runtime/trace: add simple benchmarks for user annotation

Also, avoid Region creation when tracing is disabled.
Unfortunate side-effect of this change is that we no longer trace
pre-existing regions in tracing, but we can add the feature in
the future when we find it useful and justifiable. Until then,
let's avoid the overhead from this low-level api use as much as
possible.

goos: linux
goarch: amd64
pkg: runtime/trace

// Trace disabled
BenchmarkStartRegion-12 2000000000	         0.66 ns/op	       0 B/op	       0 allocs/op
BenchmarkNewTask-12    	30000000	        40.4 ns/op	      56 B/op	       2 allocs/op

// Trace enabled, -trace=/dev/null
BenchmarkStartRegion-12  5000000	       287 ns/op	      32 B/op	       1 allocs/op
BenchmarkNewTask-12    	 5000000	       283 ns/op	      56 B/op	       2 allocs/op

Also, skip other tests if tracing is already enabled.

Change-Id: Id3028d60b5642fcab4b09a74fd7d79361a3861e5
Reviewed-on: https://go-review.googlesource.com/109115
Reviewed-by: Peter Weinberger <pjw@google.com>

4 files changed, 67 insertions, 2 deletions

diff --git a/src/runtime/trace/annotation.go b/src/runtime/trace/annotation.go
index 0261d1c162..1cf01165cc 100644
--- a/src/runtime/trace/annotation.go
+++ b/src/runtime/trace/annotation.go
@@ -158,6 +158,9 @@ func WithSpan(ctx context.Context, spanType string, fn func(ctx context.Context)
 //     defer trace.StartRegion(ctx, "myTracedRegion").End()
 //
 func StartRegion(ctx context.Context, regionType string) *Region {
+	if !IsEnabled() {
+		return noopRegion
+	}
 	id := fromContext(ctx).id
 	userRegion(id, regionStartCode, regionType)
 	return &Region{id, regionType}
@@ -175,8 +178,13 @@ type Region struct {
 	regionType string
 }
 
+var noopRegion = &Region{}
+
 // End marks the end of the traced code region.
 func (r *Region) End() {
+	if r == noopRegion {
+		return
+	}
 	userRegion(r.id, regionEndCode, r.regionType)
 }
 
diff --git a/src/runtime/trace/annotation_test.go b/src/runtime/trace/annotation_test.go
index c20b009daa..71abbfcfa6 100644
--- a/src/runtime/trace/annotation_test.go
+++ b/src/runtime/trace/annotation_test.go
@@ -12,7 +12,35 @@ import (
 	"testing"
 )
 
+func BenchmarkStartRegion(b *testing.B) {
+	b.ReportAllocs()
+	ctx, task := NewTask(context.Background(), "benchmark")
+	defer task.End()
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			StartRegion(ctx, "region").End()
+		}
+	})
+}
+
+func BenchmarkNewTask(b *testing.B) {
+	b.ReportAllocs()
+	pctx, task := NewTask(context.Background(), "benchmark")
+	defer task.End()
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			_, task := NewTask(pctx, "task")
+			task.End()
+		}
+	})
+}
+
 func TestUserTaskRegion(t *testing.T) {
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
 	bgctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
@@ -107,7 +135,8 @@ func TestUserTaskRegion(t *testing.T) {
 		{trace.EvUserRegion, []string{"task0", "region1"}, []uint64{1}, false},
 		{trace.EvUserRegion, []string{"task0", "region0"}, []uint64{1}, false},
 		{trace.EvUserTaskEnd, []string{"task0"}, nil, false},
-		{trace.EvUserRegion, []string{"", "pre-existing region"}, []uint64{1}, false},
+		//  Currently, pre-existing region is not recorded to avoid allocations.
+		//  {trace.EvUserRegion, []string{"", "pre-existing region"}, []uint64{1}, false},
 		{trace.EvUserRegion, []string{"", "post-existing region"}, []uint64{0}, false},
 	}
 	if !reflect.DeepEqual(got, want) {
diff --git a/src/runtime/trace/trace_stack_test.go b/src/runtime/trace/trace_stack_test.go
index e7b8d57659..62c06e67d9 100644
--- a/src/runtime/trace/trace_stack_test.go
+++ b/src/runtime/trace/trace_stack_test.go
@@ -24,7 +24,7 @@ import (
 // In particular that we strip bottom uninteresting frames like goexit,
 // top uninteresting frames (runtime guts).
 func TestTraceSymbolize(t *testing.T) {
-	testenv.MustHaveGoBuild(t)
+	skipTraceSymbolizeTestIfNecessary(t)
 
 	buf := new(bytes.Buffer)
 	if err := Start(buf); err != nil {
@@ -285,6 +285,13 @@ func TestTraceSymbolize(t *testing.T) {
 	}
 }
 
+func skipTraceSymbolizeTestIfNecessary(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
+}
+
 func dumpEventStacks(typ byte, events []*trace.Event) ([]byte, int) {
 	matched := 0
 	o := new(bytes.Buffer)
diff --git a/src/runtime/trace/trace_test.go b/src/runtime/trace/trace_test.go
index 997d486c65..f289bd6f85 100644
--- a/src/runtime/trace/trace_test.go
+++ b/src/runtime/trace/trace_test.go
@@ -31,6 +31,9 @@ func TestEventBatch(t *testing.T) {
 	if race.Enabled {
 		t.Skip("skipping in race mode")
 	}
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
 	if testing.Short() {
 		t.Skip("skipping in short mode")
 	}
@@ -81,6 +84,9 @@ func TestEventBatch(t *testing.T) {
 }
 
 func TestTraceStartStop(t *testing.T) {
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
 	buf := new(bytes.Buffer)
 	if err := Start(buf); err != nil {
 		t.Fatalf("failed to start tracing: %v", err)
@@ -98,6 +104,9 @@ func TestTraceStartStop(t *testing.T) {
 }
 
 func TestTraceDoubleStart(t *testing.T) {
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
 	Stop()
 	buf := new(bytes.Buffer)
 	if err := Start(buf); err != nil {
@@ -111,6 +120,9 @@ func TestTraceDoubleStart(t *testing.T) {
 }
 
 func TestTrace(t *testing.T) {
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
 	buf := new(bytes.Buffer)
 	if err := Start(buf); err != nil {
 		t.Fatalf("failed to start tracing: %v", err)
@@ -168,6 +180,9 @@ func testBrokenTimestamps(t *testing.T, data []byte) {
 }
 
 func TestTraceStress(t *testing.T) {
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
 	var wg sync.WaitGroup
 	done := make(chan bool)
 
@@ -307,6 +322,9 @@ func TestTraceStress(t *testing.T) {
 // Do a bunch of various stuff (timers, GC, network, etc) in a separate goroutine.
 // And concurrently with all that start/stop trace 3 times.
 func TestTraceStressStartStop(t *testing.T) {
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(8))
 	outerDone := make(chan bool)
 
@@ -454,6 +472,9 @@ func TestTraceStressStartStop(t *testing.T) {
 }
 
 func TestTraceFutileWakeup(t *testing.T) {
+	if IsEnabled() {
+		t.Skip("skipping because -test.trace is set")
+	}
 	buf := new(bytes.Buffer)
 	if err := Start(buf); err != nil {
 		t.Fatalf("failed to start tracing: %v", err)