runtime: Expose MemStat for last N GCs.

Change-Id: I0c85c8dd8086a942293479effcbf3bc10d0f1edd

src/runtime/mgc.go

@@ -1658,14 +1658,18 @@ func gcMarkTermination(nextTriggerRatio float64) {
 	// Free stack spans. This must be done between GC cycles.
 	systemstack(freeStackSpans)
 
-	// Print gctrace before dropping worldsema. As soon as we drop
+	// Remember and print gctrace before dropping worldsema. As soon as we drop
 	// worldsema another cycle could start and smash the stats
 	// we're trying to print.
+	var stats MemStats
+	getmemstats(&stats)
+	gcstats[memstats.numgc%uint32(len(gcstats))] = stats
 	if debug.gctrace > 0 {
 		util := int(memstats.gc_cpu_fraction * 100)
 
 		var sbuf [24]byte
 		printlock()
+
 		print("gc ", memstats.numgc,
 			" @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
 			util, "%: ")

src/runtime/mstats.go

@@ -428,6 +428,11 @@ type MemStats struct {
 	}
 }
 
+// Circular buffer of stats from past GC.
+//
+// TODO(dhananjayn): Take buffer size as input from GODEBUG?
+var gcstats [256]MemStats
+
 // Size of the trailing by_size array differs between mstats and MemStats,
 // and all data after by_size is local to runtime, not exported.
 // NumSizeClasses was changed, but we cannot change MemStats because of backward compatibility.
@@ -464,9 +469,24 @@ func ReadMemStats(m *MemStats) {
 	startTheWorld()
 }
 
+// ReadAllMemStats returns memstats for past GCs.
+func ReadAllMemStats(stats *[]MemStats) {
+	stopTheWorld("read all mem stats")
+
+	systemstack(func() {
+		copy(*stats, gcstats[:])
+	})
+
+	startTheWorld()
+}
+
 func readmemstats_m(stats *MemStats) {
 	updatememstats()
+	getmemstats(stats)
+}
 
+// converts memstats to MemStats
+func getmemstats(stats *MemStats) {
 	// The size of the trailing by_size array differs between
 	// mstats and MemStats. NumSizeClasses was changed, but we
 	// cannot change MemStats because of backward compatibility.