func newextram() {
	c := extraMWaiters.Swap(0)
	if c > 0 {
		for i := uint32(0); i < c; i++ {
			oneNewExtraM()
		}
	} else if extraMLength.Load() == 0 {
		// Make sure there is at least one extra M.
		oneNewExtraM()
	}
}

// oneNewExtraM allocates an m and puts it on the extra list.
func oneNewExtraM() {
	// Create extra goroutine locked to extra m.