Re: [PATCH v2 08/11] md: add atomic mode switching in RAID 1/10

[Paul E Luse]

On Thu, 18 Apr 2024 14:39:53 +0800
Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:

> Hi,
> 
> 在 2024/04/18 13:44, tada keisuke 写道:
> > This patch depends on patch 07.
> > 
> > All rdevs running in RAID 1/10 switch nr_pending to atomic mode.
> > The value of nr_pending is read in a normal operation
> > (choose_best_rdev()). Therefore, nr_pending must always be
> > consistent.
> > 
> > Signed-off-by: Keisuke TADA <keisuke1.tada@xxxxxxxxxx>
> > Signed-off-by: Toshifumi OHTAKE <toshifumi.ootake@xxxxxxxxxx>
> > ---
> >   drivers/md/md.h     | 14 ++++++++++++++
> >   drivers/md/raid1.c  |  7 +++++++
> >   drivers/md/raid10.c |  4 ++++
> >   3 files changed, 25 insertions(+)
> > 
> > diff --git a/drivers/md/md.h b/drivers/md/md.h
> > index ab09e312c9bb..57b09b567ffa 100644
> > --- a/drivers/md/md.h
> > +++ b/drivers/md/md.h
> > @@ -236,6 +236,20 @@ static inline unsigned long
> > nr_pending_read(struct md_rdev *rdev) return
> > atomic_long_read(&rdev->nr_pending.data->count); }
> >   
> > +static inline bool nr_pending_is_percpu_mode(struct md_rdev *rdev)
> > +{
> > +	unsigned long __percpu *percpu_count;
> > +
> > +	return __ref_is_percpu(&rdev->nr_pending, &percpu_count);
> > +}
> > +
> > +static inline bool nr_pending_is_atomic_mode(struct md_rdev *rdev)
> > +{
> > +	unsigned long __percpu *percpu_count;
> > +
> > +	return !__ref_is_percpu(&rdev->nr_pending, &percpu_count);
> > +}
> > +
> >   static inline int is_badblock(struct md_rdev *rdev, sector_t s,
> > int sectors, sector_t *first_bad, int *bad_sectors)
> >   {
> > diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> > index 12318fb15a88..c38ae13aadab 100644
> > --- a/drivers/md/raid1.c
> > +++ b/drivers/md/raid1.c
> > @@ -784,6 +784,7 @@ static int choose_best_rdev(struct r1conf
> > *conf, struct r1bio *r1_bio) if (ctl.readable_disks++ == 1)
> >   			set_bit(R1BIO_FailFast, &r1_bio->state);
> >   
> > +		WARN_ON_ONCE(nr_pending_is_percpu_mode(rdev));
> >   		pending = nr_pending_read(rdev);
> >   		dist = abs(r1_bio->sector -
> > conf->mirrors[disk].head_position); 
> > @@ -1930,6 +1931,7 @@ static int raid1_add_disk(struct mddev
> > *mddev, struct md_rdev *rdev) if (err)
> >   				return err;
> >   
> > +
> > percpu_ref_switch_to_atomic_sync(&rdev->nr_pending);
> > raid1_add_conf(conf, rdev, mirror, false); /* As all devices are
> > equivalent, we don't need a full recovery
> >   			 * if this was recently any drive of the
> > array @@ -1949,6 +1951,7 @@ static int raid1_add_disk(struct mddev
> > *mddev, struct md_rdev *rdev) set_bit(Replacement, &rdev->flags);
> >   		raid1_add_conf(conf, rdev, repl_slot, true);
> >   		err = 0;
> > +
> > percpu_ref_switch_to_atomic_sync(&rdev->nr_pending);  
> 
> I don't understand what's the point here, 'nr_pending' will be used
> when the rdev issuing IO, and it's always used as atomic mode, there
> is no difference.
> 
> Consider that 'nr_pending' must be read from IO fast path, use it as
> atomic is something we must accept. Unless someone comes up with a
> plan to avoid reading 'inflight' counter from fast path like generic
> block layer, it's not ok to me to switch to percpu_ref for now.
> 
> +CC Paul
> 
> HI, Paul, perhaps you RR mode doesn't need such 'inflight' counter
> anymore?
> 

I too am struggling to see the benefit but am curious enough that I
will run some tests on my own as I have fresh baseline RAID1 large sweep
performance data ready right now.

So my WIP round robin patch won't need nr_pedning for SSDs but I think
it will for HDD plus I need a new atomic counter for round robin for
SSDs anyway but I see no perfomrnace impact so far from adding it.

-Paul

> Thanks,
> Kuai
> 
> >   		conf->fullsync = 1;
> >   	}
> >   
> > @@ -3208,6 +3211,7 @@ static void raid1_free(struct mddev *mddev,
> > void *priv); static int raid1_run(struct mddev *mddev)
> >   {
> >   	struct r1conf *conf;
> > +	struct md_rdev *rdev;
> >   	int i;
> >   	int ret;
> >   
> > @@ -3269,6 +3273,9 @@ static int raid1_run(struct mddev *mddev)
> >   	/*
> >   	 * Ok, everything is just fine now
> >   	 */
> > +	rdev_for_each(rdev, mddev) {
> > +
> > percpu_ref_switch_to_atomic_sync(&rdev->nr_pending);
> > +	}
> >   	rcu_assign_pointer(mddev->thread, conf->thread);
> >   	rcu_assign_pointer(conf->thread, NULL);
> >   	mddev->private = conf;
> > diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> > index b91dd6c0be5a..66896a1076e1 100644
> > --- a/drivers/md/raid10.c
> > +++ b/drivers/md/raid10.c
> > @@ -808,6 +808,7 @@ static struct md_rdev *read_balance(struct
> > r10conf *conf, 
> >   		nonrot = bdev_nonrot(rdev->bdev);
> >   		has_nonrot_disk |= nonrot;
> > +		WARN_ON_ONCE(nr_pending_is_percpu_mode(rdev));
> >   		pending = nr_pending_read(rdev);
> >   		if (min_pending > pending && nonrot) {
> >   			min_pending = pending;
> > @@ -2113,6 +2114,7 @@ static int raid10_add_disk(struct mddev
> > *mddev, struct md_rdev *rdev) p->recovery_disabled =
> > mddev->recovery_disabled - 1; rdev->raid_disk = mirror;
> >   		err = 0;
> > +
> > percpu_ref_switch_to_atomic_sync(&rdev->nr_pending); if
> > (rdev->saved_raid_disk != mirror) conf->fullsync = 1;
> >   		WRITE_ONCE(p->rdev, rdev);
> > @@ -2127,6 +2129,7 @@ static int raid10_add_disk(struct mddev
> > *mddev, struct md_rdev *rdev) err = mddev_stack_new_rdev(mddev,
> > rdev); if (err)
> >   			return err;
> > +
> > percpu_ref_switch_to_atomic_sync(&rdev->nr_pending); conf->fullsync
> > = 1; WRITE_ONCE(p->replacement, rdev);
> >   	}
> > @@ -4028,6 +4031,7 @@ static int raid10_run(struct mddev *mddev)
> >   	rdev_for_each(rdev, mddev) {
> >   		long long diff;
> >   
> > +
> > percpu_ref_switch_to_atomic_sync(&rdev->nr_pending); disk_idx =
> > rdev->raid_disk; if (disk_idx < 0)
> >   			continue;
> >   
> 
>