Re: [PATCH -next v2 7/7] md/raid1-10: limit the number of plugged bio

[Xiao Ni]

Hi Kuai

There is a limitation of the memory in your test. But for most
situations, customers should not set this. Can this change introduce a
performance regression against other situations?

Best Regards
Xiao

On Wed, Apr 26, 2023 at 4:24 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> bio can be added to plug infinitely, and following writeback test can
> trigger huge amount of plugged bio:
>
> Test script:
> modprobe brd rd_nr=4 rd_size=10485760
> mdadm -CR /dev/md0 -l10 -n4 /dev/ram[0123] --assume-clean
> echo 0 > /proc/sys/vm/dirty_background_ratio
> echo 60 > /proc/sys/vm/dirty_ratio
> fio -filename=/dev/md0 -ioengine=libaio -rw=write -bs=4k -numjobs=1 -iodepth=128 -name=test
>
> Test result:
> Monitor /sys/block/md0/inflight will found that inflight keep increasing
> until fio finish writing, after running for about 2 minutes:
>
> [root@fedora ~]# cat /sys/block/md0/inflight
>        0  4474191
>
> Fix the problem by limiting the number of plugged bio based on the number
> of copies for original bio.
>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  drivers/md/raid1-10.c | 9 ++++++++-
>  drivers/md/raid1.c    | 2 +-
>  drivers/md/raid10.c   | 2 +-
>  3 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/raid1-10.c b/drivers/md/raid1-10.c
> index 98d678b7df3f..35fb80aa37aa 100644
> --- a/drivers/md/raid1-10.c
> +++ b/drivers/md/raid1-10.c
> @@ -21,6 +21,7 @@
>  #define IO_MADE_GOOD ((struct bio *)2)
>
>  #define BIO_SPECIAL(bio) ((unsigned long)bio <= 2)
> +#define MAX_PLUG_BIO 32
>
>  /* for managing resync I/O pages */
>  struct resync_pages {
> @@ -31,6 +32,7 @@ struct resync_pages {
>  struct raid1_plug_cb {
>         struct blk_plug_cb      cb;
>         struct bio_list         pending;
> +       unsigned int            count;
>  };
>
>  static void rbio_pool_free(void *rbio, void *data)
> @@ -127,7 +129,7 @@ static inline void md_submit_write(struct bio *bio)
>  }
>
>  static inline bool md_add_bio_to_plug(struct mddev *mddev, struct bio *bio,
> -                                     blk_plug_cb_fn unplug)
> +                                     blk_plug_cb_fn unplug, int copies)
>  {
>         struct raid1_plug_cb *plug = NULL;
>         struct blk_plug_cb *cb;
> @@ -147,6 +149,11 @@ static inline bool md_add_bio_to_plug(struct mddev *mddev, struct bio *bio,
>
>         plug = container_of(cb, struct raid1_plug_cb, cb);
>         bio_list_add(&plug->pending, bio);
> +       if (++plug->count / MAX_PLUG_BIO >= copies) {
> +               list_del(&cb->list);
> +               cb->callback(cb, false);
> +       }
> +
>
>         return true;
>  }
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index 639e09cecf01..c6066408a913 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -1562,7 +1562,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
>                                               r1_bio->sector);
>                 /* flush_pending_writes() needs access to the rdev so...*/
>                 mbio->bi_bdev = (void *)rdev;
> -               if (!md_add_bio_to_plug(mddev, mbio, raid1_unplug)) {
> +               if (!md_add_bio_to_plug(mddev, mbio, raid1_unplug, disks)) {
>                         spin_lock_irqsave(&conf->device_lock, flags);
>                         bio_list_add(&conf->pending_bio_list, mbio);
>                         spin_unlock_irqrestore(&conf->device_lock, flags);
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index bd9e655ca408..7135cfaf75db 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1306,7 +1306,7 @@ static void raid10_write_one_disk(struct mddev *mddev, struct r10bio *r10_bio,
>
>         atomic_inc(&r10_bio->remaining);
>
> -       if (!md_add_bio_to_plug(mddev, mbio, raid10_unplug)) {
> +       if (!md_add_bio_to_plug(mddev, mbio, raid10_unplug, conf->copies)) {
>                 spin_lock_irqsave(&conf->device_lock, flags);
>                 bio_list_add(&conf->pending_bio_list, mbio);
>                 spin_unlock_irqrestore(&conf->device_lock, flags);
> --
> 2.39.2
>