapePool) ActiveTargets() []*Target {
-	sp.mtx.Lock()
-	defer sp.mtx.Unlock()
+	sp.targetMtx.Lock()
+	defer sp.targetMtx.Unlock()
 
 	var tActive []*Target
 	for _, t := range sp.activeTargets {
@@ -284,8 +290,8 @@ func (sp *scrapePool) ActiveTargets() []*Target {
 }
 
 func (sp *scrapePool) DroppedTargets() []*Target {
-	sp.mtx.Lock()
-	defer sp.mtx.Unlock()
+	sp.targetMtx.Lock()
+	defer sp.targetMtx.Unlock()
 	return sp.droppedTargets
 }
 
@@ -294,8 +300,7 @@ func (sp *scrapePool) stop() {
 	sp.cancel()
 	var wg sync.WaitGroup
 
-	sp.mtx.Lock()
-	defer sp.mtx.Unlock()
+	sp.targetMtx.Lock()
 
 	for fp, l := range sp.loops {
 		wg.Add(1)
@@ -308,6 +313,9 @@ func (sp *scrapePool) stop() {
 		delete(sp.loops, fp)
 		delete(sp.activeTargets, fp)
 	}
+
+	sp.targetMtx.Unlock()
+
 	wg.Wait()
 	sp.client.CloseIdleConnections()
 
@@ -326,9 +334,6 @@ func (sp *scrapePool) reload(cfg *config.ScrapeConfig) error {
 	targetScrapePoolReloads.Inc()
 	start := time.Now()
 
-	sp.mtx.Lock()
-	defer sp.mtx.Unlock()
-
 	client, err := config_util.NewClientFromConfig(cfg.HTTPClientConfig, cfg.JobName, false, false)
 	if err != nil {
 		targetScrapePoolReloadsFailed.Inc()
@@ -352,6 +357,8 @@ func (sp *scrapePool) reload(cfg *config.ScrapeConfig) error {
 		mrc             = sp.config.MetricRelabelConfigs
 	)
 
+	sp.targetMtx.Lock()
+
 	forcedErr := sp.refreshTargetLimitErr()
 	for fp, oldLoop := range sp.loops {
 		var cache *scrapeCache
@@ -387,6 +394,8 @@ func (sp *scrapePool) reload(cfg *config.ScrapeConfig) error {
 		sp.loops[fp] = newLoop
 	}
 
+	sp.targetMtx.Unlock()
+
 	wg.Wait()
 	oldClient.CloseIdleConnections()
 	targetReloadIntervalLength.WithLabelValues(interval.String()).Observe(
@@ -398,11 +407,9 @@ func (sp *scrapePool) reload(cfg *config.ScrapeConfig) error {
 // Sync converts target groups into actual scrape targets and synchronizes
 // the currently running scraper with the resulting set and returns all scraped and dropped targets.
 func (sp *scrapePool) Sync(tgs []*targetgroup.Group) {
-	sp.mtx.Lock()
-	defer sp.mtx.Unlock()
-
 	start := time.Now()
 
+	sp.targetMtx.Lock()
 	var all []*Target
 	sp.droppedTargets = []*Target{}
 	for _, tg := range tgs {
@@ -419,6 +426,7 @@ func (sp *scrapePool) Sync(tgs []*targetgroup.Group) {
 			}
 		}
 	}
+	sp.targetMtx.Unlock()
 	sp.sync(all)
 
 	targetSyncIntervalLength.WithLabelValues(sp.config.JobName).Observe(
@@ -431,7 +439,6 @@ func (sp *scrapePool) Sync(tgs []*targetgroup.Group) {
 // scrape loops for new targets, and stops scrape loops for disappeared targets.
 // It returns after all stopped scrape loops terminated.
 func (sp *scrapePool) sync(targets []*Target) {
-	// This function expects that you have acquired the sp.mtx lock.
 	var (
 		uniqueLoops     = make(map[uint64]loop)
 		interval        = time.Duration(sp.config.ScrapeInterval)
@@ -442,6 +449,7 @@ func (sp *scrapePool) sync(targets []*Target) {
 		mrc             = sp.config.MetricRelabelConfigs
 	)
 
+	sp.targetMtx.Lock()
 	for _, t := range targets {
 		hash := t.hash()
 
@@ -487,6 +495,8 @@ func (sp *scrapePool) sync(targets []*Target) {
 		}
 	}
 
+	sp.targetMtx.Unlock()
+
 	targetScrapePoolTargetsAdded.WithLabelValues(sp.config.JobName).Set(float64(len(uniqueLoops)))
 	forcedErr := sp.refreshTargetLimitErr()
 	for _, l := range sp.loops {
@@ -507,7 +517,6 @@ func (sp *scrapePool) sync(targets []*Target) {
 // refreshTargetLimitErr returns an error that can be passed to the scrape loops
 // if the number of targets exceeds the configured limit.
 func (sp *scrapePool) refreshTargetLimitErr() error {
-	// This function expects that you have acquired the sp.mtx lock.
 	if sp.config == nil || sp.config.TargetLimit == 0 && !sp.targetLimitHit {
 		return nil
 	}