Adding upstream version 1.21.8.upstream/1.21.8

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 484 insertions, 0 deletions

diff --git a/src/runtime/mgclimit.go b/src/runtime/mgclimit.go
new file mode 100644
index 0000000..ef3cc08
--- /dev/null
+++ b/src/runtime/mgclimit.go
@@ -0,0 +1,484 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+import "runtime/internal/atomic"
+
+// gcCPULimiter is a mechanism to limit GC CPU utilization in situations
+// where it might become excessive and inhibit application progress (e.g.
+// a death spiral).
+//
+// The core of the limiter is a leaky bucket mechanism that fills with GC
+// CPU time and drains with mutator time. Because the bucket fills and
+// drains with time directly (i.e. without any weighting), this effectively
+// sets a very conservative limit of 50%. This limit could be enforced directly,
+// however, but the purpose of the bucket is to accommodate spikes in GC CPU
+// utilization without hurting throughput.
+//
+// Note that the bucket in the leaky bucket mechanism can never go negative,
+// so the GC never gets credit for a lot of CPU time spent without the GC
+// running. This is intentional, as an application that stays idle for, say,
+// an entire day, could build up enough credit to fail to prevent a death
+// spiral the following day. The bucket's capacity is the GC's only leeway.
+//
+// The capacity thus also sets the window the limiter considers. For example,
+// if the capacity of the bucket is 1 cpu-second, then the limiter will not
+// kick in until at least 1 full cpu-second in the last 2 cpu-second window
+// is spent on GC CPU time.
+var gcCPULimiter gcCPULimiterState
+
+type gcCPULimiterState struct {
+	lock atomic.Uint32
+
+	enabled atomic.Bool
+	bucket  struct {
+		// Invariants:
+		// - fill >= 0
+		// - capacity >= 0
+		// - fill <= capacity
+		fill, capacity uint64
+	}
+	// overflow is the cumulative amount of GC CPU time that we tried to fill the
+	// bucket with but exceeded its capacity.
+	overflow uint64
+
+	// gcEnabled is an internal copy of gcBlackenEnabled that determines
+	// whether the limiter tracks total assist time.
+	//
+	// gcBlackenEnabled isn't used directly so as to keep this structure
+	// unit-testable.
+	gcEnabled bool
+
+	// transitioning is true when the GC is in a STW and transitioning between
+	// the mark and sweep phases.
+	transitioning bool
+
+	// assistTimePool is the accumulated assist time since the last update.
+	assistTimePool atomic.Int64
+
+	// idleMarkTimePool is the accumulated idle mark time since the last update.
+	idleMarkTimePool atomic.Int64
+
+	// idleTimePool is the accumulated time Ps spent on the idle list since the last update.
+	idleTimePool atomic.Int64
+
+	// lastUpdate is the nanotime timestamp of the last time update was called.
+	//
+	// Updated under lock, but may be read concurrently.
+	lastUpdate atomic.Int64
+
+	// lastEnabledCycle is the GC cycle that last had the limiter enabled.
+	lastEnabledCycle atomic.Uint32
+
+	// nprocs is an internal copy of gomaxprocs, used to determine total available
+	// CPU time.
+	//
+	// gomaxprocs isn't used directly so as to keep this structure unit-testable.
+	nprocs int32
+
+	// test indicates whether this instance of the struct was made for testing purposes.
+	test bool
+}
+
+// limiting returns true if the CPU limiter is currently enabled, meaning the Go GC
+// should take action to limit CPU utilization.
+//
+// It is safe to call concurrently with other operations.
+func (l *gcCPULimiterState) limiting() bool {
+	return l.enabled.Load()
+}
+
+// startGCTransition notifies the limiter of a GC transition.
+//
+// This call takes ownership of the limiter and disables all other means of
+// updating the limiter. Release ownership by calling finishGCTransition.
+//
+// It is safe to call concurrently with other operations.
+func (l *gcCPULimiterState) startGCTransition(enableGC bool, now int64) {
+	if !l.tryLock() {
+		// This must happen during a STW, so we can't fail to acquire the lock.
+		// If we did, something went wrong. Throw.
+		throw("failed to acquire lock to start a GC transition")
+	}
+	if l.gcEnabled == enableGC {
+		throw("transitioning GC to the same state as before?")
+	}
+	// Flush whatever was left between the last update and now.
+	l.updateLocked(now)
+	l.gcEnabled = enableGC
+	l.transitioning = true
+	// N.B. finishGCTransition releases the lock.
+	//
+	// We don't release here to increase the chance that if there's a failure
+	// to finish the transition, that we throw on failing to acquire the lock.
+}
+
+// finishGCTransition notifies the limiter that the GC transition is complete
+// and releases ownership of it. It also accumulates STW time in the bucket.
+// now must be the timestamp from the end of the STW pause.
+func (l *gcCPULimiterState) finishGCTransition(now int64) {
+	if !l.transitioning {
+		throw("finishGCTransition called without starting one?")
+	}
+	// Count the full nprocs set of CPU time because the world is stopped
+	// between startGCTransition and finishGCTransition. Even though the GC
+	// isn't running on all CPUs, it is preventing user code from doing so,
+	// so it might as well be.
+	if lastUpdate := l.lastUpdate.Load(); now >= lastUpdate {
+		l.accumulate(0, (now-lastUpdate)*int64(l.nprocs))
+	}
+	l.lastUpdate.Store(now)
+	l.transitioning = false
+	l.unlock()
+}
+
+// gcCPULimiterUpdatePeriod dictates the maximum amount of wall-clock time
+// we can go before updating the limiter.
+const gcCPULimiterUpdatePeriod = 10e6 // 10ms
+
+// needUpdate returns true if the limiter's maximum update period has been
+// exceeded, and so would benefit from an update.
+func (l *gcCPULimiterState) needUpdate(now int64) bool {
+	return now-l.lastUpdate.Load() > gcCPULimiterUpdatePeriod
+}
+
+// addAssistTime notifies the limiter of additional assist time. It will be
+// included in the next update.
+func (l *gcCPULimiterState) addAssistTime(t int64) {
+	l.assistTimePool.Add(t)
+}
+
+// addIdleTime notifies the limiter of additional time a P spent on the idle list. It will be
+// subtracted from the total CPU time in the next update.
+func (l *gcCPULimiterState) addIdleTime(t int64) {
+	l.idleTimePool.Add(t)
+}
+
+// update updates the bucket given runtime-specific information. now is the
+// current monotonic time in nanoseconds.
+//
+// This is safe to call concurrently with other operations, except *GCTransition.
+func (l *gcCPULimiterState) update(now int64) {
+	if !l.tryLock() {
+		// We failed to acquire the lock, which means something else is currently
+		// updating. Just drop our update, the next one to update will include
+		// our total assist time.
+		return
+	}
+	if l.transitioning {
+		throw("update during transition")
+	}
+	l.updateLocked(now)
+	l.unlock()
+}
+
+// updateLocked is the implementation of update. l.lock must be held.
+func (l *gcCPULimiterState) updateLocked(now int64) {
+	lastUpdate := l.lastUpdate.Load()
+	if now < lastUpdate {
+		// Defensively avoid overflow. This isn't even the latest update anyway.
+		return
+	}
+	windowTotalTime := (now - lastUpdate) * int64(l.nprocs)
+	l.lastUpdate.Store(now)
+
+	// Drain the pool of assist time.
+	assistTime := l.assistTimePool.Load()
+	if assistTime != 0 {
+		l.assistTimePool.Add(-assistTime)
+	}
+
+	// Drain the pool of idle time.
+	idleTime := l.idleTimePool.Load()
+	if idleTime != 0 {
+		l.idleTimePool.Add(-idleTime)
+	}
+
+	if !l.test {
+		// Consume time from in-flight events. Make sure we're not preemptible so allp can't change.
+		//
+		// The reason we do this instead of just waiting for those events to finish and push updates
+		// is to ensure that all the time we're accounting for happened sometime between lastUpdate
+		// and now. This dramatically simplifies reasoning about the limiter because we're not at
+		// risk of extra time being accounted for in this window than actually happened in this window,
+		// leading to all sorts of weird transient behavior.
+		mp := acquirem()
+		for _, pp := range allp {
+			typ, duration := pp.limiterEvent.consume(now)
+			switch typ {
+			case limiterEventIdleMarkWork:
+				fallthrough
+			case limiterEventIdle:
+				idleTime += duration
+				sched.idleTime.Add(duration)
+			case limiterEventMarkAssist:
+				fallthrough
+			case limiterEventScavengeAssist:
+				assistTime += duration
+			case limiterEventNone:
+				break
+			default:
+				throw("invalid limiter event type found")
+			}
+		}
+		releasem(mp)
+	}
+
+	// Compute total GC time.
+	windowGCTime := assistTime
+	if l.gcEnabled {
+		windowGCTime += int64(float64(windowTotalTime) * gcBackgroundUtilization)
+	}
+
+	// Subtract out all idle time from the total time. Do this after computing
+	// GC time, because the background utilization is dependent on the *real*
+	// total time, not the total time after idle time is subtracted.
+	//
+	// Idle time is counted as any time that a P is on the P idle list plus idle mark
+	// time. Idle mark workers soak up time that the application spends idle.
+	//
+	// On a heavily undersubscribed system, any additional idle time can skew GC CPU
+	// utilization, because the GC might be executing continuously and thrashing,
+	// yet the CPU utilization with respect to GOMAXPROCS will be quite low, so
+	// the limiter fails to turn on. By subtracting idle time, we're removing time that
+	// we know the application was idle giving a more accurate picture of whether
+	// the GC is thrashing.
+	//
+	// Note that this can cause the limiter to turn on even if it's not needed. For
+	// instance, on a system with 32 Ps but only 1 running goroutine, each GC will have
+	// 8 dedicated GC workers. Assuming the GC cycle is half mark phase and half sweep
+	// phase, then the GC CPU utilization over that cycle, with idle time removed, will
+	// be 8/(8+2) = 80%. Even though the limiter turns on, though, assist should be
+	// unnecessary, as the GC has way more CPU time to outpace the 1 goroutine that's
+	// running.
+	windowTotalTime -= idleTime
+
+	l.accumulate(windowTotalTime-windowGCTime, windowGCTime)
+}
+
+// accumulate adds time to the bucket and signals whether the limiter is enabled.
+//
+// This is an internal function that deals just with the bucket. Prefer update.
+// l.lock must be held.
+func (l *gcCPULimiterState) accumulate(mutatorTime, gcTime int64) {
+	headroom := l.bucket.capacity - l.bucket.fill
+	enabled := headroom == 0
+
+	// Let's be careful about three things here:
+	// 1. The addition and subtraction, for the invariants.
+	// 2. Overflow.
+	// 3. Excessive mutation of l.enabled, which is accessed
+	//    by all assists, potentially more than once.
+	change := gcTime - mutatorTime
+
+	// Handle limiting case.
+	if change > 0 && headroom <= uint64(change) {
+		l.overflow += uint64(change) - headroom
+		l.bucket.fill = l.bucket.capacity
+		if !enabled {
+			l.enabled.Store(true)
+			l.lastEnabledCycle.Store(memstats.numgc + 1)
+		}
+		return
+	}
+
+	// Handle non-limiting cases.
+	if change < 0 && l.bucket.fill <= uint64(-change) {
+		// Bucket emptied.
+		l.bucket.fill = 0
+	} else {
+		// All other cases.
+		l.bucket.fill -= uint64(-change)
+	}
+	if change != 0 && enabled {
+		l.enabled.Store(false)
+	}
+}
+
+// tryLock attempts to lock l. Returns true on success.
+func (l *gcCPULimiterState) tryLock() bool {
+	return l.lock.CompareAndSwap(0, 1)
+}
+
+// unlock releases the lock on l. Must be called if tryLock returns true.
+func (l *gcCPULimiterState) unlock() {
+	old := l.lock.Swap(0)
+	if old != 1 {
+		throw("double unlock")
+	}
+}
+
+// capacityPerProc is the limiter's bucket capacity for each P in GOMAXPROCS.
+const capacityPerProc = 1e9 // 1 second in nanoseconds
+
+// resetCapacity updates the capacity based on GOMAXPROCS. Must not be called
+// while the GC is enabled.
+//
+// It is safe to call concurrently with other operations.
+func (l *gcCPULimiterState) resetCapacity(now int64, nprocs int32) {
+	if !l.tryLock() {
+		// This must happen during a STW, so we can't fail to acquire the lock.
+		// If we did, something went wrong. Throw.
+		throw("failed to acquire lock to reset capacity")
+	}
+	// Flush the rest of the time for this period.
+	l.updateLocked(now)
+	l.nprocs = nprocs
+
+	l.bucket.capacity = uint64(nprocs) * capacityPerProc
+	if l.bucket.fill > l.bucket.capacity {
+		l.bucket.fill = l.bucket.capacity
+		l.enabled.Store(true)
+		l.lastEnabledCycle.Store(memstats.numgc + 1)
+	} else if l.bucket.fill < l.bucket.capacity {
+		l.enabled.Store(false)
+	}
+	l.unlock()
+}
+
+// limiterEventType indicates the type of an event occurring on some P.
+//
+// These events represent the full set of events that the GC CPU limiter tracks
+// to execute its function.
+//
+// This type may use no more than limiterEventBits bits of information.
+type limiterEventType uint8
+
+const (
+	limiterEventNone           limiterEventType = iota // None of the following events.
+	limiterEventIdleMarkWork                           // Refers to an idle mark worker (see gcMarkWorkerMode).
+	limiterEventMarkAssist                             // Refers to mark assist (see gcAssistAlloc).
+	limiterEventScavengeAssist                         // Refers to a scavenge assist (see allocSpan).
+	limiterEventIdle                                   // Refers to time a P spent on the idle list.
+
+	limiterEventBits = 3
+)
+
+// limiterEventTypeMask is a mask for the bits in p.limiterEventStart that represent
+// the event type. The rest of the bits of that field represent a timestamp.
+const (
+	limiterEventTypeMask  = uint64((1<<limiterEventBits)-1) << (64 - limiterEventBits)
+	limiterEventStampNone = limiterEventStamp(0)
+)
+
+// limiterEventStamp is a nanotime timestamp packed with a limiterEventType.
+type limiterEventStamp uint64
+
+// makeLimiterEventStamp creates a new stamp from the event type and the current timestamp.
+func makeLimiterEventStamp(typ limiterEventType, now int64) limiterEventStamp {
+	return limiterEventStamp(uint64(typ)<<(64-limiterEventBits) | (uint64(now) &^ limiterEventTypeMask))
+}
+
+// duration computes the difference between now and the start time stored in the stamp.
+//
+// Returns 0 if the difference is negative, which may happen if now is stale or if the
+// before and after timestamps cross a 2^(64-limiterEventBits) boundary.
+func (s limiterEventStamp) duration(now int64) int64 {
+	// The top limiterEventBits bits of the timestamp are derived from the current time
+	// when computing a duration.
+	start := int64((uint64(now) & limiterEventTypeMask) | (uint64(s) &^ limiterEventTypeMask))
+	if now < start {
+		return 0
+	}
+	return now - start
+}
+
+// type extracts the event type from the stamp.
+func (s limiterEventStamp) typ() limiterEventType {
+	return limiterEventType(s >> (64 - limiterEventBits))
+}
+
+// limiterEvent represents tracking state for an event tracked by the GC CPU limiter.
+type limiterEvent struct {
+	stamp atomic.Uint64 // Stores a limiterEventStamp.
+}
+
+// start begins tracking a new limiter event of the current type. If an event
+// is already in flight, then a new event cannot begin because the current time is
+// already being attributed to that event. In this case, this function returns false.
+// Otherwise, it returns true.
+//
+// The caller must be non-preemptible until at least stop is called or this function
+// returns false. Because this is trying to measure "on-CPU" time of some event, getting
+// scheduled away during it can mean that whatever we're measuring isn't a reflection
+// of "on-CPU" time. The OS could deschedule us at any time, but we want to maintain as
+// close of an approximation as we can.
+func (e *limiterEvent) start(typ limiterEventType, now int64) bool {
+	if limiterEventStamp(e.stamp.Load()).typ() != limiterEventNone {
+		return false
+	}
+	e.stamp.Store(uint64(makeLimiterEventStamp(typ, now)))
+	return true
+}
+
+// consume acquires the partial event CPU time from any in-flight event.
+// It achieves this by storing the current time as the new event time.
+//
+// Returns the type of the in-flight event, as well as how long it's currently been
+// executing for. Returns limiterEventNone if no event is active.
+func (e *limiterEvent) consume(now int64) (typ limiterEventType, duration int64) {
+	// Read the limiter event timestamp and update it to now.
+	for {
+		old := limiterEventStamp(e.stamp.Load())
+		typ = old.typ()
+		if typ == limiterEventNone {
+			// There's no in-flight event, so just push that up.
+			return
+		}
+		duration = old.duration(now)
+		if duration == 0 {
+			// We might have a stale now value, or this crossed the
+			// 2^(64-limiterEventBits) boundary in the clock readings.
+			// Just ignore it.
+			return limiterEventNone, 0
+		}
+		new := makeLimiterEventStamp(typ, now)
+		if e.stamp.CompareAndSwap(uint64(old), uint64(new)) {
+			break
+		}
+	}
+	return
+}
+
+// stop stops the active limiter event. Throws if the
+//
+// The caller must be non-preemptible across the event. See start as to why.
+func (e *limiterEvent) stop(typ limiterEventType, now int64) {
+	var stamp limiterEventStamp
+	for {
+		stamp = limiterEventStamp(e.stamp.Load())
+		if stamp.typ() != typ {
+			print("runtime: want=", typ, " got=", stamp.typ(), "\n")
+			throw("limiterEvent.stop: found wrong event in p's limiter event slot")
+		}
+		if e.stamp.CompareAndSwap(uint64(stamp), uint64(limiterEventStampNone)) {
+			break
+		}
+	}
+	duration := stamp.duration(now)
+	if duration == 0 {
+		// It's possible that we're missing time because we crossed a
+		// 2^(64-limiterEventBits) boundary between the start and end.
+		// In this case, we're dropping that information. This is OK because
+		// at worst it'll cause a transient hiccup that will quickly resolve
+		// itself as all new timestamps begin on the other side of the boundary.
+		// Such a hiccup should be incredibly rare.
+		return
+	}
+	// Account for the event.
+	switch typ {
+	case limiterEventIdleMarkWork:
+		gcCPULimiter.addIdleTime(duration)
+	case limiterEventIdle:
+		gcCPULimiter.addIdleTime(duration)
+		sched.idleTime.Add(duration)
+	case limiterEventMarkAssist:
+		fallthrough
+	case limiterEventScavengeAssist:
+		gcCPULimiter.addAssistTime(duration)
+	default:
+		throw("limiterEvent.stop: invalid limiter event type found")
+	}
+}