Re: [PATCH md-6.9 03/10] md/raid1: fix choose next idle in read_balance()

[Yu Kuai]

Hi,

在 2024/02/27 10:23, Xiao Ni 写道:
> On Thu, Feb 22, 2024 at 4:04 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > Commit 12cee5a8a29e ("md/raid1: prevent merging too large request") add
> > the case choose next idle in read_balance():
> >
> > read_balance:
> >   for_each_rdev
> >    if(next_seq_sect == this_sector || disk == 0)
> >    -> sequential reads
> >     best_disk = disk;
> >     if (...)
> >      choose_next_idle = 1
> >      continue;
> >
> >   for_each_rdev
> >   -> iterate next rdev
> >    if (pending == 0)
> >     best_disk = disk;
> >     -> choose the next idle disk
> >     break;
> >
> >    if (choose_next_idle)
> >     -> keep using this rdev if there are no other idle disk
> >     contine
> >
> > However, commit 2e52d449bcec ("md/raid1: add failfast handling for reads.")
> > remove the code:
> >
> > -               /* If device is idle, use it */
> > -               if (pending == 0) {
> > -                       best_disk = disk;
> > -                       break;
> > -               }
> >
> > Hence choose next idle will never work now, fix this problem by
> > following:
> >
> > 1) don't set best_disk in this case, read_balance() will choose the best
> >     disk after iterating all the disks;
> > 2) add 'pending' so that other idle disk will be chosen;
> > 3) set 'dist' to 0 so that if there is no other idle disk, and all disks
> >     are rotational, this disk will still be chosen;
> >
> > Fixes: 2e52d449bcec ("md/raid1: add failfast handling for reads.")
> > Co-developed-by: Paul Luse <paul.e.luse@xxxxxxxxxxxxxxx>
> > Signed-off-by: Paul Luse <paul.e.luse@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   drivers/md/raid1.c | 21 ++++++++++++---------
> >   1 file changed, 12 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> > index c60ea58ae8c5..d0bc67e6d068 100644
> > --- a/drivers/md/raid1.c
> > +++ b/drivers/md/raid1.c
> > @@ -604,7 +604,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
> >          unsigned int min_pending;
> >          struct md_rdev *rdev;
> >          int choose_first;
> > -       int choose_next_idle;
> >
> >          /*
> >           * Check if we can balance. We can balance on the whole
> > @@ -619,7 +618,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
> >          best_pending_disk = -1;
> >          min_pending = UINT_MAX;
> >          best_good_sectors = 0;
> > -       choose_next_idle = 0;
> >          clear_bit(R1BIO_FailFast, &r1_bio->state);
> >
> >          if ((conf->mddev->recovery_cp < this_sector + sectors) ||
> > @@ -712,7 +710,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
> >                          int opt_iosize = bdev_io_opt(rdev->bdev) >> 9;
> >                          struct raid1_info *mirror = &conf->mirrors[disk];
> >
> > -                       best_disk = disk;
> >                          /*
> >                           * If buffered sequential IO size exceeds optimal
> >                           * iosize, check if there is idle disk. If yes, choose
> > @@ -731,15 +728,21 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
> >                              mirror->next_seq_sect > opt_iosize &&
> >                              mirror->next_seq_sect - opt_iosize >=
> >                              mirror->seq_start) {
> > -                               choose_next_idle = 1;
> > -                               continue;
> > +                               /*
> > +                                * Add 'pending' to avoid choosing this disk if
> > +                                * there is other idle disk.
> > +                                * Set 'dist' to 0, so that if there is no other
> > +                                * idle disk and all disks are rotational, this
> > +                                * disk will still be chosen.
> > +                                */
> > +                               pending++;
> > +                               dist = 0;
> > +                       } else {
> > +                               best_disk = disk;
> > +                               break;
> >                          }
> > -                       break;
> >                  }
>
> Hi Kuai
>
> I noticed something. In patch 12cee5a8a29e, it sets best_disk if it's
> a sequential read. If there are no other idle disks, it will read from
> the sequential disk. With this patch, it reads from the
> best_pending_disk even min_pending is not 0. It looks like a wrong
> behaviour?

Yes, nice catch, I didn't notice this yet... So there is a hidden
logical, sequential IO priority is higher than minimal 'pending'
selection, it's only less than 'choose_next_idle' where idle disk
exist.

Looks like if we want to keep this behaviour, we can add a 'sequential
disk':

if (is_sequential())
 if (!should_choose_next())
  return disk;
 ctl.sequential_disk = disk;

..

if (ctl.min_pending != 0 && ctl.sequential_disk != -1)
 return ctl.sequential_disk;

Thanks,
Kuai

> Best Regards
> Xiao
> > -               if (choose_next_idle)
> > -                       continue;
> > -
> >                  if (min_pending > pending) {
> >                          min_pending = pending;
> >                          best_pending_disk = disk;
> > --
> > 2.39.2
>
> .