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/12579Reviewed-by: Austin Clements <austin@google.com>

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{}},
 }

src/internal/trace/parser_test.go

@@ -16,6 +16,10 @@ func TestCorruptedInputs(t *testing.T) {
 		"gotrace\x00Q00\x020",
 		"gotrace\x00T00\x020",
 		"gotrace\x00\xc3\x0200",
+		"go 1.5 trace\x00\x00\x00\x00\x020",
+		"go 1.5 trace\x00\x00\x00\x00Q00\x020",
+		"go 1.5 trace\x00\x00\x00\x00T00\x020",
+		"go 1.5 trace\x00\x00\x00\x00\xc3\x0200",
 	}
 	for _, data := range tests {
 		events, err := Parse(strings.NewReader(data))

src/runtime/proc1.go

@@ -1348,7 +1348,7 @@ func execute(gp *g, inheritTime bool) {
 		// GoSysExit has to happen when we have a P, but before GoStart.
 		// So we emit it here.
 		if gp.syscallsp != 0 && gp.sysblocktraced {
-			traceGoSysExit(gp.sysexitticks)
+			traceGoSysExit(gp.sysexitseq, gp.sysexitticks)
 		}
 		traceGoStart()
 	}
@@ -1986,6 +1986,7 @@ func exitsyscall(dummy int32) {
 	}
 
 	_g_.sysexitticks = 0
+	_g_.sysexitseq = 0
 	if trace.enabled {
 		// Wait till traceGoSysBlock event is emitted.
 		// This ensures consistency of the trace (the goroutine is started after it is blocked).
@@ -1996,7 +1997,7 @@ func exitsyscall(dummy int32) {
 		// Tracing code can invoke write barriers that cannot run without a P.
 		// So instead we remember the syscall exit time and emit the event
 		// in execute when we have a P.
-		_g_.sysexitticks = cputicks()
+		_g_.sysexitseq, _g_.sysexitticks = tracestamp()
 	}
 
 	_g_.m.locks--
@@ -2044,7 +2045,7 @@ func exitsyscallfast() bool {
 					// Denote blocking of the new syscall.
 					traceGoSysBlock(_g_.m.p.ptr())
 					// Denote completion of the current syscall.
-					traceGoSysExit(0)
+					traceGoSysExit(tracestamp())
 				})
 			}
 			_g_.m.p.ptr().syscalltick++
@@ -2068,7 +2069,7 @@ func exitsyscallfast() bool {
 						osyield()
 					}
 				}
-				traceGoSysExit(0)
+				traceGoSysExit(tracestamp())
 			}
 		})
 		if ok {

src/runtime/runtime2.go

@@ -235,16 +235,17 @@ type g struct {
 	waitsince      int64  // approx time when the g become blocked
 	waitreason     string // if status==Gwaiting
 	schedlink      guintptr
-	preempt        bool  // preemption signal, duplicates stackguard0 = stackpreempt
-	paniconfault   bool  // panic (instead of crash) on unexpected fault address
-	preemptscan    bool  // preempted g does scan for gc
-	gcscandone     bool  // g has scanned stack; protected by _Gscan bit in status
-	gcscanvalid    bool  // false at start of gc cycle, true if G has not run since last scan
-	throwsplit     bool  // must not split stack
-	raceignore     int8  // ignore race detection events
-	sysblocktraced bool  // StartTrace has emitted EvGoInSyscall about this goroutine
-	sysexitticks   int64 // cputicks when syscall has returned (for tracing)
-	m              *m    // for debuggers, but offset not hard-coded
+	preempt        bool   // preemption signal, duplicates stackguard0 = stackpreempt
+	paniconfault   bool   // panic (instead of crash) on unexpected fault address
+	preemptscan    bool   // preempted g does scan for gc
+	gcscandone     bool   // g has scanned stack; protected by _Gscan bit in status
+	gcscanvalid    bool   // false at start of gc cycle, true if G has not run since last scan
+	throwsplit     bool   // must not split stack
+	raceignore     int8   // ignore race detection events
+	sysblocktraced bool   // StartTrace has emitted EvGoInSyscall about this goroutine
+	sysexitticks   int64  // cputicks when syscall has returned (for tracing)
+	sysexitseq     uint64 // trace seq when syscall has returned (for tracing)
+	m              *m     // for debuggers, but offset not hard-coded
 	lockedm        *m
 	sig            uint32
 	writebuf       []byte

src/runtime/trace.go

@@ -95,6 +95,7 @@ var trace struct {
 	headerWritten bool      // whether ReadTrace has emitted trace header
 	footerWritten bool      // whether ReadTrace has emitted trace footer
 	shutdownSema  uint32    // used to wait for ReadTrace completion
+	seqStart      uint64    // sequence number when tracing was started
 	ticksStart    int64     // cputicks when tracing was started
 	ticksEnd      int64     // cputicks when tracing was stopped
 	timeStart     int64     // nanotime when tracing was started
@@ -110,9 +111,31 @@ var trace struct {
 	buf     *traceBuf // global trace buffer, used when running without a p
 }
 
+var traceseq uint64 // global trace sequence number
+
+// tracestamp returns a consistent sequence number, time stamp pair
+// for use in a trace. We need to make sure that time stamp ordering
+// (assuming synchronized CPUs) and sequence ordering match.
+// To do that, we increment traceseq, grab ticks, and increment traceseq again.
+// We treat odd traceseq as a sign that another thread is in the middle
+// of the sequence and spin until it is done.
+// Not splitting stack to avoid preemption, just in case the call sites
+// that used to call xadd64 and cputicks are sensitive to that.
+//go:nosplit
+func tracestamp() (seq uint64, ts int64) {
+	seq = atomicload64(&traceseq)
+	for seq&1 != 0 || !cas64(&traceseq, seq, seq+1) {
+		seq = atomicload64(&traceseq)
+	}
+	ts = cputicks()
+	atomicstore64(&traceseq, seq+2)
+	return seq >> 1, ts
+}
+
 // traceBufHeader is per-P tracing buffer.
 type traceBufHeader struct {
 	link      *traceBuf               // in trace.empty/full
+	lastSeq   uint64                  // sequence number of last event
 	lastTicks uint64                  // when we wrote the last event
 	buf       []byte                  // trace data, always points to traceBuf.arr
 	stk       [traceStackSize]uintptr // scratch buffer for traceback
@@ -147,7 +170,7 @@ func StartTrace() error {
 		return errorString("tracing is already enabled")
 	}
 
-	trace.ticksStart = cputicks()
+	trace.seqStart, trace.ticksStart = tracestamp()
 	trace.timeStart = nanotime()
 	trace.headerWritten = false
 	trace.footerWritten = false
@@ -308,7 +331,7 @@ func ReadTrace() []byte {
 		trace.headerWritten = true
 		trace.lockOwner = nil
 		unlock(&trace.lock)
-		return []byte("gotrace\x00")
+		return []byte("go 1.5 trace\x00\x00\x00\x00")
 	}
 	// Wait for new data.
 	if trace.fullHead == nil && !trace.shutdown {
@@ -334,9 +357,11 @@ func ReadTrace() []byte {
 		var data []byte
 		data = append(data, traceEvFrequency|0<<traceArgCountShift)
 		data = traceAppend(data, uint64(freq))
+		data = traceAppend(data, 0)
 		if timers.gp != nil {
 			data = append(data, traceEvTimerGoroutine|0<<traceArgCountShift)
 			data = traceAppend(data, uint64(timers.gp.goid))
+			data = traceAppend(data, 0)
 		}
 		return data
 	}
@@ -435,22 +460,27 @@ func traceEvent(ev byte, skip int, args ...uint64) {
 		return
 	}
 	buf := *bufp
-	const maxSize = 2 + 4*traceBytesPerNumber // event type, length, timestamp, stack id and two add params
+	const maxSize = 2 + 5*traceBytesPerNumber // event type, length, sequence, timestamp, stack id and two add params
 	if buf == nil || cap(buf.buf)-len(buf.buf) < maxSize {
 		buf = traceFlush(buf)
 		*bufp = buf
 	}
 
-	ticks := uint64(cputicks()) / traceTickDiv
+	seq, ticksraw := tracestamp()
+	seqDiff := seq - buf.lastSeq
+	ticks := uint64(ticksraw) / traceTickDiv
 	tickDiff := ticks - buf.lastTicks
 	if len(buf.buf) == 0 {
 		data := buf.buf
 		data = append(data, traceEvBatch|1<<traceArgCountShift)
 		data = traceAppend(data, uint64(pid))
+		data = traceAppend(data, seq)
 		data = traceAppend(data, ticks)
 		buf.buf = data
+		seqDiff = 0
 		tickDiff = 0
 	}
+	buf.lastSeq = seq
 	buf.lastTicks = ticks
 	narg := byte(len(args))
 	if skip >= 0 {
@@ -469,6 +499,7 @@ func traceEvent(ev byte, skip int, args ...uint64) {
 		data = append(data, 0)
 		lenp = &data[len(data)-1]
 	}
+	data = traceAppend(data, seqDiff)
 	data = traceAppend(data, tickDiff)
 	for _, a := range args {
 		data = traceAppend(data, a)
@@ -800,12 +831,12 @@ func traceGoSysCall() {
 	traceEvent(traceEvGoSysCall, 4)
 }
 
-func traceGoSysExit(ts int64) {
-	if ts != 0 && ts < trace.ticksStart {
+func traceGoSysExit(seq uint64, ts int64) {
+	if int64(seq)-int64(trace.seqStart) < 0 {
 		// The timestamp was obtained during a previous tracing session, ignore.
 		return
 	}
-	traceEvent(traceEvGoSysExit, -1, uint64(getg().m.curg.goid), uint64(ts)/traceTickDiv)
+	traceEvent(traceEvGoSysExit, -1, uint64(getg().m.curg.goid), seq, uint64(ts)/traceTickDiv)
 }
 
 func traceGoSysBlock(pp *p) {

src/runtime/trace/trace_stack_test.go

@@ -125,7 +125,7 @@ func TestTraceSymbolize(t *testing.T) {
 	<-pipeReadDone
 
 	Stop()
-	events, _, err := parseTrace(buf)
+	events, _, err := parseTrace(t, buf)
 	if err != nil {
 		t.Fatalf("failed to parse trace: %v", err)
 	}

src/runtime/trace/trace_test.go

@@ -75,13 +75,19 @@ func TestTrace(t *testing.T) {
 	}
 	Stop()
 	_, err := trace.Parse(buf)
+	if err == trace.ErrTimeOrder {
+		t.Skipf("skipping trace: %v", err)
+	}
 	if err != nil {
 		t.Fatalf("failed to parse trace: %v", err)
 	}
 }
 
-func parseTrace(r io.Reader) ([]*trace.Event, map[uint64]*trace.GDesc, error) {
+func parseTrace(t *testing.T, r io.Reader) ([]*trace.Event, map[uint64]*trace.GDesc, error) {
 	events, err := trace.Parse(r)
+	if err == trace.ErrTimeOrder {
+		t.Skipf("skipping trace: %v", err)
+	}
 	if err != nil {
 		return nil, nil, err
 	}
@@ -221,7 +227,7 @@ func TestTraceStress(t *testing.T) {
 	runtime.GOMAXPROCS(procs)
 
 	Stop()
-	_, _, err = parseTrace(buf)
+	_, _, err = parseTrace(t, buf)
 	if err != nil {
 		t.Fatalf("failed to parse trace: %v", err)
 	}
@@ -361,7 +367,7 @@ func TestTraceStressStartStop(t *testing.T) {
 		}
 		time.Sleep(time.Millisecond)
 		Stop()
-		if _, _, err := parseTrace(buf); err != nil {
+		if _, _, err := parseTrace(t, buf); err != nil {
 			t.Fatalf("failed to parse trace: %v", err)
 		}
 	}
@@ -428,7 +434,7 @@ func TestTraceFutileWakeup(t *testing.T) {
 	done.Wait()
 
 	Stop()
-	events, _, err := parseTrace(buf)
+	events, _, err := parseTrace(t, buf)
 	if err != nil {
 		t.Fatalf("failed to parse trace: %v", err)
 	}