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

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Mar 18 2021 at 12:48pm -0400,
Ming Lei <ming.lei@redhat.com> wrote:

> Currently bio based IO poll needs to poll all hw queue blindly, this way
> is very inefficient, and the big reason is that we can't pass bio
> submission result to io poll task.
> 
> In IO submission context, track associated underlying bios by per-task
> submission queue and save 'cookie' poll data in bio->bi_iter.bi_private_data,
> and return current->pid to caller of submit_bio() for any bio based
> driver's IO, which is submitted from FS.
> 
> In IO poll context, the passed cookie tells us the PID of submission
> context, and we can find the bio from that submission context. Moving
> bio from submission queue to poll queue of the poll context, and keep
> polling until these bios are ended. Remove bio from poll queue if the
> bio is ended. Add BIO_DONE and BIO_END_BY_POLL for such purpose.
> 
> In previous version, kfifo is used to implement submission queue, and
> Jeffle Xu found that kfifo can't scale well in case of high queue depth.
> So far bio's size is close to 2 cacheline size, and it may not be
> accepted to add new field into bio for solving the scalability issue by
> tracking bios via linked list, switch to bio group list for tracking bio,
> the idea is to reuse .bi_end_io for linking bios into a linked list for
> all sharing same .bi_end_io(call it bio group), which is recovered before
> really end bio, since BIO_END_BY_POLL is added for enhancing this point.
> Usually .bi_end_bio is same for all bios in same layer, so it is enough to
> provide very limited groups, such as 32 for fixing the scalability issue.
> 
> Usually submission shares context with io poll. The per-task poll context
> is just like stack variable, and it is cheap to move data between the two
> per-task queues.
> 
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/bio.c               |   5 ++
>  block/blk-core.c          | 149 +++++++++++++++++++++++++++++++-
>  block/blk-mq.c            | 173 +++++++++++++++++++++++++++++++++++++-
>  block/blk.h               |   9 ++
>  include/linux/blk_types.h |  16 +++-
>  5 files changed, 348 insertions(+), 4 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 26b7f721cda8..04c043dc60fc 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1402,6 +1402,11 @@ static inline bool bio_remaining_done(struct bio *bio)
>   **/
>  void bio_endio(struct bio *bio)
>  {
> +	/* BIO_END_BY_POLL has to be set before calling submit_bio */
> +	if (bio_flagged(bio, BIO_END_BY_POLL)) {
> +		bio_set_flag(bio, BIO_DONE);
> +		return;
> +	}
>  again:
>  	if (!bio_remaining_done(bio))
>  		return;
> diff --git a/block/blk-core.c b/block/blk-core.c
> index efc7a61a84b4..778d25a7e76c 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -805,6 +805,77 @@ static inline unsigned int bio_grp_list_size(unsigned int nr_grps)
>  		sizeof(struct bio_grp_list_data);
>  }
>  
> +static inline void *bio_grp_data(struct bio *bio)
> +{
> +	return bio->bi_poll;
> +}
> +
> +/* add bio into bio group list, return true if it is added */
> +static bool bio_grp_list_add(struct bio_grp_list *list, struct bio *bio)
> +{
> +	int i;
> +	struct bio_grp_list_data *grp;
> +
> +	for (i = 0; i < list->nr_grps; i++) {
> +		grp = &list->head[i];
> +		if (grp->grp_data == bio_grp_data(bio)) {
> +			__bio_grp_list_add(&grp->list, bio);
> +			return true;
> +		}
> +	}
> +
> +	if (i == list->max_nr_grps)
> +		return false;
> +
> +	/* create a new group */
> +	grp = &list->head[i];
> +	bio_list_init(&grp->list);
> +	grp->grp_data = bio_grp_data(bio);
> +	__bio_grp_list_add(&grp->list, bio);
> +	list->nr_grps++;
> +
> +	return true;
> +}
> +
> +static int bio_grp_list_find_grp(struct bio_grp_list *list, void *grp_data)
> +{
> +	int i;
> +	struct bio_grp_list_data *grp;
> +
> +	for (i = 0; i < list->max_nr_grps; i++) {
> +		grp = &list->head[i];
> +		if (grp->grp_data == grp_data)
> +			return i;
> +	}
> +	for (i = 0; i < list->max_nr_grps; i++) {
> +		grp = &list->head[i];
> +		if (bio_grp_list_grp_empty(grp))
> +			return i;
> +	}
> +	return -1;
> +}
> +
> +/* Move as many as possible groups from 'src' to 'dst' */
> +void bio_grp_list_move(struct bio_grp_list *dst, struct bio_grp_list *src)
> +{
> +	int i, j, cnt = 0;
> +	struct bio_grp_list_data *grp;
> +
> +	for (i = src->nr_grps - 1; i >= 0; i--) {
> +		grp = &src->head[i];
> +		j = bio_grp_list_find_grp(dst, grp->grp_data);
> +		if (j < 0)
> +			break;
> +		if (bio_grp_list_grp_empty(&dst->head[j]))
> +			dst->head[j].grp_data = grp->grp_data;
> +		__bio_grp_list_merge(&dst->head[j].list, &grp->list);
> +		bio_list_init(&grp->list);
> +		cnt++;
> +	}
> +
> +	src->nr_grps -= cnt;
> +}
> +
>  static void bio_poll_ctx_init(struct blk_bio_poll_ctx *pc)
>  {
>  	pc->sq = (void *)pc + sizeof(*pc);
> @@ -866,6 +937,46 @@ static inline void blk_bio_poll_preprocess(struct request_queue *q,
>  		bio->bi_opf |= REQ_TAG;
>  }
>  
> +static bool blk_bio_poll_prep_submit(struct io_context *ioc, struct bio *bio)
> +{
> +	struct blk_bio_poll_ctx *pc = ioc->data;
> +	unsigned int queued;
> +
> +	/*
> +	 * We rely on immutable .bi_end_io between blk-mq bio submission
> +	 * and completion. However, bio crypt may update .bi_end_io during
> +	 * submitting, so simply not support bio based polling for this
> +	 * setting.
> +	 */
> +	if (likely(!bio_has_crypt_ctx(bio))) {
> +		/* track this bio via bio group list */
> +		spin_lock(&pc->sq_lock);
> +		queued = bio_grp_list_add(pc->sq, bio);
> +		spin_unlock(&pc->sq_lock);
> +	} else {
> +		queued = false;
> +	}
> +
> +	/*
> +	 * Now the bio is added per-task fifo, mark it as END_BY_POLL,
> +	 * and the bio is always completed from the pair poll context.
> +	 *
> +	 * One invariant is that if bio isn't completed, blk_poll() will
> +	 * be called by passing cookie returned from submitting this bio.
> +	 */
> +	if (!queued)
> +		bio->bi_opf &= ~(REQ_HIPRI | REQ_TAG);
> +	else
> +		bio_set_flag(bio, BIO_END_BY_POLL);
> +
> +	return queued;
> +}
> +
> +static void blk_bio_poll_post_submit(struct bio *bio, blk_qc_t cookie)
> +{
> +	bio->bi_iter.bi_private_data = cookie;
> +}
> +
>  static noinline_for_stack bool submit_bio_checks(struct bio *bio)
>  {
>  	struct block_device *bdev = bio->bi_bdev;
> @@ -1020,7 +1131,7 @@ static blk_qc_t __submit_bio(struct bio *bio)
>   * bio_list_on_stack[1] contains bios that were submitted before the current
>   *	->submit_bio_bio, but that haven't been processed yet.
>   */
> -static blk_qc_t __submit_bio_noacct(struct bio *bio)
> +static blk_qc_t __submit_bio_noacct_int(struct bio *bio, struct io_context *ioc)
>  {
>  	struct bio_list bio_list_on_stack[2];
>  	blk_qc_t ret = BLK_QC_T_NONE;
> @@ -1043,7 +1154,16 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio)
>  		bio_list_on_stack[1] = bio_list_on_stack[0];
>  		bio_list_init(&bio_list_on_stack[0]);
>  
> -		ret = __submit_bio(bio);
> +		if (ioc && queue_is_mq(q) &&
> +				(bio->bi_opf & (REQ_HIPRI | REQ_TAG))) {
> +			bool queued = blk_bio_poll_prep_submit(ioc, bio);
> +
> +			ret = __submit_bio(bio);
> +			if (queued)
> +				blk_bio_poll_post_submit(bio, ret);
> +		} else {
> +			ret = __submit_bio(bio);
> +		}

So you're only supporting bio-based polling if the bio-based device is
stacked _directly_ ontop of blk-mq?  Severely limits the utility of
bio-based IO polling support if such shallow stacking is required.

Mike