Add the GC_GLOBAL_SYNC event marking that all caps are stopped for GC

Quoting design rationale by dcoutts: The event indicates that we're doing
a stop-the-world GC and all other HECs should be between their GC_START
and GC_END events at that moment. We don't want to use GC_STATS_GHC
for that, because GC_STATS_GHC is for extra GHC-specific info,
not something we have to rely on to be able to match the GC pauses
across HECs to a particular global GC.

5 files changed, 23 insertions, 1 deletions

diff --git a/rts/RtsProbes.d b/rts/RtsProbes.d
index 8d24f3a9ca..f5470dfe0b 100644
--- a/rts/RtsProbes.d
+++ b/rts/RtsProbes.d
@@ -55,6 +55,7 @@ provider HaskellEvent {
   probe gc__idle (EventCapNo);
   probe gc__work (EventCapNo);
   probe gc__done (EventCapNo);
+  probe gc__sync (EventCapNo);
   probe gc__stats (CapsetID, StgWord, StgWord, StgWord, StgWord, StgWord, StgWord, StgWord);
   probe heap__info (CapsetID, StgWord, StgWord, StgWord, StgWord, StgWord);
   probe heap__allocated (EventCapNo, CapsetID, StgWord64);
diff --git a/rts/Stats.c b/rts/Stats.c
index ddc9bb96ae..cebb75343f 100644
--- a/rts/Stats.c
+++ b/rts/Stats.c
@@ -349,6 +349,13 @@ stat_endGC (Capability *cap, gc_thread *gct,
     {
         Time cpu, elapsed, gc_cpu, gc_elapsed;
 
+        // Has to be emitted while all caps stopped for GC, but before GC_END.
+        // See trac.haskell.org/ThreadScope/wiki/RTSsummaryEvents
+        // for a detailed design rationale of the current setup
+        // of GC eventlog events.
+        traceEventGcGlobalSync(cap);
+	
+        // Emitted before GC_END on all caps, which simplifies tools code.
         traceEventGcStats(cap,
                           CAPSET_HEAP_DEFAULT,
                           gen,
@@ -360,7 +367,7 @@ stat_endGC (Capability *cap, gc_thread *gct,
                           par_n_threads,
                           par_max_copied * sizeof(W_),
                           par_tot_copied * sizeof(W_));
-	
+
         getProcessTimes(&cpu, &elapsed);
 
         // Post EVENT_GC_END with the same timestamp as used for stats
diff --git a/rts/Trace.c b/rts/Trace.c
index 995f8a2200..089bf24423 100644
--- a/rts/Trace.c
+++ b/rts/Trace.c
@@ -282,6 +282,9 @@ static void traceGcEvent_stderr (Capability *cap, EventTypeNum tag)
       case EVENT_GC_DONE:         // (cap)
           debugBelch("cap %d: GC done\n", cap->no);
           break;
+      case EVENT_GC_GLOBAL_SYNC:  // (cap)
+          debugBelch("cap %d: all caps stopped for GC\n", cap->no);
+          break;
       default:
           barf("traceGcEvent: unknown event tag %d", tag);
           break;
diff --git a/rts/Trace.h b/rts/Trace.h
index 062666b503..c4c4e41608 100644
--- a/rts/Trace.h
+++ b/rts/Trace.h
@@ -350,6 +350,8 @@ INLINE_HEADER void dtraceStartup (int num_caps) {
     HASKELLEVENT_GC_WORK(cap)
 #define dtraceGcDone(cap)                               \
     HASKELLEVENT_GC_DONE(cap)
+#define dtraceGcGlobalSync(cap)                         \
+    HASKELLEVENT_GC_GLOBAL_SYNC(cap)
 #define dtraceEventGcStats(heap_capset, gens,           \
                            copies, slop, fragmentation, \
                            par_n_threads,               \
@@ -419,6 +421,7 @@ INLINE_HEADER void dtraceStartup (int num_caps STG_UNUSED) {};
 #define dtraceGcIdle(cap)                               /* nothing */
 #define dtraceGcWork(cap)                               /* nothing */
 #define dtraceGcDone(cap)                               /* nothing */
+#define dtraceGcGlobalSync(cap)                         /* nothing */
 #define dtraceEventGcStats(heap_capset, gens,           \
                            copies, slop, fragmentation, \
                            par_n_threads,               \
@@ -610,6 +613,12 @@ INLINE_HEADER void traceEventGcDone(Capability *cap STG_UNUSED)
     dtraceGcDone((EventCapNo)cap->no);
 }
 
+INLINE_HEADER void traceEventGcGlobalSync(Capability *cap STG_UNUSED)
+{
+    traceGcEvent(cap, EVENT_GC_GLOBAL_SYNC);
+    dtraceGcGlobalSync((EventCapNo)cap->no);
+}
+
 INLINE_HEADER void traceEventGcStats(Capability *cap            STG_UNUSED,
                                      CapsetID    heap_capset    STG_UNUSED,
                                      nat         gen            STG_UNUSED,
diff --git a/rts/eventlog/EventLog.c b/rts/eventlog/EventLog.c
index aab93bbd92..a3103775d9 100644
--- a/rts/eventlog/EventLog.c
+++ b/rts/eventlog/EventLog.c
@@ -71,6 +71,7 @@ char *EventDesc[] = {
   [EVENT_GC_END]              = "Finished GC",
   [EVENT_REQUEST_SEQ_GC]      = "Request sequential GC",
   [EVENT_REQUEST_PAR_GC]      = "Request parallel GC",
+  [EVENT_GC_GLOBAL_SYNC]      = "Synchronise stop-the-world GC",
   [EVENT_GC_STATS_GHC]        = "GC statistics",
   [EVENT_HEAP_INFO_GHC]       = "Heap static parameters",
   [EVENT_HEAP_ALLOCATED]      = "Total heap mem ever allocated",
@@ -341,6 +342,7 @@ initEventLogging(void)
         case EVENT_GC_IDLE:
         case EVENT_GC_WORK:
         case EVENT_GC_DONE:
+        case EVENT_GC_GLOBAL_SYNC:  // (cap)
         case EVENT_SPARK_CREATE:    // (cap)
         case EVENT_SPARK_DUD:       // (cap)
         case EVENT_SPARK_OVERFLOW:  // (cap)