internal/trace: implement MutatorUtilizationV2

This change adds a new MutatorUtilization for traces for Go 1.22+.

To facilitate testing, it also generates a short trace with the
gc-stress.go test program (shortening its duration to 10ms) and adds it
to the tests for the internal/trace/v2 package. Notably, we make sure
this trace has a GCMarkAssistActive event to test that codepath.

For #63960.
For #60773.

Change-Id: I2e61f545988677be716818e2a08641c54c4c201f
Reviewed-on: https://go-review.googlesource.com/c/go/+/540256
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/go/build/deps_test.go

@@ -608,9 +608,6 @@ var depsRules = `
 	FMT
 	< internal/diff, internal/txtar;
 
-	FMT, container/heap, math/rand
-	< internal/trace;
-
 	# v2 execution trace parser.
 	FMT
 	< internal/trace/v2/event;
@@ -633,6 +630,9 @@ var depsRules = `
 	regexp, internal/txtar, internal/trace/v2, internal/trace/v2/raw
 	< internal/trace/v2/internal/testgen/go122;
 
+	FMT, container/heap, math/rand, internal/trace/v2
+	< internal/trace;
+
 	# Coverage.
 	FMT, crypto/md5, encoding/binary, regexp, sort, text/tabwriter, unsafe,
 	internal/coverage, internal/coverage/uleb128

src/internal/trace/gc.go

@@ -6,6 +6,8 @@ package trace
 
 import (
 	"container/heap"
+	tracev2 "internal/trace/v2"
+	"io"
 	"math"
 	"sort"
 	"strings"
@@ -202,6 +204,268 @@ func MutatorUtilization(events []*Event, flags UtilFlags) [][]MutatorUtil {
 	return out
 }
 
+// MutatorUtilizationV2 returns a set of mutator utilization functions
+// for the given v2 trace, passed as an io.Reader. Each function will
+// always end with 0 utilization. The bounds of each function are implicit
+// in the first and last event; outside of these bounds each function is
+// undefined.
+//
+// If the UtilPerProc flag is not given, this always returns a single
+// utilization function. Otherwise, it returns one function per P.
+func MutatorUtilizationV2(trace io.Reader, flags UtilFlags) ([][]MutatorUtil, error) {
+	// Create a reader.
+	r, err := tracev2.NewReader(trace)
+	if err != nil {
+		return nil, err
+	}
+
+	// Set up a bunch of analysis state.
+	type perP struct {
+		// gc > 0 indicates that GC is active on this P.
+		gc int
+		// series the logical series number for this P. This
+		// is necessary because Ps may be removed and then
+		// re-added, and then the new P needs a new series.
+		series int
+	}
+	type procsCount struct {
+		// time at which procs changed.
+		time int64
+		// n is the number of procs at that point.
+		n int
+	}
+	out := [][]MutatorUtil{}
+	stw := 0
+	ps := []perP{}
+	inGC := make(map[tracev2.GoID]bool)
+	states := make(map[tracev2.GoID]tracev2.GoState)
+	bgMark := make(map[tracev2.GoID]bool)
+	procs := []procsCount{}
+	seenSync := false
+
+	// Helpers.
+	handleSTW := func(r tracev2.Range) bool {
+		return flags&UtilSTW != 0 && isGCSTW(r)
+	}
+	handleMarkAssist := func(r tracev2.Range) bool {
+		return flags&UtilAssist != 0 && isGCMarkAssist(r)
+	}
+	handleSweep := func(r tracev2.Range) bool {
+		return flags&UtilSweep != 0 && isGCSweep(r)
+	}
+
+	// Iterate through the trace, tracking mutator utilization.
+	var lastEv tracev2.Event
+	for {
+		// Read a single event.
+		ev, err := r.ReadEvent()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return nil, err
+		}
+		lastEv = ev
+
+		// Process the event.
+		switch ev.Kind() {
+		case tracev2.EventSync:
+			seenSync = true
+		case tracev2.EventMetric:
+			m := ev.Metric()
+			if m.Name != "/sched/gomaxprocs:threads" {
+				break
+			}
+			gomaxprocs := int(m.Value.Uint64())
+			if len(ps) > gomaxprocs {
+				if flags&UtilPerProc != 0 {
+					// End each P's series.
+					for _, p := range ps[gomaxprocs:] {
+						out[p.series] = addUtil(out[p.series], MutatorUtil{int64(ev.Time()), 0})
+					}
+				}
+				ps = ps[:gomaxprocs]
+			}
+			for len(ps) < gomaxprocs {
+				// Start new P's series.
+				series := 0
+				if flags&UtilPerProc != 0 || len(out) == 0 {
+					series = len(out)
+					out = append(out, []MutatorUtil{{int64(ev.Time()), 1}})
+				}
+				ps = append(ps, perP{series: series})
+			}
+			if len(procs) == 0 || gomaxprocs != procs[len(procs)-1].n {
+				procs = append(procs, procsCount{time: int64(ev.Time()), n: gomaxprocs})
+			}
+		}
+		if len(ps) == 0 {
+			// We can't start doing any analysis until we see what GOMAXPROCS is.
+			// It will show up very early in the trace, but we need to be robust to
+			// something else being emitted beforehand.
+			continue
+		}
+
+		switch ev.Kind() {
+		case tracev2.EventRangeActive:
+			if seenSync {
+				// If we've seen a sync, then we can be sure we're not finding out about
+				// something late; we have complete information after that point, and these
+				// active events will just be redundant.
+				break
+			}
+			// This range is active back to the start of the trace. We're failing to account
+			// for this since we just found out about it now. Fix up the mutator utilization.
+			//
+			// N.B. A trace can't start during a STW, so we don't handle it here.
+			r := ev.Range()
+			switch {
+			case handleMarkAssist(r):
+				if !states[ev.Goroutine()].Executing() {
+					// If the goroutine isn't executing, then the fact that it was in mark
+					// assist doesn't actually count.
+					break
+				}
+				// This G has been in a mark assist *and running on its P* since the start
+				// of the trace.
+				fallthrough
+			case handleSweep(r):
+				// This P has been in sweep (or mark assist, from above) in the start of the trace.
+				//
+				// We don't need to do anything if UtilPerProc is set. If we get an event like
+				// this for a running P, it must show up the first time a P is mentioned. Therefore,
+				// this P won't actually have any MutatorUtils on its list yet.
+				//
+				// However, if UtilPerProc isn't set, then we probably have data from other procs
+				// and from previous events. We need to fix that up.
+				if flags&UtilPerProc != 0 {
+					break
+				}
+				// Subtract out 1/gomaxprocs mutator utilization for all time periods
+				// from the beginning of the trace until now.
+				mi, pi := 0, 0
+				for mi < len(out[0]) {
+					if pi < len(procs)-1 && procs[pi+1].time < out[0][mi].Time {
+						pi++
+						continue
+					}
+					out[0][mi].Util -= float64(1) / float64(procs[pi].n)
+					if out[0][mi].Util < 0 {
+						out[0][mi].Util = 0
+					}
+					mi++
+				}
+			}
+			// After accounting for the portion we missed, this just acts like the
+			// beginning of a new range.
+			fallthrough
+		case tracev2.EventRangeBegin:
+			r := ev.Range()
+			if handleSTW(r) {
+				stw++
+			} else if handleSweep(r) {
+				ps[ev.Proc()].gc++
+			} else if handleMarkAssist(r) {
+				ps[ev.Proc()].gc++
+				if g := r.Scope.Goroutine(); g != tracev2.NoGoroutine {
+					inGC[g] = true
+				}
+			}
+		case tracev2.EventRangeEnd:
+			r := ev.Range()
+			if handleSTW(r) {
+				stw--
+			} else if handleSweep(r) {
+				ps[ev.Proc()].gc--
+			} else if handleMarkAssist(r) {
+				ps[ev.Proc()].gc--
+				if g := r.Scope.Goroutine(); g != tracev2.NoGoroutine {
+					delete(inGC, g)
+				}
+			}
+		case tracev2.EventStateTransition:
+			st := ev.StateTransition()
+			if st.Resource.Kind != tracev2.ResourceGoroutine {
+				break
+			}
+			old, new := st.Goroutine()
+			g := st.Resource.Goroutine()
+			if inGC[g] || bgMark[g] {
+				if !old.Executing() && new.Executing() {
+					// Started running while doing GC things.
+					ps[ev.Proc()].gc++
+				} else if old.Executing() && !new.Executing() {
+					// Stopped running while doing GC things.
+					ps[ev.Proc()].gc--
+				}
+			}
+			states[g] = new
+		case tracev2.EventLabel:
+			l := ev.Label()
+			if flags&UtilBackground != 0 && strings.HasPrefix(l.Label, "GC ") && l.Label != "GC (idle)" {
+				// Background mark worker.
+				//
+				// If we're in per-proc mode, we don't
+				// count dedicated workers because
+				// they kick all of the goroutines off
+				// that P, so don't directly
+				// contribute to goroutine latency.
+				if !(flags&UtilPerProc != 0 && l.Label == "GC (dedicated)") {
+					bgMark[ev.Goroutine()] = true
+					ps[ev.Proc()].gc++
+				}
+			}
+		}
+
+		if flags&UtilPerProc == 0 {
+			// Compute the current average utilization.
+			if len(ps) == 0 {
+				continue
+			}
+			gcPs := 0
+			if stw > 0 {
+				gcPs = len(ps)
+			} else {
+				for i := range ps {
+					if ps[i].gc > 0 {
+						gcPs++
+					}
+				}
+			}
+			mu := MutatorUtil{int64(ev.Time()), 1 - float64(gcPs)/float64(len(ps))}
+
+			// Record the utilization change. (Since
+			// len(ps) == len(out), we know len(out) > 0.)
+			out[0] = addUtil(out[0], mu)
+		} else {
+			// Check for per-P utilization changes.
+			for i := range ps {
+				p := &ps[i]
+				util := 1.0
+				if stw > 0 || p.gc > 0 {
+					util = 0.0
+				}
+				out[p.series] = addUtil(out[p.series], MutatorUtil{int64(ev.Time()), util})
+			}
+		}
+	}
+
+	// No events in the stream.
+	if lastEv.Kind() == tracev2.EventBad {
+		return nil, nil
+	}
+
+	// Add final 0 utilization event to any remaining series. This
+	// is important to mark the end of the trace. The exact value
+	// shouldn't matter since no window should extend beyond this,
+	// but using 0 is symmetric with the start of the trace.
+	mu := MutatorUtil{int64(lastEv.Time()), 0}
+	for i := range ps {
+		out[ps[i].series] = addUtil(out[ps[i].series], mu)
+	}
+	return out, nil
+}
+
 func addUtil(util []MutatorUtil, mu MutatorUtil) []MutatorUtil {
 	if len(util) > 0 {
 		if mu.Util == util[len(util)-1].Util {
@@ -824,3 +1088,15 @@ func (in *integrator) next(time int64) int64 {
 	}
 	return 1<<63 - 1
 }
+
+func isGCSTW(r tracev2.Range) bool {
+	return strings.HasPrefix(r.Name, "stop-the-world") && strings.Contains(r.Name, "GC")
+}
+
+func isGCMarkAssist(r tracev2.Range) bool {
+	return r.Name == "GC mark assist"
+}
+
+func isGCSweep(r tracev2.Range) bool {
+	return r.Name == "GC incremental sweep"
+}