runtime: doubly-linked sched.midle list

This will be used by CL 714801 to remove Ms from the middle of the list.

We could simply convert schedlink to the doubly-linked list, bringing
along all other uses of schedlink.

However, CL 714801 removes Ms from the middle of the midle list. It
would be an easy mistake to make to accidentally remove an M from
schedlink, assuming that it is on the midle list when it is actually on
a completely different list. Using separate a list node makes this
impossible.

For #65694.

Change-Id: I6a6a636c223d925fdc30c0c648460cbf5c2af4d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/714800
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

src/runtime/proc.go

@@ -849,6 +849,8 @@ func schedinit() {
 
 	lockVerifyMSize()
 
+	sched.midle.init(unsafe.Offsetof(m{}.idleNode))
+
 	// raceinit must be the first call to race detector.
 	// In particular, it must be done before mallocinit below calls racemapshadow.
 	gp := getg()
@@ -6976,8 +6978,7 @@ func schedEnabled(gp *g) bool {
 func mput(mp *m) {
 	assertLockHeld(&sched.lock)
 
-	mp.schedlink = sched.midle
-	sched.midle.set(mp)
+	sched.midle.push(unsafe.Pointer(mp))
 	sched.nmidle++
 	checkdead()
 }
@@ -6990,9 +6991,8 @@ func mput(mp *m) {
 func mget() *m {
 	assertLockHeld(&sched.lock)
 
-	mp := sched.midle.ptr()
+	mp := (*m)(sched.midle.pop())
 	if mp != nil {
-		sched.midle = mp.schedlink
 		sched.nmidle--
 	}
 	return mp

src/runtime/runtime2.go

@@ -665,6 +665,7 @@ type m struct {
 	park            note
 	alllink         *m // on allm
 	schedlink       muintptr
+	idleNode        listNodeManual
 	lockedg         guintptr
 	createstack     [32]uintptr // stack that created this thread, it's used for StackRecord.Stack0, so it must align with it.
 	lockedExt       uint32      // tracking for external LockOSThread
@@ -875,7 +876,7 @@ type schedt struct {
 	// When increasing nmidle, nmidlelocked, nmsys, or nmfreed, be
 	// sure to call checkdead().
 
-	midle        muintptr // idle m's waiting for work
+	midle        listHeadManual // idle m's waiting for work
 	nmidle       int32    // number of idle m's waiting for work
 	nmidlelocked int32    // number of locked m's waiting for work
 	mnext        int64    // number of m's that have been created and next M ID