Refactor some code related to backend statistics

This commit changes the way pending backend statistics are tracked by
moving them into a new structure called PgStat_BackendPending, removing
PgStat_BackendPendingIO.  PgStat_BackendPending currently only includes
PgStat_PendingIO for the pending I/O stats.

pgstat_flush_backend() is extended with a "flags" argument to control
which parts of the stats of a backend should be flushed.

With this refactoring, it becomes easier to plug into backend statistics
more data.  A patch to add information related to WAL in this stats kind
is under discussion.

Author: Bertrand Drouvot
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/utils/activity/pgstat.c

@@ -370,7 +370,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_size = sizeof(PgStatShared_Backend),
 		.shared_data_off = offsetof(PgStatShared_Backend, stats),
 		.shared_data_len = sizeof(((PgStatShared_Backend *) 0)->stats),
-		.pending_size = sizeof(PgStat_BackendPendingIO),
+		.pending_size = sizeof(PgStat_BackendPending),
 
 		.flush_pending_cb = pgstat_backend_flush_cb,
 		.reset_timestamp_cb = pgstat_backend_reset_timestamp_cb,

src/backend/utils/activity/pgstat_backend.c

@@ -39,23 +39,21 @@ pgstat_fetch_stat_backend(ProcNumber procNumber)
 }
 
 /*
- * Flush out locally pending backend statistics
- *
- * If no stats have been recorded, this function returns false.
+ * Flush out locally pending backend IO statistics.  Locking is managed
+ * by the caller.
  */
-bool
-pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+static void
+pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 {
-	PgStatShared_Backend *shbackendioent;
-	PgStat_BackendPendingIO *pendingent;
+	PgStatShared_Backend *shbackendent;
+	PgStat_BackendPending *pendingent;
 	PgStat_BktypeIO *bktype_shstats;
+	PgStat_PendingIO *pending_io;
 
-	if (!pgstat_lock_entry(entry_ref, nowait))
-		return false;
-
-	shbackendioent = (PgStatShared_Backend *) entry_ref->shared_stats;
-	bktype_shstats = &shbackendioent->stats.stats;
-	pendingent = (PgStat_BackendPendingIO *) entry_ref->pending;
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	pendingent = (PgStat_BackendPending *) entry_ref->pending;
+	bktype_shstats = &shbackendent->stats.io_stats;
+	pending_io = &pendingent->pending_io;
 
 	for (int io_object = 0; io_object < IOOBJECT_NUM_TYPES; io_object++)
 	{
@@ -66,15 +64,33 @@ pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 				instr_time	time;
 
 				bktype_shstats->counts[io_object][io_context][io_op] +=
-					pendingent->counts[io_object][io_context][io_op];
+					pending_io->counts[io_object][io_context][io_op];
 
-				time = pendingent->pending_times[io_object][io_context][io_op];
+				time = pending_io->pending_times[io_object][io_context][io_op];
 
 				bktype_shstats->times[io_object][io_context][io_op] +=
 					INSTR_TIME_GET_MICROSEC(time);
 			}
 		}
 	}
+}
+
+/*
+ * Wrapper routine to flush backend statistics.
+ */
+static bool
+pgstat_flush_backend_entry(PgStat_EntryRef *entry_ref, bool nowait,
+						   bits32 flags)
+{
+	if (!pgstat_tracks_backend_bktype(MyBackendType))
+		return false;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	/* Flush requested statistics */
+	if (flags & PGSTAT_BACKEND_FLUSH_IO)
+		pgstat_flush_backend_entry_io(entry_ref);
 
 	pgstat_unlock_entry(entry_ref);
 
@@ -82,10 +98,23 @@ pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 }
 
 /*
- * Simpler wrapper of pgstat_backend_flush_cb()
+ * Callback to flush out locally pending backend statistics.
+ *
+ * If no stats have been recorded, this function returns false.
+ */
+bool
+pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	return pgstat_flush_backend_entry(entry_ref, nowait, PGSTAT_BACKEND_FLUSH_ALL);
+}
+
+/*
+ * Flush out locally pending backend statistics
+ *
+ * "flags" parameter controls which statistics to flush.
  */
 void
-pgstat_flush_backend(bool nowait)
+pgstat_flush_backend(bool nowait, bits32 flags)
 {
 	PgStat_EntryRef *entry_ref;
 
@@ -94,7 +123,7 @@ pgstat_flush_backend(bool nowait)
 
 	entry_ref = pgstat_get_entry_ref(PGSTAT_KIND_BACKEND, InvalidOid,
 									 MyProcNumber, false, NULL);
-	(void) pgstat_backend_flush_cb(entry_ref, nowait);
+	(void) pgstat_flush_backend_entry(entry_ref, nowait, flags);
 }
 
 /*
@@ -119,9 +148,9 @@ pgstat_create_backend(ProcNumber procnum)
 }
 
 /*
- * Find or create a local PgStat_BackendPendingIO entry for proc number.
+ * Find or create a local PgStat_BackendPending entry for proc number.
  */
-PgStat_BackendPendingIO *
+PgStat_BackendPending *
 pgstat_prep_backend_pending(ProcNumber procnum)
 {
 	PgStat_EntryRef *entry_ref;

src/backend/utils/activity/pgstat_io.c

@@ -81,10 +81,10 @@ pgstat_count_io_op_n(IOObject io_object, IOContext io_context, IOOp io_op, uint3
 
 	if (pgstat_tracks_backend_bktype(MyBackendType))
 	{
-		PgStat_PendingIO *entry_ref;
+		PgStat_BackendPending *entry_ref;
 
 		entry_ref = pgstat_prep_backend_pending(MyProcNumber);
-		entry_ref->counts[io_object][io_context][io_op] += cnt;
+		entry_ref->pending_io.counts[io_object][io_context][io_op] += cnt;
 	}
 
 	PendingIOStats.counts[io_object][io_context][io_op] += cnt;
@@ -151,10 +151,10 @@ pgstat_count_io_op_time(IOObject io_object, IOContext io_context, IOOp io_op,
 
 		if (pgstat_tracks_backend_bktype(MyBackendType))
 		{
-			PgStat_PendingIO *entry_ref;
+			PgStat_BackendPending *entry_ref;
 
 			entry_ref = pgstat_prep_backend_pending(MyProcNumber);
-			INSTR_TIME_ADD(entry_ref->pending_times[io_object][io_context][io_op],
+			INSTR_TIME_ADD(entry_ref->pending_io.pending_times[io_object][io_context][io_op],
 						   io_time);
 		}
 	}

src/backend/utils/activity/pgstat_relation.c

@@ -264,7 +264,7 @@ pgstat_report_vacuum(Oid tableoid, bool shared,
 	 * VACUUM command has processed all tables and committed.
 	 */
 	pgstat_flush_io(false);
-	pgstat_flush_backend(false);
+	pgstat_flush_backend(false, PGSTAT_BACKEND_FLUSH_IO);
 }
 
 /*
@@ -351,7 +351,7 @@ pgstat_report_analyze(Relation rel,
 
 	/* see pgstat_report_vacuum() */
 	pgstat_flush_io(false);
-	pgstat_flush_backend(false);
+	pgstat_flush_backend(false, PGSTAT_BACKEND_FLUSH_IO);
 }
 
 /*

src/backend/utils/adt/pgstatfuncs.c

@@ -1544,7 +1544,7 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 	if (bktype == B_INVALID)
 		return (Datum) 0;
 
-	bktype_stats = &backend_stats->stats;
+	bktype_stats = &backend_stats->io_stats;
 
 	/*
 	 * In Assert builds, we can afford an extra loop through all of the

src/include/pgstat.h

@@ -375,15 +375,24 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-/* Backend statistics store the same amount of IO data as PGSTAT_KIND_IO */
-typedef PgStat_PendingIO PgStat_BackendPendingIO;
-
 typedef struct PgStat_Backend
 {
 	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO stats;
+	PgStat_BktypeIO io_stats;
 } PgStat_Backend;
 
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;

src/include/utils/pgstat_internal.h

@@ -613,9 +613,12 @@ extern void pgstat_archiver_snapshot_cb(void);
  * Functions in pgstat_backend.c
  */
 
-extern void pgstat_flush_backend(bool nowait);
+/* flags for pgstat_flush_backend() */
+#define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
 
-extern PgStat_BackendPendingIO *pgstat_prep_backend_pending(ProcNumber procnum);
+extern void pgstat_flush_backend(bool nowait, bits32 flags);
+extern PgStat_BackendPending *pgstat_prep_backend_pending(ProcNumber procnum);
 extern bool pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
 extern void pgstat_backend_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
 

src/tools/pgindent/typedefs.list

@@ -2140,7 +2140,7 @@ PgStatShared_Subscription
 PgStatShared_Wal
 PgStat_ArchiverStats
 PgStat_Backend
-PgStat_BackendPendingIO
+PgStat_BackendPending
 PgStat_BackendSubEntry
 PgStat_BgWriterStats
 PgStat_BktypeIO