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

[Xiao Ni]

On Mon, Feb 26, 2024 at 5:12 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> 在 2024/02/26 16:55, Xiao Ni 写道:
> > Hi Kuai
> >
> > Thanks for the effort!
> >
> > 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)
> >
> > typo error: s/disk/dist/g
> >
> >>    -> 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
> >>     continue
> >>
> >> 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;
> >
> > There is a problem. If all disks are not idle and there is a disk with
> > dist=0 before the seq disk, it can't read from the seq disk. It will
> > read from the first disk with dist=0. Maybe we can only add the codes
> > which are removed from 2e52d449bcec?
>
> If there is a disk with disk=0, then best_dist_disk will be updated to
> the disk, and best_dist will be updated to 0 already:
>
> // in each iteration
> if (dist < best_dist) {
>         best_dist = dist;
>         btest_disk_disk = disk;
> }
>
> In this case, best_dist will be set to the first disk with dist=0, and
> at last, the disk will be chosen:
>
> if (best_disk == -1) {
>          if (has_nonrot_disk || min_pending == 0)
>                  best_disk = best_pending_disk;
>          else
>                  best_disk = best_dist_disk;
>                 -> the first disk with dist=0;
> }
>
> So, the problem that you concerned should not exist.

Hi Kuai

Thanks for the explanation. You're right. It chooses the first disk
which has dist==0. In the above, you made the same typo error disk=0
as the comment. I guess you want to use dist=0, right? Beside this,
this patch is good to me.

Best Regards
Xiao
>
> Thanks,
> Kuai
> >
> > Best Regards
> > Xiao
> >
> >> +                       } else {
> >> +                               best_disk = disk;
> >> +                               break;
> >>                          }
> >> -                       break;
> >>                  }
> >>
> >> -               if (choose_next_idle)
> >> -                       continue;
> >> -
> >>                  if (min_pending > pending) {
> >>                          min_pending = pending;
> >>                          best_pending_disk = disk;
> >> --
> >> 2.39.2
> >>
> >>
> >
> > .
> >
>