[PATCH 3/4] dm: Compute average flush time from component devices

["Darrick J. Wong" <djwong@us.ibm.com>]

For dm devices which are composed of other block devices, a flush is mapped out
to those other block devices.  Therefore, the average flush time can be
computed as the average flush time of whichever device flushes most slowly.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
---
 drivers/md/dm.c |   28 ++++++++++++++++++++++++++++
 1 files changed, 28 insertions(+), 0 deletions(-)


diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 7cb1352..62aeeb9 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -846,12 +846,38 @@ static void start_queue(struct request_queue *q)
 	spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
+static void measure_flushes(struct mapped_device *md)
+{
+	struct dm_table *t;
+	struct dm_dev_internal *dd;
+	struct list_head *devices;
+	u64 max = 0, samples = 0;
+
+	t = dm_get_live_table(md);
+	devices = dm_table_get_devices(t);
+	list_for_each_entry(dd, devices, list) {
+		if (dd->dm_dev.bdev->bd_disk->avg_flush_time_ns <= max)
+			continue;
+		max = dd->dm_dev.bdev->bd_disk->avg_flush_time_ns;
+		samples = dd->dm_dev.bdev->bd_disk->flush_samples;
+	}
+	dm_table_put(t);
+
+	spin_lock(&md->disk->flush_time_lock);
+	md->disk->avg_flush_time_ns = max;
+	md->disk->flush_samples = samples;
+	spin_unlock(&md->disk->flush_time_lock);
+}
+
 static void dm_done(struct request *clone, int error, bool mapped)
 {
 	int r = error;
 	struct dm_rq_target_io *tio = clone->end_io_data;
 	dm_request_endio_fn rq_end_io = tio->ti->type->rq_end_io;
 
+	if (clone->cmd_flags & REQ_FLUSH)
+		measure_flushes(tio->md);
+
 	if (mapped && rq_end_io)
 		r = rq_end_io(tio->ti, clone, error, &tio->info);
 
@@ -2310,6 +2336,8 @@ static void dm_wq_work(struct work_struct *work)
 		if (dm_request_based(md))
 			generic_make_request(c);
 		else
+			if (c->bi_rw & REQ_FLUSH)
+				measure_flushes(md);
 			__split_and_process_bio(md, c);
 
 		down_read(&md->io_lock);