Parallelize stat calls in Linux filesystem collector.

This change adds the ability to process multiple stat calls in parallel.
Processing is rate-limited based on the new flag
`collector.filesystem.stat-workers` (default 4).

Caveat: filesystem stats information is no longer in the same order as
returned by `/proc/1/mounts`.  This should not be an issue.

Caveat: This change currently uses unbuffered channels to prove
correctness without reliance on buffers.  Buffered channels will yield
superior performance.

Signed-off-by: Erica Mays <erica@emays.dev>

collector/filesystem_linux.go

@@ -40,6 +40,9 @@ const (
 var mountTimeout = kingpin.Flag("collector.filesystem.mount-timeout",
 	"how long to wait for a mount to respond before marking it as stale").
 	Hidden().Default("5s").Duration()
+var statWorkerCount = kingpin.Flag("collector.filesystem.stat-workers",
+	"how many stat calls to process simultaneously").
+	Hidden().Default("4").Int()
 var stuckMounts = make(map[string]struct{})
 var stuckMountsMtx = &sync.Mutex{}
 
@@ -50,6 +53,26 @@ func (c *filesystemCollector) GetStats() ([]filesystemStats, error) {
 		return nil, err
 	}
 	stats := []filesystemStats{}
+	labelChan := make(chan filesystemLabels)
+	statChan := make(chan filesystemStats)
+	wg := sync.WaitGroup{}
+
+	workerCount := *statWorkerCount
+	if workerCount < 1 {
+		workerCount = 1
+	}
+
+	for i := 0; i < workerCount; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for labels := range labelChan {
+				statChan <- c.processStat(labels)
+			}
+		}()
+	}
+
+	go func() {
 		for _, labels := range mps {
 			if c.excludedMountPointsPattern.MatchString(labels.mountPoint) {
 				level.Debug(c.logger).Log("msg", "Ignoring mount point", "mountpoint", labels.mountPoint)
@@ -59,6 +82,7 @@ func (c *filesystemCollector) GetStats() ([]filesystemStats, error) {
 				level.Debug(c.logger).Log("msg", "Ignoring fs", "type", labels.fsType)
 				continue
 			}
+
 			stuckMountsMtx.Lock()
 			if _, ok := stuckMounts[labels.mountPoint]; ok {
 				stats = append(stats, filesystemStats{
@@ -69,17 +93,30 @@ func (c *filesystemCollector) GetStats() ([]filesystemStats, error) {
 				stuckMountsMtx.Unlock()
 				continue
 			}
-		stuckMountsMtx.Unlock()
 
-		// The success channel is used do tell the "watcher" that the stat
+			stuckMountsMtx.Unlock()
-		// finished successfully. The channel is closed on success.
+			labelChan <- labels
+		}
+		close(labelChan)
+		wg.Wait()
+		close(statChan)
+	}()
+
+	for stat := range statChan {
+		stats = append(stats, stat)
+	}
+	return stats, nil
+}
+
+func (c *filesystemCollector) processStat(labels filesystemLabels) filesystemStats {
 	success := make(chan struct{})
 	go stuckMountWatcher(labels.mountPoint, success, c.logger)
 
 	buf := new(unix.Statfs_t)
-		err = unix.Statfs(rootfsFilePath(labels.mountPoint), buf)
+	err := unix.Statfs(rootfsFilePath(labels.mountPoint), buf)
 	stuckMountsMtx.Lock()
 	close(success)
+
 	// If the mount has been marked as stuck, unmark it and log it's recovery.
 	if _, ok := stuckMounts[labels.mountPoint]; ok {
 		level.Debug(c.logger).Log("msg", "Mount point has recovered, monitoring will resume", "mountpoint", labels.mountPoint)
@@ -88,13 +125,11 @@ func (c *filesystemCollector) GetStats() ([]filesystemStats, error) {
 	stuckMountsMtx.Unlock()
 
 	if err != nil {
-			stats = append(stats, filesystemStats{
+		level.Debug(c.logger).Log("msg", "Error on statfs() system call", "rootfs", rootfsFilePath(labels.mountPoint), "err", err)
+		return filesystemStats{
 			labels:      labels,
 			deviceError: 1,
-			})
+		}
-
-			level.Debug(c.logger).Log("msg", "Error on statfs() system call", "rootfs", rootfsFilePath(labels.mountPoint), "err", err)
-			continue
 	}
 
 	var ro float64
@@ -104,8 +139,7 @@ func (c *filesystemCollector) GetStats() ([]filesystemStats, error) {
 			break
 		}
 	}
-
+	return filesystemStats{
-		stats = append(stats, filesystemStats{
 		labels:    labels,
 		size:      float64(buf.Blocks) * float64(buf.Bsize),
 		free:      float64(buf.Bfree) * float64(buf.Bsize),
@@ -113,9 +147,7 @@ func (c *filesystemCollector) GetStats() ([]filesystemStats, error) {
 		files:     float64(buf.Files),
 		filesFree: float64(buf.Ffree),
 		ro:        ro,
-		})
 	}
-	return stats, nil
 }
 
 // stuckMountWatcher listens on the given success channel and if the channel closes