Re: [PATCH -next v3 4/7] md/raid1-10: submit write io directly if bitmap is not enabled

[Xiao Ni]

On Mon, May 29, 2023 at 9:14 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Commit 6cce3b23f6f8 ("[PATCH] md: write intent bitmap support for raid10")
> add bitmap support, and it changed that write io is submitted through
> daemon thread because bitmap need to be updated before write io. And
> later, plug is used to fix performance regression because all the write io
> will go to demon thread, which means io can't be issued concurrently.
>
> However, if bitmap is not enabled, the write io should not go to daemon
> thread in the first place, and plug is not needed as well.
>
> Fixes: 6cce3b23f6f8 ("[PATCH] md: write intent bitmap support for raid10")
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  drivers/md/md-bitmap.c |  4 +---
>  drivers/md/md-bitmap.h |  7 +++++++
>  drivers/md/raid1-10.c  | 13 +++++++++++--
>  3 files changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
> index ad5a3456cd8a..3ee590cf12a7 100644
> --- a/drivers/md/md-bitmap.c
> +++ b/drivers/md/md-bitmap.c
> @@ -1016,7 +1016,6 @@ static int md_bitmap_file_test_bit(struct bitmap *bitmap, sector_t block)
>         return set;
>  }
>
> -
>  /* this gets called when the md device is ready to unplug its underlying
>   * (slave) device queues -- before we let any writes go down, we need to
>   * sync the dirty pages of the bitmap file to disk */
> @@ -1026,8 +1025,7 @@ void md_bitmap_unplug(struct bitmap *bitmap)
>         int dirty, need_write;
>         int writing = 0;
>
> -       if (!bitmap || !bitmap->storage.filemap ||
> -           test_bit(BITMAP_STALE, &bitmap->flags))
> +       if (!md_bitmap_enabled(bitmap))
>                 return;
>
>         /* look at each page to see if there are any set bits that need to be
> diff --git a/drivers/md/md-bitmap.h b/drivers/md/md-bitmap.h
> index cfd7395de8fd..3a4750952b3a 100644
> --- a/drivers/md/md-bitmap.h
> +++ b/drivers/md/md-bitmap.h
> @@ -273,6 +273,13 @@ int md_bitmap_copy_from_slot(struct mddev *mddev, int slot,
>                              sector_t *lo, sector_t *hi, bool clear_bits);
>  void md_bitmap_free(struct bitmap *bitmap);
>  void md_bitmap_wait_behind_writes(struct mddev *mddev);
> +
> +static inline bool md_bitmap_enabled(struct bitmap *bitmap)
> +{
> +       return bitmap && bitmap->storage.filemap &&
> +              !test_bit(BITMAP_STALE, &bitmap->flags);
> +}
> +
>  #endif
>
>  #endif
> diff --git a/drivers/md/raid1-10.c b/drivers/md/raid1-10.c
> index 506299bd55cb..73cc3cb9154d 100644
> --- a/drivers/md/raid1-10.c
> +++ b/drivers/md/raid1-10.c
> @@ -131,9 +131,18 @@ static inline bool raid1_add_bio_to_plug(struct mddev *mddev, struct bio *bio,
>                                       blk_plug_cb_fn unplug)
>  {
>         struct raid1_plug_cb *plug = NULL;
> -       struct blk_plug_cb *cb = blk_check_plugged(unplug, mddev,
> -                                                  sizeof(*plug));
> +       struct blk_plug_cb *cb;
> +
> +       /*
> +        * If bitmap is not enabled, it's safe to submit the io directly, and
> +        * this can get optimal performance.
> +        */
> +       if (!md_bitmap_enabled(mddev->bitmap)) {
> +               raid1_submit_write(bio);
> +               return true;
> +       }

Can we check this out of raid1_add_bio_to_plug and call
raid1_submit_write directly in make_request function?

Regards
Xiao
>
> +       cb = blk_check_plugged(unplug, mddev, sizeof(*plug));
>         if (!cb)
>                 return false;
>
> --
> 2.39.2
>