Re: [UPDATE PATCH V2 3/7] md: the latest try for improve io stats accounting

[Song Liu <song@kernel.org>]

On Fri, May 21, 2021 at 1:01 AM Guoqing Jiang <jgq516@gmail.com> wrote:
>
> From: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
>
> Use generic io accounting functions to manage io stats. There was an
> attempt to do this earlier in commit 18c0b223cf990172 ("md: use generic
> io stats accounting functions to simplify io stat accounting"), but it
> did not include a call to generic_end_io_acct() and caused issues with
> tracking in-flight IOs, so it was later removed in commit 74672d069b29
> ("md: fix md io stats accounting broken").
>
> This patch attempts to fix this by using both generic_start_io_acct()
> and generic_end_io_acct(). To make it possible, in md_make_request() a
> bio is cloned with additional data - struct md_io, which includes the io
> start_time. A new bioset is introduced for this purpose. We call
> generic_start_io_acct() and pass the clone instead of the original to
> md_handle_request(). When it completes, we call generic_end_io_acct()
> and complete the original bio.
>
> This adds correct statistics about in-flight IOs and IO processing time,
> interpreted e.g. in iostat as await, svctm, aqu-sz and %util.
>
> It also fixes a situation where too many IOs where reported if a bio was
> re-submitted to the mddev, because io accounting is now performed only
> on newly arriving bios.
>
> Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
> [Guoqing: rebase and make generic accounting applies to personalities
>           which don't have clone infrastructure]
> Signed-off-by: Guoqing Jiang <jiangguoqing@kylinos.cn>
> ---
> Delete not necessary bioset_exit in md_integrity_register, thanks for
> Artur's review.
>
>  drivers/md/md.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  drivers/md/md.h |  1 +
>  2 files changed, 55 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 7ba00e4c862d..d8823db843db 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -441,6 +441,25 @@ void md_handle_request(struct mddev *mddev, struct bio *bio)
>  }
>  EXPORT_SYMBOL(md_handle_request);
>
> +struct md_io {
> +       struct mddev *mddev;
> +       struct bio *orig_bio;
> +       unsigned long start_time;
> +       struct bio bio_clone;
> +};
> +
> +static void md_end_io(struct bio *bio)
> +{
> +       struct md_io *md_io = bio->bi_private;
> +       struct bio *orig_bio = md_io->orig_bio;
> +
> +       orig_bio->bi_status = bio->bi_status;
> +
> +       bio_end_io_acct(orig_bio, md_io->start_time);
> +       bio_put(bio);
> +       bio_endio(orig_bio);
> +}
> +
>  static blk_qc_t md_submit_bio(struct bio *bio)
>  {
>         const int rw = bio_data_dir(bio);
> @@ -465,6 +484,30 @@ static blk_qc_t md_submit_bio(struct bio *bio)
>                 return BLK_QC_T_NONE;
>         }
>
> +       /*
> +        * clone bio under conditions:
> +        * 1. QUEUE_FLAG_IO_STAT flag is set.
> +        * 2. bio just enters md and it is not split from personality.
> +        */

We had iostat on by default. So, let set QUEUE_FLAG_IO_STAT in
md_run().

Thanks,
Song