Re: [PATCH -next 01/13] md: remove rdev flag 'RemoveSynchronized'

From: NeilBrown
Date: Mon Aug 14 2023 - 18:37:35 EST

On Mon, 14 Aug 2023, Song Liu wrote:
> + Neil
>
> RemoveSynchronized was added by Neil not too long ago. I wonder whether
> we still need it in some cases (large raid10 losing a large set of disks).

We have customers with thousands of members in a raid10. They do
sometimes need to remove half of them. Dropping RemoveSynchronized
would significantly increase the time it takes to do that.

Removing an rdev is not often a hot path. But sometimes it can be.

NeilBrown


>
> Thanks,
> Song
>
>
> On Thu, Aug 3, 2023 at 9:27 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> >
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > When multiple rdevs are removed from array in remove_and_add_spares(),
> > which is only possible from daemon thread(user can only remove one rdev
> > through ioctl/sysfs at one time), the flag is used to avoid calling
> > synchronize_rcu() multiple times for each rdev.
> >
> > However, remove rdev from daemon thread really is super cold path, while
> > removing n rdevs, it doesn't matter sync rcu n times or just one time.
> > Hence remove this flag and make code simpler.
> >
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> > drivers/md/md-multipath.c | 15 +++++++--------
> > drivers/md/md.c | 26 ++------------------------
> > drivers/md/md.h | 5 -----
> > drivers/md/raid1.c | 15 +++++++--------
> > drivers/md/raid10.c | 15 +++++++--------
> > drivers/md/raid5.c | 15 +++++++--------
> > 6 files changed, 30 insertions(+), 61 deletions(-)
> >
> > diff --git a/drivers/md/md-multipath.c b/drivers/md/md-multipath.c
> > index d22276870283..9e4ddd5240cd 100644
> > --- a/drivers/md/md-multipath.c
> > +++ b/drivers/md/md-multipath.c
> > @@ -258,14 +258,13 @@ static int multipath_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
> > goto abort;
> > }
> > p->rdev = NULL;
> > - 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;
> > - }
> > +
> > + 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/md.c b/drivers/md/md.c
> > index a3d98273b295..cd7ac1dee3b8 100644
> > --- a/drivers/md/md.c
> > +++ b/drivers/md/md.c
> > @@ -9155,7 +9155,6 @@ static int remove_and_add_spares(struct mddev *mddev,
> > struct md_rdev *rdev;
> > int spares = 0;
> > int removed = 0;
> > - bool remove_some = false;
> >
> > if (this && test_bit(MD_RECOVERY_RUNNING, &mddev->recovery))
> > /* Mustn't remove devices when resync thread is running */
> > @@ -9165,28 +9164,9 @@ static int remove_and_add_spares(struct mddev *mddev,
> > if ((this == NULL || rdev == this) &&
> > rdev->raid_disk >= 0 &&
> > !test_bit(Blocked, &rdev->flags) &&
> > - test_bit(Faulty, &rdev->flags) &&
> > + !test_bit(In_sync, &rdev->flags) &&
> > + !test_bit(Journal, &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)) {
> > if (mddev->pers->hot_remove_disk(
> > mddev, rdev) == 0) {
> > sysfs_unlink_rdev(mddev, rdev);
> > @@ -9195,8 +9175,6 @@ 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)
> > diff --git a/drivers/md/md.h b/drivers/md/md.h
> > index 8ae957480976..b25b6d061372 100644
> > --- a/drivers/md/md.h
> > +++ b/drivers/md/md.h
> > @@ -189,11 +189,6 @@ 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.
> > - */
> > ExternalBbl, /* External metadata provides bad
> > * block management for a disk
> > */
> > diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> > index 23d211969565..acb6d6542619 100644
> > --- a/drivers/md/raid1.c
> > +++ b/drivers/md/raid1.c
> > @@ -1859,15 +1859,14 @@ static int raid1_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
> > goto abort;
> > }
> > p->rdev = NULL;
> > - 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;
> > - }
> > + 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
> > diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> > index 7704a4c7f469..64dd5cb6133e 100644
> > --- a/drivers/md/raid10.c
> > +++ b/drivers/md/raid10.c
> > @@ -2247,15 +2247,14 @@ static int raid10_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
> > goto abort;
> > }
> > *rdevp = NULL;
> > - 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;
> > - }
> > + 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;
> > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> > index 4cdb35e54251..37d9865b180a 100644
> > --- a/drivers/md/raid5.c
> > +++ b/drivers/md/raid5.c
> > @@ -8267,15 +8267,14 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
> > goto abort;
> > }
> > *rdevp = NULL;
> > - if (!test_bit(RemoveSynchronized, &rdev->flags)) {
> > - lockdep_assert_held(&mddev->reconfig_mutex);
> > - synchronize_rcu();
> > - if (atomic_read(&rdev->nr_pending)) {
> > - /* lost the race, try later */
> > - err = -EBUSY;
> > - rcu_assign_pointer(*rdevp, rdev);
> > - }
> > + lockdep_assert_held(&mddev->reconfig_mutex);
> > + synchronize_rcu();
> > + if (atomic_read(&rdev->nr_pending)) {
> > + /* lost the race, try later */
> > + err = -EBUSY;
> > + rcu_assign_pointer(*rdevp, rdev);
> > }
> > +
> > if (!err) {
> > err = log_modify(conf, rdev, false);
> > if (err)
> > --
> > 2.39.2
> >
>