runtime: track the number of free unscavenged huge pages

This change tracks the number of potential free and unscavenged huge
pages which will be used to inform the rate at which scavenging should
occur.

For #30333.

Change-Id: I47663e5ffb64cac44ffa10db158486783f707479
Reviewed-on: https://go-review.googlesource.com/c/go/+/170860
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 852f37409e..3c3e110f89 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -36,6 +36,8 @@ var Atoi32 = atoi32
 
 var Nanotime = nanotime
 
+var PhysHugePageSize = physHugePageSize
+
 type LFNode struct {
 	Next    uint64
 	Pushcnt uintptr
@@ -516,6 +518,26 @@ func MapTombstoneCheck(m map[int]int) {
 	}
 }
 
+// UnscavHugePagesSlow returns the value of mheap_.freeHugePages
+// and the number of unscavenged huge pages calculated by
+// scanning the heap.
+func UnscavHugePagesSlow() (uintptr, uintptr) {
+	var base, slow uintptr
+	// Run on the system stack to avoid deadlock from stack growth
+	// trying to acquire the heap lock.
+	systemstack(func() {
+		lock(&mheap_.lock)
+		base = mheap_.free.unscavHugePages
+		for _, s := range mheap_.allspans {
+			if s.state == mSpanFree && !s.scavenged {
+				slow += s.hugePages()
+			}
+		}
+		unlock(&mheap_.lock)
+	})
+	return base, slow
+}
+
 // Span is a safe wrapper around an mspan, whose memory
 // is managed manually.
 type Span struct {