runtime: add testing framework and basic tests for GC pacer

This change creates a formal exported interface for the GC pacer and
creates tests for it that simulate some series of GC cycles. The tests
are completely driven by the real pacer implementation, except for
assists, which are idealized (though revise is called repeatedly).

For #44167.

Change-Id: I0112242b07e7702595ca71001d781ad6c1fddd2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/353354
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

Author: mknyszek

src/runtime/export_test.go

@@ -1230,3 +1230,81 @@ func GCTestPointerClass(p unsafe.Pointer) string {
 }
 
 const Raceenabled = raceenabled
+
+const (
+	GCBackgroundUtilization = gcBackgroundUtilization
+	GCGoalUtilization       = gcGoalUtilization
+)
+
+type GCController struct {
+	gcControllerState
+}
+
+func NewGCController(gcPercent int) *GCController {
+	// Force the controller to escape. We're going to
+	// do 64-bit atomics on it, and if it gets stack-allocated
+	// on a 32-bit architecture, it may get allocated unaligned
+	// space.
+	g := escape(new(GCController)).(*GCController)
+	g.init(int32(gcPercent))
+	return g
+}
+
+func (c *GCController) StartCycle(stackSize, globalsSize uint64, scannableFrac float64, gomaxprocs int) {
+	c.scannableStackSize = stackSize
+	c.globalsScan = globalsSize
+	c.heapLive = c.trigger
+	c.heapScan += uint64(float64(c.trigger-c.heapMarked) * scannableFrac)
+	c.startCycle(0, gomaxprocs)
+}
+
+func (c *GCController) AssistWorkPerByte() float64 {
+	return c.assistWorkPerByte.Load()
+}
+
+func (c *GCController) HeapGoal() uint64 {
+	return c.heapGoal
+}
+
+func (c *GCController) HeapLive() uint64 {
+	return c.heapLive
+}
+
+func (c *GCController) HeapMarked() uint64 {
+	return c.heapMarked
+}
+
+func (c *GCController) Trigger() uint64 {
+	return c.trigger
+}
+
+type GCControllerReviseDelta struct {
+	HeapLive        int64
+	HeapScan        int64
+	HeapScanWork    int64
+	StackScanWork   int64
+	GlobalsScanWork int64
+}
+
+func (c *GCController) Revise(d GCControllerReviseDelta) {
+	c.heapLive += uint64(d.HeapLive)
+	c.heapScan += uint64(d.HeapScan)
+	c.scanWork += d.HeapScanWork + d.StackScanWork + d.GlobalsScanWork
+	c.revise()
+}
+
+func (c *GCController) EndCycle(bytesMarked uint64, assistTime, elapsed int64, gomaxprocs int) {
+	c.assistTime = assistTime
+	triggerRatio := c.endCycle(elapsed, gomaxprocs, false)
+	c.resetLive(bytesMarked)
+	c.commit(triggerRatio)
+}
+
+var escapeSink interface{}
+
+//go:noinline
+func escape(x interface{}) interface{} {
+	escapeSink = x
+	escapeSink = nil
+	return x
+}