runtime: eliminate mheap.busy* lists

The old whole-page reclaimer was the only thing that used the busy
span lists. Remove them so nothing uses them any more.

Change-Id: I4007dd2be08b9ef41bfdb0c387215c73c392cc4c
Reviewed-on: https://go-review.googlesource.com/c/138960
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mheap.go

@@ -32,7 +32,6 @@ type mheap struct {
 	lock      mutex
 	free      mTreap // free and non-scavenged spans
 	scav      mTreap // free and scavenged spans
-	busy      mSpanList // busy list of spans
 	sweepgen  uint32 // sweep generation, see comment in mspan
 	sweepdone uint32 // all spans are swept
 	sweepers  uint32 // number of active sweepone calls
@@ -676,7 +675,7 @@ func (h *mheap) init() {
 	h.spanalloc.zero = false
 
 	// h->mapcache needs no init
-	h.busy.init()
+
 	for i := range h.central {
 		h.central[i].mcentral.init(spanClass(i))
 	}
@@ -893,8 +892,6 @@ func (h *mheap) alloc_m(npage uintptr, spanclass spanClass, large bool) *mspan {
 			mheap_.largealloc += uint64(s.elemsize)
 			mheap_.nlargealloc++
 			atomic.Xadd64(&memstats.heap_live, int64(npage<<_PageShift))
-			// Swept spans are at the end of lists.
-			h.busy.insertBack(s)
 		}
 	}
 	// heap_scan and heap_live were updated.
@@ -1199,9 +1196,6 @@ func (h *mheap) freeSpanLocked(s *mspan, acctinuse, acctidle bool, unusedsince i
 		memstats.heap_idle += uint64(s.npages << _PageShift)
 	}
 	s.state = mSpanFree
-	if s.inList() {
-		h.busy.remove(s)
-	}
 
 	// Stamp newly unused spans. The scavenger will use that
 	// info to potentially give back some pages to the OS.