Re: [PATCH md-6.9 v2 03/10] md/raid1: fix choose next idle in read_balance()

[Xiao Ni]

On Tue, Feb 27, 2024 at 8:09 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 || dist == 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) add a new local variable 'sequential_disk' to record the disk, and if
>    there is no other idle disk, 'sequential_disk' will 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 | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index 0fed01b06de9..fc5899fb08c1 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -598,13 +598,12 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
>         const sector_t this_sector = r1_bio->sector;
>         int sectors;
>         int best_good_sectors;
> -       int best_disk, best_dist_disk, best_pending_disk;
> +       int best_disk, best_dist_disk, best_pending_disk, sequential_disk;
>         int disk;
>         sector_t best_dist;
>         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
> @@ -615,11 +614,11 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
>         sectors = r1_bio->sectors;
>         best_disk = -1;
>         best_dist_disk = -1;
> +       sequential_disk = -1;
>         best_dist = MaxSector;
>         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 +711,6 @@ 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 +729,22 @@ 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.
> +                                */
> +                               pending++;
> +                               /*
> +                                * If there is no other idle disk, this disk
> +                                * will be chosen.
> +                                */
> +                               sequential_disk = disk;
> +                       } else {
> +                               best_disk = disk;
> +                               break;
>                         }
> -                       break;
>                 }
>
> -               if (choose_next_idle)
> -                       continue;
> -
>                 if (min_pending > pending) {
>                         min_pending = pending;
>                         best_pending_disk = disk;
> @@ -751,6 +756,13 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
>                 }
>         }
>
> +       /*
> +        * sequential IO size exceeds optimal iosize, however, there is no other
> +        * idle disk, so choose the sequential disk.
> +        */
> +       if (best_disk == -1 && min_pending != 0)
> +               best_disk = sequential_disk;
> +
>         /*
>          * If all disks are rotational, choose the closest disk. If any disk is
>          * non-rotational, choose the disk with less pending request even the
> --
> 2.39.2
>
Hi all
This patch looks good to me.
Reviewed-by: Xiao Ni <xni@xxxxxxxxxx>