runtime: access the assist ratio atomically

This change makes it so that the GC assist ratio (the pair of
gcControllerState fields assistBytesPerWork and assistWorkPerByte) is
updated atomically. Note that the pair of fields are not updated
together atomically, but that's OK. The code here was already racy for
some time and in practice the assist ratio moves very slowly.

The purpose of this change is so that we can document
gcController.revise to be safe for concurrent use, which will be useful
in further changes.

Change-Id: Ie25d630207c88e4f85f2b8953f6a0051ebf1b4ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/246963
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>

1 files changed, 43 insertions, 8 deletions

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 5c565a5853..c54f893689 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -388,10 +388,24 @@ type gcControllerState struct {
 	// bytes that should be performed by mutator assists. This is
 	// computed at the beginning of each cycle and updated every
 	// time heap_scan is updated.
-	assistWorkPerByte float64
+	//
+	// Stored as a uint64, but it's actually a float64. Use
+	// float64frombits to get the value.
+	//
+	// Read and written atomically.
+	assistWorkPerByte uint64
 
 	// assistBytesPerWork is 1/assistWorkPerByte.
-	assistBytesPerWork float64
+	//
+	// Stored as a uint64, but it's actually a float64. Use
+	// float64frombits to get the value.
+	//
+	// Read and written atomically.
+	//
+	// Note that because this is read and written independently
+	// from assistWorkPerByte users may notice a skew between
+	// the two values, and such a state should be safe.
+	assistBytesPerWork uint64
 
 	// fractionalUtilizationGoal is the fraction of wall clock
 	// time that should be spent in the fractional mark worker on
@@ -470,7 +484,8 @@ func (c *gcControllerState) startCycle() {
 	c.revise()
 
 	if debug.gcpacertrace > 0 {
-		print("pacer: assist ratio=", c.assistWorkPerByte,
+		assistRatio := float64frombits(atomic.Load64(&c.assistWorkPerByte))
+		print("pacer: assist ratio=", assistRatio,
 			" (scan ", memstats.heap_scan>>20, " MB in ",
 			work.initialHeapLive>>20, "->",
 			memstats.next_gc>>20, " MB)",
@@ -480,9 +495,22 @@ func (c *gcControllerState) startCycle() {
 }
 
 // revise updates the assist ratio during the GC cycle to account for
-// improved estimates. This should be called either under STW or
-// whenever memstats.heap_scan, memstats.heap_live, or
-// memstats.next_gc is updated (with mheap_.lock held).
+// improved estimates. This should be called whenever memstats.heap_scan,
+// memstats.heap_live, or memstats.next_gc is updated. It is safe to
+// call concurrently, but it may race with other calls to revise.
+//
+// The result of this race is that the two assist ratio values may not line
+// up or may be stale. In practice this is OK because the assist ratio
+// moves slowly throughout a GC cycle, and the assist ratio is a best-effort
+// heuristic anyway. Furthermore, no part of the heuristic depends on
+// the two assist ratio values being exact reciprocals of one another, since
+// the two values are used to convert values from different sources.
+//
+// The worst case result of this raciness is that we may miss a larger shift
+// in the ratio (say, if we decide to pace more aggressively against the
+// hard heap goal) but even this "hard goal" is best-effort (see #40460).
+// The dedicated GC should ensure we don't exceed the hard goal by too much
+// in the rare case we do exceed it.
 //
 // It should only be called when gcBlackenEnabled != 0 (because this
 // is when assists are enabled and the necessary statistics are
@@ -555,8 +583,15 @@ func (c *gcControllerState) revise() {
 	// Compute the mutator assist ratio so by the time the mutator
 	// allocates the remaining heap bytes up to next_gc, it will
 	// have done (or stolen) the remaining amount of scan work.
-	c.assistWorkPerByte = float64(scanWorkRemaining) / float64(heapRemaining)
-	c.assistBytesPerWork = float64(heapRemaining) / float64(scanWorkRemaining)
+	// Note that the assist ratio values are updated atomically
+	// but not together. This means there may be some degree of
+	// skew between the two values. This is generally OK as the
+	// values shift relatively slowly over the course of a GC
+	// cycle.
+	assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
+	assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
+	atomic.Store64(&c.assistWorkPerByte, float64bits(assistWorkPerByte))
+	atomic.Store64(&c.assistBytesPerWork, float64bits(assistBytesPerWork))
 }
 
 // endCycle computes the trigger ratio for the next cycle.