Re: [RFC PATCH V2 09/13] block: use per-task poll context to implement bio based io poll

[Ming Lei <ming.lei@redhat.com>]

On Mon, Mar 22, 2021 at 08:46:04PM -0700, Sagi Grimberg wrote:
> 
> > +static void blk_bio_poll_post_submit(struct bio *bio, blk_qc_t cookie)
> > +{
> > +	bio->bi_iter.bi_private_data = cookie;
> > +}
> > +
> 
> Hey Ming, thinking about nvme-mpath, I'm thinking that this should be
> an exported function for failover. nvme-mpath updates bio.bi_dev
> when re-submitting I/Os to an alternate path, so I'm thinking
> that if this function is exported then nvme-mpath could do as little
> as the below to allow polling?
> 
> --
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 92adebfaf86f..e562e296153b 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -345,6 +345,7 @@ static void nvme_requeue_work(struct work_struct *work)
>         struct nvme_ns_head *head =
>                 container_of(work, struct nvme_ns_head, requeue_work);
>         struct bio *bio, *next;
> +       blk_qc_t cookie;
> 
>         spin_lock_irq(&head->requeue_lock);
>         next = bio_list_get(&head->requeue_list);
> @@ -359,7 +360,8 @@ static void nvme_requeue_work(struct work_struct *work)
>                  * path.
>                  */
>                 bio_set_dev(bio, head->disk->part0);
> -               submit_bio_noacct(bio);
> +               cookie = submit_bio_noacct(bio);
> +               blk_bio_poll_post_submit(bio, cookie);
>         }
>  }
> --
> 
> I/O failover will create misalignment from the polling context cpu and
> the submission cpu (running requeue_work), but I don't see if there is
> something that would break here...

I understand requeue shouldn't be one usual event, and I guess it is just
fine to fallback to IRQ based mode?

This patchset actually doesn't cover such bio submission from kernel context.

-- 
Ming