feat: fvm: optimize FVM lanes a bit (#11875)

This is showing up in profiles so I figured I'd optimize it a bit:

1. Avoid holding locks while recording metrics.
2. Slightly reduce allocations by re-using the metrics "mutators".

Also, use the passed context for better tracing.

This is unlikely to make a huge difference, but it may help RPC
providers a _tiny_ bit and doesn't really move the complexity needle.

chain/vm/execution.go

@@ -41,14 +41,14 @@ func newVMExecutor(vmi Interface, lane ExecutionLane) Interface {
 }
 
 func (e *vmExecutor) ApplyMessage(ctx context.Context, cmsg types.ChainMsg) (*ApplyRet, error) {
-	token := execution.getToken(e.lane)
+	token := execution.getToken(ctx, e.lane)
 	defer token.Done()
 
 	return e.vmi.ApplyMessage(ctx, cmsg)
 }
 
 func (e *vmExecutor) ApplyImplicitMessage(ctx context.Context, msg *types.Message) (*ApplyRet, error) {
-	token := execution.getToken(e.lane)
+	token := execution.getToken(ctx, e.lane)
 	defer token.Done()
 
 	return e.vmi.ApplyImplicitMessage(ctx, msg)
@@ -61,6 +61,7 @@ func (e *vmExecutor) Flush(ctx context.Context) (cid.Cid, error) {
 type executionToken struct {
 	lane     ExecutionLane
 	reserved int
+	ctx      context.Context
 }
 
 func (token *executionToken) Done() {
@@ -77,78 +78,69 @@ type executionEnv struct {
 	reserved int
 }
 
-func (e *executionEnv) getToken(lane ExecutionLane) *executionToken {
-	metricsUp(metrics.VMExecutionWaiting, lane)
-	defer metricsDown(metrics.VMExecutionWaiting, lane)
+func (e *executionEnv) getToken(ctx context.Context, lane ExecutionLane) *executionToken {
+	metricsUp(ctx, metrics.VMExecutionWaiting, lane)
+	defer metricsDown(ctx, metrics.VMExecutionWaiting, lane)
 
 	e.mx.Lock()
-	defer e.mx.Unlock()
 
-	switch lane {
-	case ExecutionLaneDefault:
+	reserving := 0
+	if lane == ExecutionLaneDefault {
 		for e.available <= e.reserved {
 			e.cond.Wait()
 		}
 
-		e.available--
-
-		metricsUp(metrics.VMExecutionRunning, lane)
-		return &executionToken{lane: lane, reserved: 0}
-
-	case ExecutionLanePriority:
+	} else {
 		for e.available == 0 {
 			e.cond.Wait()
 		}
-
-		e.available--
-
-		reserving := 0
 		if e.reserved > 0 {
 			e.reserved--
 			reserving = 1
 		}
+	}
 
-		metricsUp(metrics.VMExecutionRunning, lane)
-		return &executionToken{lane: lane, reserved: reserving}
+	e.available--
+	e.mx.Unlock()
 
-	default:
-		// already checked at interface boundary in NewVM, so this is appropriate
-		panic("bogus execution lane")
-	}
+	metricsUp(ctx, metrics.VMExecutionRunning, lane)
+	return &executionToken{lane: lane, reserved: reserving, ctx: ctx}
 }
 
 func (e *executionEnv) putToken(token *executionToken) {
 	e.mx.Lock()
-	defer e.mx.Unlock()
 
 	e.available++
 	e.reserved += token.reserved
 
 	// Note: Signal is unsound, because a priority token could wake up a non-priority
-	// goroutnie and lead to deadlock. So Broadcast it must be.
+	// goroutine and lead to deadlock. So Broadcast it must be.
 	e.cond.Broadcast()
+	e.mx.Unlock()
 
-	metricsDown(metrics.VMExecutionRunning, token.lane)
+	metricsDown(token.ctx, metrics.VMExecutionRunning, token.lane)
 }
 
-func metricsUp(metric *stats.Int64Measure, lane ExecutionLane) {
-	metricsAdjust(metric, lane, 1)
+func metricsUp(ctx context.Context, metric *stats.Int64Measure, lane ExecutionLane) {
+	metricsAdjust(ctx, metric, lane, 1)
 }
 
-func metricsDown(metric *stats.Int64Measure, lane ExecutionLane) {
-	metricsAdjust(metric, lane, -1)
+func metricsDown(ctx context.Context, metric *stats.Int64Measure, lane ExecutionLane) {
+	metricsAdjust(ctx, metric, lane, -1)
 }
 
-func metricsAdjust(metric *stats.Int64Measure, lane ExecutionLane, delta int) {
-	laneName := "default"
+var (
+	defaultLaneTag  = tag.Upsert(metrics.ExecutionLane, "default")
+	priorityLaneTag = tag.Upsert(metrics.ExecutionLane, "priority")
+)
+
+func metricsAdjust(ctx context.Context, metric *stats.Int64Measure, lane ExecutionLane, delta int) {
+	laneTag := defaultLaneTag
 	if lane > ExecutionLaneDefault {
-		laneName = "priority"
+		laneTag = priorityLaneTag
 	}
 
-	ctx, _ := tag.New(
-		context.Background(),
-		tag.Upsert(metrics.ExecutionLane, laneName),
-	)
+	ctx, _ = tag.New(ctx, laneTag)
 	stats.Record(ctx, metric.M(int64(delta)))
 }