runtime: create initializer for gcControllerState

Now that gcControllerState contains almost all of the pacer state,
create an initializer for it instead of haphazardly setting some fields.

For #44167.

Change-Id: I4ce1d5dd82003cb7c263fa46697851bb22a32544
Reviewed-on: https://go-review.googlesource.com/c/go/+/306601
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/mgc.go

@@ -153,22 +153,12 @@ func gcinit() {
 	if unsafe.Sizeof(workbuf{}) != _WorkbufSize {
 		throw("size of Workbuf is suboptimal")
 	}
-	gcController.heapMinimum = defaultHeapMinimum
-
 	// No sweep on the first cycle.
 	mheap_.sweepDrained = 1
 
-	// Set a reasonable initial GC trigger.
+	// Initialize GC pacer state.
-	gcController.triggerRatio = 7 / 8.0
+	// Use the environment variable GOGC for the initial gcPercent value.
-
+	gcController.init(readGOGC())
-	// Fake a heapMarked value so it looks like a trigger at
-	// heapMinimum is the appropriate growth from heapMarked.
-	// This will go into computing the initial GC goal.
-	gcController.heapMarked = uint64(float64(gcController.heapMinimum) / (1 + gcController.triggerRatio))
-
-	// Set gcPercent from the environment. This will also compute
-	// and set the GC trigger and goal.
-	_ = setGCPercent(readGOGC())
 
 	work.startSema = 1
 	work.markDoneSema = 1

src/runtime/mgcpacer.go

@@ -243,6 +243,21 @@ type gcControllerState struct {
 	_ cpu.CacheLinePad
 }
 
+func (c *gcControllerState) init(gcPercent int32) {
+	c.heapMinimum = defaultHeapMinimum
+
+	// Set a reasonable initial GC trigger.
+	c.triggerRatio = 7 / 8.0
+
+	// Fake a heapMarked value so it looks like a trigger at
+	// heapMinimum is the appropriate growth from heapMarked.
+	// This will go into computing the initial GC goal.
+	c.heapMarked = uint64(float64(c.heapMinimum) / (1 + c.triggerRatio))
+
+	// This will also compute and set the GC trigger and goal.
+	_ = setGCPercent(gcPercent)
+}
+
 // startCycle resets the GC controller's state and computes estimates
 // for a new GC cycle. The caller must hold worldsema and the world
 // must be stopped.