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

[Paul E Luse]

On Tue, 16 Apr 2024 07:38:06 -0700
Paul E Luse <paul.e.luse@xxxxxxxxxxxxxxx> wrote:

> 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
> 

I can run more if others are interested (RAID5 or HDD for example) but
at least for RAID1 there's really no difference.  This was a quick run,
just 40 sec each, 16 jobs and the rest ofthe fio params are on the
charts. 2 disk RAID1. THe baseline is 6.8.0 from the md repo.

Using my favorite drives, of course, KIOXIA KCMYDVUG3T20 :)

Here's the results: https://photos.app.goo.gl/Avyw64eXCqWFWrs78

NOTE:  There are few small randoms where it appears to help but I
assumed that was because these are small randoms with very short run
times.  SO I reran the 4K mixed rw randoms with 5 minute run time and
that chart is at the very bottom and shows that over longer duration
its a wash, there's no clear winner.  I'm sure an even longer run would
show more consistently close results.

> > 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;
> > >     
> > 
> >   
>