runtime/pprof: fix period information

The period recorded in CPU profiles is in nanoseconds, but was being
computed incorrectly as hz * 1000. As a result, many absolute times
displayed by pprof were incorrect.

Fix this by computing the period correctly.

Change-Id: I6fadd6d8ad3e57f31e8cc7a25a24fcaec510d8d4
Reviewed-on: https://go-review.googlesource.com/40995
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/pprof/proto.go

@@ -270,7 +270,9 @@ func (b *profileBuilder) addCPUData(data []uint64, tags []unsafe.Pointer) error
 		if data[0] != 3 || data[2] == 0 {
 			return fmt.Errorf("malformed profile")
 		}
-		b.period = int64(data[2]) * 1000
+		// data[2] is sampling rate in Hz. Convert to sampling
+		// period in nanoseconds.
+		b.period = 1e9 / int64(data[2])
 		b.havePeriod = true
 		data = data[3:]
 	}

src/runtime/pprof/proto_test.go

@@ -40,7 +40,7 @@ func TestConvertCPUProfileEmpty(t *testing.T) {
 	// A test server with mock cpu profile data.
 	var buf bytes.Buffer
 
-	b := []uint64{3, 0, 2000} // empty profile with 2ms sample period
+	b := []uint64{3, 0, 500} // empty profile at 500 Hz (2ms sample period)
 	p, err := translateCPUProfile(b)
 	if err != nil {
 		t.Fatalf("translateCPUProfile: %v", err)
@@ -103,7 +103,7 @@ func TestConvertCPUProfile(t *testing.T) {
 	addr1, addr2, map1, map2 := testPCs(t)
 
 	b := []uint64{
-		3, 0, 2000, // periodMs = 2000
+		3, 0, 500, // hz = 500
 		5, 0, 10, uint64(addr1), uint64(addr1 + 2), // 10 samples in addr1
 		5, 0, 40, uint64(addr2), uint64(addr2 + 2), // 40 samples in addr2
 		5, 0, 10, uint64(addr1), uint64(addr1 + 2), // 10 samples in addr1