runtime/trace: record event sequence numbers explicitly

Nearly all the flaky failures we've seen in trace tests have been
due to the use of time stamps to determine relative event ordering.
This is tricky for many reasons, including:
 - different cores might not have exactly synchronized clocks
 - VMs are worse than real hardware
 - non-x86 chips have different timer resolution than x86 chips
 - on fast systems two events can end up with the same time stamp

Stop trying to make time reliable. It's clearly not going to be for Go 1.5.
Instead, record an explicit event sequence number for ordering.
Using our own counter solves all of the above problems.

The trace still contains time stamps, of course. The sequence number
is just used for ordering.

Should alleviate #10554 somewhat. Then tickDiv can be chosen to
be a useful time unit instead of having to be exact for ordering.

Separating ordering and time stamps lets the trace parser diagnose
systems where the time stamp order and actual order do not match
for one reason or another. This CL adds that check to the end of
trace.Parse, after all other sequence order-based checking.
If that error is found, we skip the test instead of failing it.
Putting the check in trace.Parse means that cmd/trace will pick
up the same check, refusing to display a trace where the time stamps
do not match actual ordering.

Using net/http's BenchmarkClientServerParallel4 on various CPU counts,
not tracing vs tracing:

name                      old time/op    new time/op    delta
ClientServerParallel4       50.4µs ± 4%    80.2µs ± 4%  +59.06%        (p=0.000 n=10+10)
ClientServerParallel4-2     33.1µs ± 7%    57.8µs ± 5%  +74.53%        (p=0.000 n=10+10)
ClientServerParallel4-4     18.5µs ± 4%    32.6µs ± 3%  +75.77%        (p=0.000 n=10+10)
ClientServerParallel4-6     12.9µs ± 5%    24.4µs ± 2%  +89.33%        (p=0.000 n=10+10)
ClientServerParallel4-8     11.4µs ± 6%    21.0µs ± 3%  +83.40%        (p=0.000 n=10+10)
ClientServerParallel4-12    14.4µs ± 4%    23.8µs ± 4%  +65.67%        (p=0.000 n=10+10)

Fixes #10512.

Change-Id: I173eecf8191e86feefd728a5aad25bf1bc094b12
Reviewed-on: https://go-review.googlesource.com/12579
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 47 insertions, 27 deletions

diff --git a/src/internal/trace/parser.go b/src/internal/trace/parser.go
index bfe6b73015..294a52f976 100644
--- a/src/internal/trace/parser.go
+++ b/src/internal/trace/parser.go
@@ -20,12 +20,13 @@ import (
 type Event struct {
 	Off   int       // offset in input file (for debugging and error reporting)
 	Type  byte      // one of Ev*
+	Seq   int64     // sequence number
 	Ts    int64     // timestamp in nanoseconds
 	P     int       // P on which the event happened (can be one of TimerP, NetpollP, SyscallP)
 	G     uint64    // G on which the event happened
 	StkID uint64    // unique stack ID
 	Stk   []*Frame  // stack trace (can be empty)
-	Args  [2]uint64 // event-type-specific arguments
+	Args  [3]uint64 // event-type-specific arguments
 	// linked event (can be nil), depends on event type:
 	// for GCStart: the GCStop
 	// for GCScanStart: the GCScanDone
@@ -88,12 +89,12 @@ type rawEvent struct {
 // It does not care about specific event types and argument meaning.
 func readTrace(r io.Reader) ([]rawEvent, error) {
 	// Read and validate trace header.
-	var buf [8]byte
+	var buf [16]byte
 	off, err := r.Read(buf[:])
-	if off != 8 || err != nil {
+	if off != 16 || err != nil {
 		return nil, fmt.Errorf("failed to read header: read %v, err %v", off, err)
 	}
-	if bytes.Compare(buf[:], []byte("gotrace\x00")) != 0 {
+	if bytes.Compare(buf[:], []byte("go 1.5 trace\x00\x00\x00\x00")) != 0 {
 		return nil, fmt.Errorf("not a trace file")
 	}
 
@@ -111,10 +112,10 @@ func readTrace(r io.Reader) ([]rawEvent, error) {
 		}
 		off += n
 		typ := buf[0] << 2 >> 2
-		narg := buf[0]>>6 + 1
+		narg := buf[0] >> 6
 		ev := rawEvent{typ: typ, off: off0}
-		if narg <= 3 {
-			for i := 0; i < int(narg); i++ {
+		if narg < 3 {
+			for i := 0; i < int(narg)+2; i++ { // sequence number and time stamp are present but not counted in narg
 				var v uint64
 				v, off, err = readVal(r, off)
 				if err != nil {
@@ -123,7 +124,7 @@ func readTrace(r io.Reader) ([]rawEvent, error) {
 				ev.args = append(ev.args, v)
 			}
 		} else {
-			// If narg == 4, the first value is length of the event in bytes.
+			// If narg == 3, the first value is length of the event in bytes.
 			var v uint64
 			v, off, err = readVal(r, off)
 			if err != nil {
@@ -150,7 +151,7 @@ func readTrace(r io.Reader) ([]rawEvent, error) {
 // Parse events transforms raw events into events.
 // It does analyze and verify per-event-type arguments.
 func parseEvents(rawEvents []rawEvent) (events []*Event, err error) {
-	var ticksPerSec, lastTs int64
+	var ticksPerSec, lastSeq, lastTs int64
 	var lastG, timerGoid uint64
 	var lastP int
 	lastGs := make(map[int]uint64) // last goroutine running on P
@@ -171,6 +172,7 @@ func parseEvents(rawEvents []rawEvent) (events []*Event, err error) {
 				narg++
 			}
 			if raw.typ != EvBatch && raw.typ != EvFrequency && raw.typ != EvTimerGoroutine {
+				narg++ // sequence number
 				narg++ // timestamp
 			}
 			if len(raw.args) != narg {
@@ -184,7 +186,8 @@ func parseEvents(rawEvents []rawEvent) (events []*Event, err error) {
 			lastGs[lastP] = lastG
 			lastP = int(raw.args[0])
 			lastG = lastGs[lastP]
-			lastTs = int64(raw.args[1])
+			lastSeq = int64(raw.args[1])
+			lastTs = int64(raw.args[2])
 		case EvFrequency:
 			ticksPerSec = int64(raw.args[0])
 			if ticksPerSec <= 0 {
@@ -221,13 +224,15 @@ func parseEvents(rawEvents []rawEvent) (events []*Event, err error) {
 			}
 		default:
 			e := &Event{Off: raw.off, Type: raw.typ, P: lastP, G: lastG}
-			e.Ts = lastTs + int64(raw.args[0])
+			e.Seq = lastSeq + int64(raw.args[0])
+			e.Ts = lastTs + int64(raw.args[1])
+			lastSeq = e.Seq
 			lastTs = e.Ts
 			for i := range desc.Args {
-				e.Args[i] = raw.args[i+1]
+				e.Args[i] = raw.args[i+2]
 			}
 			if desc.Stack {
-				e.StkID = raw.args[len(desc.Args)+1]
+				e.StkID = raw.args[len(desc.Args)+2]
 			}
 			switch raw.typ {
 			case EvGoStart:
@@ -241,10 +246,11 @@ func parseEvents(rawEvents []rawEvent) (events []*Event, err error) {
 				EvGoSysBlock:
 				lastG = 0
 			case EvGoSysExit:
-				if e.Args[1] != 0 {
-					// EvGoSysExit emission is delayed until the thread has a P.
-					// Give it the real timestamp.
-					e.Ts = int64(e.Args[1])
+				// EvGoSysExit emission is delayed until the thread has a P.
+				// Give it the real sequence number and time stamp.
+				e.Seq = int64(e.Args[1])
+				if e.Args[2] != 0 {
+					e.Ts = int64(e.Args[2])
 				}
 			}
 			events = append(events, e)
@@ -262,10 +268,8 @@ func parseEvents(rawEvents []rawEvent) (events []*Event, err error) {
 		}
 	}
 
-	// Sort by time and translate cpu ticks to real time.
-	// Use stable sort because adjacent events in a batch can have the same timestamp
-	// (this was observed on some VMs).
-	sort.Stable(eventList(events))
+	// Sort by sequence number and translate cpu ticks to real time.
+	sort.Sort(eventList(events))
 	if ticksPerSec == 0 {
 		err = fmt.Errorf("no EvFrequency event")
 		return
@@ -341,6 +345,10 @@ func removeFutile(events []*Event) ([]*Event, error) {
 	return newEvents, nil
 }
 
+// ErrTimeOrder is returned by Parse when the trace contains
+// time stamps that do not respect actual event ordering.
+var ErrTimeOrder = fmt.Errorf("time stamps out of order")
+
 // postProcessTrace does inter-event verification and information restoration.
 // The resulting trace is guaranteed to be consistent
 // (for example, a P does not run two Gs at the same time, or a G is indeed
@@ -555,6 +563,18 @@ func postProcessTrace(events []*Event) error {
 	// TODO(dvyukov): restore stacks for EvGoStart events.
 	// TODO(dvyukov): test that all EvGoStart events has non-nil Link.
 
+	// Last, after all the other consistency checks,
+	// make sure time stamps respect sequence numbers.
+	// The tests will skip (not fail) the test case if they see this error,
+	// so check everything else that could possibly be wrong first.
+	lastTs := int64(0)
+	for _, ev := range events {
+		if ev.Ts < lastTs {
+			return ErrTimeOrder
+		}
+		lastTs = ev.Ts
+	}
+
 	return nil
 }
 
@@ -639,7 +659,7 @@ func readVal(r io.Reader, off0 int) (v uint64, off int, err error) {
 		var n int
 		n, err = r.Read(buf[:])
 		if err != nil || n != 1 {
-			return 0, 0, fmt.Errorf("failed to read trace at offset: read %v, error %v", off0, n, err)
+			return 0, 0, fmt.Errorf("failed to read trace at offset %d: read %v, error %v", off0, n, err)
 		}
 		off++
 		v |= uint64(buf[0]&0x7f) << (uint(i) * 7)
@@ -657,7 +677,7 @@ func (l eventList) Len() int {
 }
 
 func (l eventList) Less(i, j int) bool {
-	return l[i].Ts < l[j].Ts
+	return l[i].Seq < l[j].Seq
 }
 
 func (l eventList) Swap(i, j int) {
@@ -725,8 +745,8 @@ var EventDescriptions = [EvCount]struct {
 	Args  []string
 }{
 	EvNone:           {"None", false, []string{}},
-	EvBatch:          {"Batch", false, []string{"p", "ticks"}},
-	EvFrequency:      {"Frequency", false, []string{"freq"}},
+	EvBatch:          {"Batch", false, []string{"p", "seq", "ticks"}},
+	EvFrequency:      {"Frequency", false, []string{"freq", "unused"}},
 	EvStack:          {"Stack", false, []string{"id", "siz"}},
 	EvGomaxprocs:     {"Gomaxprocs", true, []string{"procs"}},
 	EvProcStart:      {"ProcStart", false, []string{"thread"}},
@@ -753,12 +773,12 @@ var EventDescriptions = [EvCount]struct {
 	EvGoBlockCond:    {"GoBlockCond", true, []string{}},
 	EvGoBlockNet:     {"GoBlockNet", true, []string{}},
 	EvGoSysCall:      {"GoSysCall", true, []string{}},
-	EvGoSysExit:      {"GoSysExit", false, []string{"g", "ts"}},
+	EvGoSysExit:      {"GoSysExit", false, []string{"g", "seq", "ts"}},
 	EvGoSysBlock:     {"GoSysBlock", false, []string{}},
 	EvGoWaiting:      {"GoWaiting", false, []string{"g"}},
 	EvGoInSyscall:    {"GoInSyscall", false, []string{"g"}},
 	EvHeapAlloc:      {"HeapAlloc", false, []string{"mem"}},
 	EvNextGC:         {"NextGC", false, []string{"mem"}},
-	EvTimerGoroutine: {"TimerGoroutine", false, []string{"g"}},
+	EvTimerGoroutine: {"TimerGoroutine", false, []string{"g", "unused"}},
 	EvFutileWakeup:   {"FutileWakeup", false, []string{}},
 }