Add callbacks to control flush of fixed-numbered stats

This commit adds two callbacks in pgstats to have a better control of
the flush timing of pgstat_report_stat(), whose operation depends on the
three PGSTAT_*_INTERVAL variables:
- have_fixed_pending_cb(), to check if a stats kind has any pending
data waiting for a flush.  This is used as a fast path if there are no
pending statistics to flush, and this check is done for fixed-numbered
statistics only if there are no variable-numbered statistics to flush.
A flush will need to happen if at least one callback reports any pending
data.
- flush_fixed_cb(), to do the actual flush.

These callbacks are currently used by the SLRU, WAL and IO statistics,
generalizing the concept for all stats kinds (builtin and custom).

The SLRU and IO stats relied each on one global variable to determine
whether a flush should happen; these are now local to pgstat_slru.c and
pgstat_io.c, cleaning up a bit how the pending flush states are tracked
in pgstat.c.

pgstat_flush_io() and pgstat_flush_wal() are still required, but we do
not need to check their return result anymore.

Reviewed-by: Bertrand Drouvot, Kyotaro Horiguchi
Discussion: https://postgr.es/m/ZtaVO0N-aTwiAk3w@paquier.xyz

1 files changed, 51 insertions, 12 deletions

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 178b5ef65aa..a7f2dfc744c 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -411,6 +411,8 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_data_off = offsetof(PgStatShared_IO, stats),
 		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
 
+		.flush_fixed_cb = pgstat_io_flush_cb,
+		.have_fixed_pending_cb = pgstat_io_have_pending_cb,
 		.init_shmem_cb = pgstat_io_init_shmem_cb,
 		.reset_all_cb = pgstat_io_reset_all_cb,
 		.snapshot_cb = pgstat_io_snapshot_cb,
@@ -426,6 +428,8 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
 		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
 
+		.flush_fixed_cb = pgstat_slru_flush_cb,
+		.have_fixed_pending_cb = pgstat_slru_have_pending_cb,
 		.init_shmem_cb = pgstat_slru_init_shmem_cb,
 		.reset_all_cb = pgstat_slru_reset_all_cb,
 		.snapshot_cb = pgstat_slru_snapshot_cb,
@@ -442,6 +446,8 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
 
 		.init_backend_cb = pgstat_wal_init_backend_cb,
+		.flush_fixed_cb = pgstat_wal_flush_cb,
+		.have_fixed_pending_cb = pgstat_wal_have_pending_cb,
 		.init_shmem_cb = pgstat_wal_init_shmem_cb,
 		.reset_all_cb = pgstat_wal_reset_all_cb,
 		.snapshot_cb = pgstat_wal_snapshot_cb,
@@ -671,13 +677,37 @@ pgstat_report_stat(bool force)
 	}
 
 	/* Don't expend a clock check if nothing to do */
-	if (dlist_is_empty(&pgStatPending) &&
-		!have_iostats &&
-		!have_slrustats &&
-		!pgstat_have_pending_wal())
+	if (dlist_is_empty(&pgStatPending))
 	{
-		Assert(pending_since == 0);
-		return 0;
+		bool		do_flush = false;
+
+		/* Check for pending fixed-numbered stats */
+		for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
+		{
+			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+			if (!kind_info)
+				continue;
+			if (!kind_info->fixed_amount)
+			{
+				Assert(kind_info->have_fixed_pending_cb == NULL);
+				continue;
+			}
+			if (!kind_info->have_fixed_pending_cb)
+				continue;
+
+			if (kind_info->have_fixed_pending_cb())
+			{
+				do_flush = true;
+				break;
+			}
+		}
+
+		if (!do_flush)
+		{
+			Assert(pending_since == 0);
+			return 0;
+		}
 	}
 
 	/*
@@ -730,14 +760,23 @@ pgstat_report_stat(bool force)
 	/* flush database / relation / function / ... stats */
 	partial_flush |= pgstat_flush_pending_entries(nowait);
 
-	/* flush IO stats */
-	partial_flush |= pgstat_flush_io(nowait);
+	/* flush of fixed-numbered stats */
+	for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-	/* flush wal stats */
-	partial_flush |= pgstat_flush_wal(nowait);
+		if (!kind_info)
+			continue;
+		if (!kind_info->fixed_amount)
+		{
+			Assert(kind_info->flush_fixed_cb == NULL);
+			continue;
+		}
+		if (!kind_info->flush_fixed_cb)
+			continue;
 
-	/* flush SLRU stats */
-	partial_flush |= pgstat_slru_flush(nowait);
+		partial_flush |= kind_info->flush_fixed_cb(nowait);
+	}
 
 	last_flush = now;