[PATCH 2/4] md: Compute average flush time from component devices

[Darrick J. Wong]

For md devices which are composed of other block devices, a flush is spread 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@xxxxxxxxxx>
---
 drivers/md/md.c |   23 +++++++++++++++++++++++
 1 files changed, 23 insertions(+), 0 deletions(-)


diff --git a/drivers/md/md.c b/drivers/md/md.c
index 43243a4..af25c96 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -357,6 +357,28 @@ EXPORT_SYMBOL(mddev_congested);
  * Generic flush handling for md
  */
 
+static void measure_flushes(mddev_t *mddev)
+{
+	mdk_rdev_t *rdev;
+	u64 max = 0, samples = 0;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(rdev, &mddev->disks, same_set)
+		if (rdev->raid_disk >= 0 &&
+		    !test_bit(Faulty, &rdev->flags)) {
+			if (rdev->bdev->bd_disk->avg_flush_time_ns <= max)
+				continue;
+			max = rdev->bdev->bd_disk->avg_flush_time_ns;
+			samples = rdev->bdev->bd_disk->flush_samples;
+		}
+	rcu_read_unlock();
+
+	spin_lock(&mddev->gendisk->flush_time_lock);
+	mddev->gendisk->avg_flush_time_ns = max;
+	mddev->gendisk->flush_samples = samples;
+	spin_unlock(&mddev->gendisk->flush_time_lock);
+}
+
 static void md_end_flush(struct bio *bio, int err)
 {
 	mdk_rdev_t *rdev = bio->bi_private;
@@ -365,6 +387,7 @@ static void md_end_flush(struct bio *bio, int err)
 	rdev_dec_pending(rdev, mddev);
 
 	if (atomic_dec_and_test(&mddev->flush_pending)) {
+		measure_flushes(mddev);
 		/* The pre-request flush has finished */
 		queue_work(md_wq, &mddev->flush_work);
 	}

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