runtime: make newproc1 not start the goroutine

Currently, newproc1 allocates, initializes, and schedules a new
goroutine. We're about to change debug call injection in a way that
will need to create a new goroutine without immediately scheduling it.
To prepare for that, make scheduling the responsibility of newproc1's
caller. Currently, there's exactly one caller (newproc), so this
simply shifts that responsibility.

For #36365.

Change-Id: Idacd06b63e738982e840fe995d891bfd377ce23b
Reviewed-on: https://go-review.googlesource.com/c/go/+/229298
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/proc.go

@@ -3454,23 +3454,44 @@ func malg(stacksize int32) *g {
 // Create a new g running fn with siz bytes of arguments.
 // Put it on the queue of g's waiting to run.
 // The compiler turns a go statement into a call to this.
-// Cannot split the stack because it assumes that the arguments
-// are available sequentially after &fn; they would not be
-// copied if a stack split occurred.
+//
+// The stack layout of this call is unusual: it assumes that the
+// arguments to pass to fn are on the stack sequentially immediately
+// after &fn. Hence, they are logically part of newproc's argument
+// frame, even though they don't appear in its signature (and can't
+// because their types differ between call sites).
+//
+// This must be nosplit because this stack layout means there are
+// untyped arguments in newproc's argument frame. Stack copies won't
+// be able to adjust them and stack splits won't be able to copy them.
+//
 //go:nosplit
 func newproc(siz int32, fn *funcval) {
 	argp := add(unsafe.Pointer(&fn), sys.PtrSize)
 	gp := getg()
 	pc := getcallerpc()
 	systemstack(func() {
-		newproc1(fn, argp, siz, gp, pc)
+		newg := newproc1(fn, argp, siz, gp, pc)
+
+		_p_ := getg().m.p.ptr()
+		runqput(_p_, newg, true)
+
+		if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 && mainStarted {
+			wakep()
+		}
 	})
 }
 
-// Create a new g running fn with narg bytes of arguments starting
-// at argp. callerpc is the address of the go statement that created
-// this. The new g is put on the queue of g's waiting to run.
-func newproc1(fn *funcval, argp unsafe.Pointer, narg int32, callergp *g, callerpc uintptr) {
+// Create a new g in state _Grunnable, starting at fn, with narg bytes
+// of arguments starting at argp. callerpc is the address of the go
+// statement that created this. The caller is responsible for adding
+// the new g to the scheduler.
+//
+// This must run on the system stack because it's the continuation of
+// newproc, which cannot split the stack.
+//
+//go:systemstack
+func newproc1(fn *funcval, argp unsafe.Pointer, narg int32, callergp *g, callerpc uintptr) *g {
 	_g_ := getg()
 
 	if fn == nil {
@@ -3566,12 +3587,9 @@ func newproc1(fn *funcval, argp unsafe.Pointer, narg int32, callergp *g, callerp
 	if trace.enabled {
 		traceGoCreate(newg, newg.startpc)
 	}
-	runqput(_p_, newg, true)
-
-	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 && mainStarted {
-		wakep()
-	}
 	releasem(_g_.m)
+
+	return newg
 }
 
 // saveAncestors copies previous ancestors of the given caller g and