cmd/trace: move process-wide GC events to their own row

Currently, the process-wide GC state is attributed to the P that
happened to perform the allocation that exceeded the GC trigger. This
is pretty arbitrary and makes it hard to see when GC is running since
the GC spans are intermingled with a lot of other trace noise.

The current display is particularly confusing because it creates three
sub-rows in the P row that can overlap each other. Usually a P has
just two sub-rows: one showing the current G and another showing that
G's activity. However, because GC is attributed to a proc, it winds up
as a third row that neither subsumes nor is subsumed by any other row.
This in turn screws up the trace's layout and results in overlapping
events.

Fix these problems by creating a new dedicated row like the existing
"Network" and "Timer" rows and displaying process-wide GC events in
this row. Mark termination and sweep events still appear in their
respective P rows because these are meaningfully attributed.

Change-Id: Ie1a1c6cf8c446e4b043f10f3968f91ff1b546d15
Reviewed-on: https://go-review.googlesource.com/30017Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/cmd/trace/trace.go

@@ -429,6 +429,9 @@ func generateTrace(params *traceParams) ViewerData {
 	ctx.emit(&ViewerEvent{Name: "process_name", Phase: "M", Pid: 1, Arg: &NameArg{"STATS"}})
 	ctx.emit(&ViewerEvent{Name: "process_sort_index", Phase: "M", Pid: 1, Arg: &SortIndexArg{0}})
 
+	ctx.emit(&ViewerEvent{Name: "thread_name", Phase: "M", Pid: 0, Tid: trace.GCP, Arg: &NameArg{"GC"}})
+	ctx.emit(&ViewerEvent{Name: "thread_sort_index", Phase: "M", Pid: 0, Tid: trace.GCP, Arg: &SortIndexArg{-6}})
+
 	ctx.emit(&ViewerEvent{Name: "thread_name", Phase: "M", Pid: 0, Tid: trace.NetpollP, Arg: &NameArg{"Network"}})
 	ctx.emit(&ViewerEvent{Name: "thread_sort_index", Phase: "M", Pid: 0, Tid: trace.NetpollP, Arg: &SortIndexArg{-5}})
 

src/internal/trace/parser.go

@@ -56,6 +56,7 @@ const (
 	TimerP   // depicts timer unblocks
 	NetpollP // depicts network unblocks
 	SyscallP // depicts returns from syscalls
+	GCP      // depicts GC state
 )
 
 // Parse parses, post-processes and verifies the trace.
@@ -548,6 +549,8 @@ func postProcessTrace(ver int, events []*Event) error {
 				return fmt.Errorf("previous GC is not ended before a new one (offset %v, time %v)", ev.Off, ev.Ts)
 			}
 			evGC = ev
+			// Attribute this to the global GC state.
+			ev.P = GCP
 		case EvGCDone:
 			if evGC == nil {
 				return fmt.Errorf("bogus GC end (offset %v, time %v)", ev.Off, ev.Ts)