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