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

[Xiao Ni]

On Mon, Feb 26, 2024 at 5:40 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> 在 2024/02/26 17:24, Xiao Ni 写道:
> > 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.
>
> Yes, and Paul change the name 'best_dist' to 'closest_dist_disk',
> and 'btest_disk_disk' to 'closest_dist' in the last patch to avoid typo
> like this. :)

Ah, thanks :)  I haven't been there.

Regards
Xiao
>
> Thanks,
> Kuai
>
>
> >
> > 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
> >>>>
> >>>>
> >>>
> >>> .
> >>>
> >>
> >
> > .
> >
>