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Date: Tue, 15 Aug 2023 09:04:54 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: NeilBrown <neil@...wn.name>, Song Liu <song@...nel.org>
Cc: Yu Kuai <yukuai1@...weicloud.com>, xni@...hat.com,
linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
yi.zhang@...wei.com, yangerkun@...wei.com,
"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH -next 01/13] md: remove rdev flag 'RemoveSynchronized'
Hi,
在 2023/08/15 5:36, NeilBrown 写道:
> 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.
>
Thanks for the explanation, and soory that I didn't mention that in this
commit message, the way that rdev is protected through rcu is not safe,
and refer to the implementation of 'q_usage_counter' in block layer, I'm
planing to refactor the way that rdev is protected and rcu is not needed
anymore:
1. refactor mddev_suspend(), and make sure it's called by all the caller
of remove_and_add_spares().
2. factor out a helper md_array_enter() from md_handle_request, this
helper will grab 'active_io' and wait for mddev_suspend() to be done.
3. in the fastpath, before accessing rdev, make sure md_array_enter() is
called.
Thanks,
Kuai
> NeilBrown
>
>
>>
>> Thanks,
>> Song
>>
>>
>> On Thu, Aug 3, 2023 at 9:27 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>>
>>> From: Yu Kuai <yukuai3@...wei.com>
>>>
>>> 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@...wei.com>
>>> ---
>>> 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
>>>
>>
>
>
> .
>
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