runtime: capture pause stack for late gcWork put debugging

This captures the stack trace where mark completion observed that each
P had no work, and then dumps this if that P later discovers more
work. Hopefully this will help bound where the work was created.

For #27993.

Change-Id: I4f29202880d22c433482dc1463fb50ab693b6de6
Reviewed-on: https://go-review.googlesource.com/c/154599
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mgc.go

@@ -1453,6 +1453,11 @@ top:
 				// there's a paused gcWork, then
 				// that's a bug.
 				_p_.gcw.pauseGen = gcWorkPauseGen
+				// Capture the G's stack.
+				for i := range _p_.gcw.pauseStack {
+					_p_.gcw.pauseStack[i] = 0
+				}
+				callers(1, _p_.gcw.pauseStack[:])
 			}
 		})
 		casgstatus(gp, _Gwaiting, _Grunning)

src/runtime/mgcwork.go

@@ -97,6 +97,10 @@ type gcWork struct {
 	// pauseGen causes put operations to spin while pauseGen ==
 	// gcWorkPauseGen if debugCachedWork is true.
 	pauseGen uint32
+
+	// pauseStack is the stack at which this P was paused if
+	// debugCachedWork is true.
+	pauseStack [16]uintptr
 }
 
 // Most of the methods of gcWork are go:nowritebarrierrec because the
@@ -128,6 +132,23 @@ func (w *gcWork) checkPut(ptr uintptr, ptrs []uintptr) {
 			for _, ptr := range ptrs {
 				gcDumpObject("ptrs", ptr, ^uintptr(0))
 			}
+			println("runtime: paused at")
+			for _, pc := range w.pauseStack {
+				if pc == 0 {
+					break
+				}
+				f := findfunc(pc)
+				if f.valid() {
+					// Obviously this doesn't
+					// relate to ancestor
+					// tracebacks, but this
+					// function prints what we
+					// want.
+					printAncestorTracebackFuncInfo(f, pc)
+				} else {
+					println("\tunknown PC ", hex(pc), "\n")
+				}
+			}
 			throw("throwOnGCWork")
 		}
 	}