[md PATCH 18/18] md: reduce the number of synchronize_rcu() calls when multiple devices fail.

NeilBrown
2016-06-02T06:19:53Z

Every time a device is removed with ->hot_remove_disk() a synchronize_rcu() call is made
which can delay several milliseconds in some case.
If lots of devices fail at once - as could happen with a large RAID10 where one set
of devices are removed all at once - these delays can add up to be very inconcenient.

As failure is not reversible we can check for that first, setting a
separate flag if it is found, and then all synchronize_rcu() once for
all the flagged devices. Then ->hot_remove_disk() function can skip the
synchronize_rcu() step if the flag is set.

Signed-off-by: NeilBrown <neilb@suse.com>
---
drivers/md/md.c | 27 +++++++++++++++++++++++++--
drivers/md/md.h | 5 +++++
drivers/md/multipath.c | 14 ++++++++------
drivers/md/raid1.c | 17 ++++++++++-------
drivers/md/raid10.c | 19 +++++++++++--------
drivers/md/raid5.c | 15 +++++++++------
6 files changed, 68 insertions(+), 29 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 2d26099e1160..1af69d15d0df 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8186,12 +8186,31 @@ static int remove_and_add_spares(struct mddev *mddev,
struct md_rdev *rdev;
int spares = 0;
int removed = 0;
+ bool remove_some = false;

- rdev_for_each(rdev, mddev)
+ rdev_for_each(rdev, mddev) {
if ((this == NULL || rdev == this) &&
rdev->raid_disk >= 0 &&
!test_bit(Blocked, &rdev->flags) &&
- (test_bit(Faulty, &rdev->flags) ||
+ test_bit(Faulty, &rdev->flags) &&
+ atomic_read(&rdev->nr_pending)==0) {
+ /* Faulty non-Blocked devices with nr_pending == 0
+ * never get nr_pending incremented,
+ * never get Faulty cleared, and never get Blocked set.
+ * So we can synchronize_rcu now rather than once per device
+ */
+ remove_some = true;
+ set_bit(RemoveSynchronized, &rdev->flags);
+ }
+ }
+
+ if (remove_some)
+ synchronize_rcu();
+ rdev_for_each(rdev, mddev) {
+ if ((this == NULL || rdev == this) &&
+ rdev->raid_disk >= 0 &&
+ !test_bit(Blocked, &rdev->flags) &&
+ ((test_bit(RemoveSynchronized, &rdev->flags) ||
(!test_bit(In_sync, &rdev->flags) &&
!test_bit(Journal, &rdev->flags))) &&
atomic_read(&rdev->nr_pending)==0) {
@@ -8202,6 +8221,10 @@ static int remove_and_add_spares(struct mddev *mddev,
removed++;
}
}
+ if (remove_some && test_bit(RemoveSynchronized, &rdev->flags))
+ clear_bit(RemoveSynchronized, &rdev->flags);
+ }
+
if (removed && mddev->kobj.sd)
sysfs_notify(&mddev->kobj, NULL, "degraded");

diff --git a/drivers/md/md.h b/drivers/md/md.h
index b5c4be73e6e4..cfd883e96a38 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -163,6 +163,11 @@ enum flag_bits {
* than other devices in the array
*/
ClusterRemove,
+ RemoveSynchronized, /* synchronize_rcu() was called after
+ * this device was known to be faulty,
+ * so it is safe to remove without
+ * another synchronize_rcu() call.
+ */
};

static inline int is_badblock(struct md_rdev *rdev, sector_t s, int sectors,
diff --git a/drivers/md/multipath.c b/drivers/md/multipath.c
index 7eb9972a37e6..c145a5a114eb 100644
--- a/drivers/md/multipath.c
+++ b/drivers/md/multipath.c
@@ -298,12 +298,14 @@ static int multipath_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
goto abort;
}
p->rdev = NULL;
- synchronize_rcu();
- if (atomic_read(&rdev->nr_pending)) {
- /* lost the race, try later */
- err = -EBUSY;
- p->rdev = rdev;
- goto abort;
+ if (!test_bit(RemoveSynchronized, &rdev->flags)) {
+ synchronize_rcu();
+ if (atomic_read(&rdev->nr_pending)) {
+ /* lost the race, try later */
+ err = -EBUSY;
+ p->rdev = rdev;
+ goto abort;
+ }
}
err = md_integrity_register(mddev);
}
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 0ee901ccc9c5..8b27904c4f37 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1656,13 +1656,16 @@ static int raid1_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
goto abort;
}
p->rdev = NULL;
- synchronize_rcu();
- if (atomic_read(&rdev->nr_pending)) {
- /* lost the race, try later */
- err = -EBUSY;
- p->rdev = rdev;
- goto abort;
- } else if (conf->mirrors[conf->raid_disks + number].rdev) {
+ if (!test_bit(RemoveSynchronized, &rdev->flags)) {
+ synchronize_rcu();
+ if (atomic_read(&rdev->nr_pending)) {
+ /* lost the race, try later */
+ err = -EBUSY;
+ p->rdev = rdev;
+ goto abort;
+ }
+ }
+ if (conf->mirrors[conf->raid_disks + number].rdev) {
/* We just removed a device that is being replaced.
* Move down the replacement. We drain all IO before
* doing this to avoid confusion.
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 78016667ec00..5e8fa7751920 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1768,7 +1768,7 @@ static int raid10_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
err = -EBUSY;
goto abort;
}
- /* Only remove faulty devices if recovery
+ /* Only remove non-faulty devices if recovery
* is not possible.
*/
if (!test_bit(Faulty, &rdev->flags) &&
@@ -1780,13 +1780,16 @@ static int raid10_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
goto abort;
}
*rdevp = NULL;
- synchronize_rcu();
- if (atomic_read(&rdev->nr_pending)) {
- /* lost the race, try later */
- err = -EBUSY;
- *rdevp = rdev;
- goto abort;
- } else if (p->replacement) {
+ if (!test_bit(RemoveSynchronized, &rdev->flags)) {
+ synchronize_rcu();
+ if (atomic_read(&rdev->nr_pending)) {
+ /* lost the race, try later */
+ err = -EBUSY;
+ *rdevp = rdev;
+ goto abort;
+ }
+ }
+ if (p->replacement) {
/* We must have just cleared 'rdev' */
p->rdev = p->replacement;
clear_bit(Replacement, &p->replacement->flags);
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 94c180f16294..4c6270e10b12 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -7197,12 +7197,15 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
goto abort;
}
*rdevp = NULL;
- synchronize_rcu();
- if (atomic_read(&rdev->nr_pending)) {
- /* lost the race, try later */
- err = -EBUSY;
- *rdevp = rdev;
- } else if (p->replacement) {
+ if (!test_bit(RemoveSynchronized, &rdev->flags)) {
+ synchronize_rcu();
+ if (atomic_read(&rdev->nr_pending)) {
+ /* lost the race, try later */
+ err = -EBUSY;
+ *rdevp = rdev;
+ }
+ }
+ if (p->replacement) {
/* We must have just cleared 'rdev' */
p->rdev = p->replacement;
clear_bit(Replacement, &p->replacement->flags);