btrfs: add a trace point for reserve tickets

While debugging a ENOSPC related performance problem I needed to see the
time difference between start and end of a reserve ticket, so add a
trace point to report when we handle a reserve ticket.

I opted to spit out start_ns itself without calculating the difference
because there could be a gap between enabling the tracepoint and setting
start_ns.  Doing it this way allows us to filter on 0 start_ns so we
don't get bogus entries, and we can easily calculate the time difference
with bpftrace or something else.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -1220,6 +1220,8 @@ static void wait_reserve_ticket(struct btrfs_fs_info *fs_info,
  * @fs_info:    the filesystem
  * @space_info: space info for the reservation
  * @ticket:     ticket for the reservation
+ * @start_ns:   timestamp when the reservation started
+ * @orig_bytes: amount of bytes originally reserved
  * @flush:      how much we can flush
  *
  * This does the work of figuring out how to flush for the ticket, waiting for
@@ -1228,6 +1230,7 @@ static void wait_reserve_ticket(struct btrfs_fs_info *fs_info,
 static int handle_reserve_ticket(struct btrfs_fs_info *fs_info,
 				 struct btrfs_space_info *space_info,
 				 struct reserve_ticket *ticket,
+				 u64 start_ns, u64 orig_bytes,
 				 enum btrfs_reserve_flush_enum flush)
 {
 	int ret;
@@ -1283,6 +1286,8 @@ static int handle_reserve_ticket(struct btrfs_fs_info *fs_info,
 	 * space wasn't reserved at all).
 	 */
 	ASSERT(!(ticket->bytes == 0 && ticket->error));
+	trace_btrfs_reserve_ticket(fs_info, space_info->flags, orig_bytes,
+				   start_ns, flush, ticket->error);
 	return ret;
 }
 
@@ -1317,6 +1322,7 @@ static int __reserve_bytes(struct btrfs_fs_info *fs_info,
 {
 	struct work_struct *async_work;
 	struct reserve_ticket ticket;
+	u64 start_ns = 0;
 	u64 used;
 	int ret = 0;
 	bool pending_tickets;
@@ -1369,6 +1375,9 @@ static int __reserve_bytes(struct btrfs_fs_info *fs_info,
 		space_info->reclaim_size += ticket.bytes;
 		init_waitqueue_head(&ticket.wait);
 		ticket.steal = (flush == BTRFS_RESERVE_FLUSH_ALL_STEAL);
+		if (trace_btrfs_reserve_ticket_enabled())
+			start_ns = ktime_get_ns();
+
 		if (flush == BTRFS_RESERVE_FLUSH_ALL ||
 		    flush == BTRFS_RESERVE_FLUSH_ALL_STEAL ||
 		    flush == BTRFS_RESERVE_FLUSH_DATA) {
@@ -1405,7 +1414,8 @@ static int __reserve_bytes(struct btrfs_fs_info *fs_info,
 	if (!ret || flush == BTRFS_RESERVE_NO_FLUSH)
 		return ret;
 
-	return handle_reserve_ticket(fs_info, space_info, &ticket, flush);
+	return handle_reserve_ticket(fs_info, space_info, &ticket, start_ns,
+				     orig_bytes, flush);
 }
 
 /**

include/trace/events/btrfs.h

@@ -2026,6 +2026,35 @@ TRACE_EVENT(btrfs_convert_extent_bit,
 		  __print_flags(__entry->clear_bits, "|", EXTENT_FLAGS))
 );
 
+TRACE_EVENT(btrfs_reserve_ticket,
+	TP_PROTO(const struct btrfs_fs_info *fs_info, u64 flags, u64 bytes,
+		 u64 start_ns, int flush, int error),
+
+	TP_ARGS(fs_info, flags, bytes, start_ns, flush, error),
+
+	TP_STRUCT__entry_btrfs(
+		__field(	u64,	flags		)
+		__field(	u64,	bytes		)
+		__field(	u64,	start_ns	)
+		__field(	int,	flush		)
+		__field(	int,	error		)
+	),
+
+	TP_fast_assign_btrfs(fs_info,
+		__entry->flags		= flags;
+		__entry->bytes		= bytes;
+		__entry->start_ns	= start_ns;
+		__entry->flush		= flush;
+		__entry->error		= error;
+	),
+
+	TP_printk_btrfs("flags=%s bytes=%llu start_ns=%llu flush=%s error=%d",
+			__print_flags(__entry->flags, "|", BTRFS_GROUP_FLAGS),
+			__entry->bytes, __entry->start_ns,
+			__print_symbolic(__entry->flush, FLUSH_ACTIONS),
+			__entry->error)
+);
+
 DECLARE_EVENT_CLASS(btrfs_sleep_tree_lock,
 	TP_PROTO(const struct extent_buffer *eb, u64 start_ns),