jbd2: trace when lock_buffer in do_get_write_access takes a long time

While investigating interactivity problems it was clear that processes
sometimes stall for long periods of times if an attempt is made to
lock a buffer which is undergoing writeback. It would stall in
a trace looking something like

[<ffffffff811a39de>] __lock_buffer+0x2e/0x30
[<ffffffff8123a60f>] do_get_write_access+0x43f/0x4b0
[<ffffffff8123a7cb>] jbd2_journal_get_write_access+0x2b/0x50
[<ffffffff81220f79>] __ext4_journal_get_write_access+0x39/0x80
[<ffffffff811f3198>] ext4_reserve_inode_write+0x78/0xa0
[<ffffffff811f3209>] ext4_mark_inode_dirty+0x49/0x220
[<ffffffff811f57d1>] ext4_dirty_inode+0x41/0x60
[<ffffffff8119ac3e>] __mark_inode_dirty+0x4e/0x2d0
[<ffffffff8118b9b9>] update_time+0x79/0xc0
[<ffffffff8118ba98>] file_update_time+0x98/0x100
[<ffffffff81110ffc>] __generic_file_aio_write+0x17c/0x3b0
[<ffffffff811112aa>] generic_file_aio_write+0x7a/0xf0
[<ffffffff811ea853>] ext4_file_write+0x83/0xd0
[<ffffffff81172b23>] do_sync_write+0xa3/0xe0
[<ffffffff811731ae>] vfs_write+0xae/0x180
[<ffffffff8117361d>] sys_write+0x4d/0x90
[<ffffffff8159d62d>] system_call_fastpath+0x1a/0x1f
[<ffffffffffffffff>] 0xffffffffffffffff

Signed-off-by: Mel Gorman <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/jbd2/transaction.c

···
 	int error;
 	char *frozen_buffer = NULL;
 	int need_copy = 0;
+	unsigned long start_lock, time_lock;
 
 	if (is_handle_aborted(handle))
 		return -EROFS;
···
 
 	/* @@@ Need to check for errors here at some point. */
 
+ 	start_lock = jiffies;
 	lock_buffer(bh);
 	jbd_lock_bh_state(bh);
+
+	/* If it takes too long to lock the buffer, trace it */
+	time_lock = jbd2_time_diff(start_lock, jiffies);
+	if (time_lock > HZ/10)
+		trace_jbd2_lock_buffer_stall(bh->b_bdev->bd_dev,
+			jiffies_to_msecs(time_lock));
 
 	/* We now hold the buffer lock so it is safe to query the buffer
 	 * state.  Is the buffer dirty?

include/trace/events/jbd2.h

···
 		  MINOR(__entry->dev), __entry->write_op)
 );
 
+TRACE_EVENT(jbd2_lock_buffer_stall,
+
+	TP_PROTO(dev_t dev, unsigned long stall_ms),
+
+	TP_ARGS(dev, stall_ms),
+
+	TP_STRUCT__entry(
+		__field(        dev_t, dev	)
+		__field(unsigned long, stall_ms	)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= dev;
+		__entry->stall_ms	= stall_ms;
+	),
+
+	TP_printk("dev %d,%d stall_ms %lu",
+		MAJOR(__entry->dev), MINOR(__entry->dev),
+		__entry->stall_ms)
+);
+
 #endif /* _TRACE_JBD2_H */
 
 /* This part must be outside protection */