On Fri, Mar 19, 2021 at 02:38:11PM -0400, Mike Snitzer wrote:
> On Thu, Mar 18 2021 at 12:48pm -0400,
> Ming Lei <ming.lei@xxxxxxxxxx> 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.
>
> This is awkward because bio-based IO polling doesn't exist upstream yet,
> so this header should be covering your approach as a clean slate, e.g.:
>
> The complexity associated with frequent bio splitting with bio-based
> devices makes it difficult to implement IO polling efficiently because
> the fan-out of underlying hw queues that need to be polled (as a
> side-effect of bios being split) creates a need for more easily mapping
> a group of bios to the hw queues that need to be polled.
>
> > 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
>
> Maybe be more precise by covering how all bios from that task's
> submission context will be moved to poll queue of poll context?
Strictly speaking, it isn't a must to add poll context which is just
more efficient for polling locally after taking bio grouping list in V2.
>
> > 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.
>
> Awkward to reference "previous version", maybe instead say:
>
> In was found that kfifo doesn't scale well for a submission queue as
> queue depth is increased, so a new mechanism for tracking bios is
> needed.
OK.
>
> > 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@xxxxxxxxxx>
> > ---
> > 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
>
> s/submitting/submission/
> s/not/don't/
>
> > + * 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.
>
> This reads awkwardly.. "pair poll context"?
Actually I meant that poll context in which blk_poll(cookie) is called
and 'cookie' is the exact return value of submit_bio(bio based bio) and
this blk-mq bio is for complete the bio driver's bio, and share same
pool context with the bio driver's bio.
>
> > + *
> > + * 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;
>
> I mentioned this in previous mail, but what is it you're trying to
> convey with _int?
>
> Think we need a better function name here.
I will think about one better name, not got one yet.
>
> > @@ -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);
> > + }
> >
> > /*
> > * Sort new bios into those for a lower level and those for the
> > @@ -1069,6 +1189,31 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio)
> > return ret;
> > }
> >
> > +static inline blk_qc_t __submit_bio_noacct_poll(struct bio *bio,
> > + struct io_context *ioc)
> > +{
> > + struct blk_bio_poll_ctx *pc = ioc->data;
> > +
> > + __submit_bio_noacct_int(bio, ioc);
> > +
> > + /* bio submissions queued to per-task poll context */
> > + if (READ_ONCE(pc->sq->nr_grps))
> > + return current->pid;
> > +
> > + /* swapper's pid is 0, but it can't submit poll IO for us */
> > + return 0;
> > +}
> > +
> > +static inline blk_qc_t __submit_bio_noacct(struct bio *bio)
> > +{
> > + struct io_context *ioc = current->io_context;
> > +
> > + if (ioc && ioc->data && (bio->bi_opf & REQ_HIPRI))
> > + return __submit_bio_noacct_poll(bio, ioc);
> > +
> > + return __submit_bio_noacct_int(bio, NULL);
> > +}
> > +
> > static blk_qc_t __submit_bio_noacct_mq(struct bio *bio)
> > {
> > struct bio_list bio_list[2] = { };
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 03f59915fe2c..f26950a51f4a 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -3865,14 +3865,185 @@ static inline int blk_mq_poll_hctx(struct request_queue *q,
> > return ret;
> > }
> >
> > +static blk_qc_t bio_get_poll_cookie(struct bio *bio)
> > +{
> > + return bio->bi_iter.bi_private_data;
> > +}
> > +
> > +static int blk_mq_poll_io(struct bio *bio)
> > +{
> > + struct request_queue *q = bio->bi_bdev->bd_disk->queue;
> > + blk_qc_t cookie = bio_get_poll_cookie(bio);
> > + int ret = 0;
> > +
> > + if (!bio_flagged(bio, BIO_DONE) && blk_qc_t_valid(cookie)) {
> > + struct blk_mq_hw_ctx *hctx =
> > + q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)];
> > +
> > + ret += blk_mq_poll_hctx(q, hctx);
> > + }
> > + return ret;
> > +}
> > +
> > +static int blk_bio_poll_and_end_io(struct request_queue *q,
> > + struct blk_bio_poll_ctx *poll_ctx)
> > +{
> > + int ret = 0;
> > + int i;
> > +
> > + /*
> > + * Poll hw queue first.
> > + *
> > + * TODO: limit max poll times and make sure to not poll same
> > + * hw queue one more time.
> > + */
> > + for (i = 0; i < poll_ctx->pq->max_nr_grps; i++) {
> > + struct bio_grp_list_data *grp = &poll_ctx->pq->head[i];
> > + struct bio *bio;
> > +
> > + if (bio_grp_list_grp_empty(grp))
> > + continue;
> > +
> > + for (bio = grp->list.head; bio; bio = bio->bi_poll)
> > + ret += blk_mq_poll_io(bio);
> > + }
> > +
> > + /* reap bios */
> > + for (i = 0; i < poll_ctx->pq->max_nr_grps; i++) {
> > + struct bio_grp_list_data *grp = &poll_ctx->pq->head[i];
> > + struct bio *bio;
> > + struct bio_list bl;
> > +
> > + if (bio_grp_list_grp_empty(grp))
> > + continue;
> > +
> > + bio_list_init(&bl);
> > +
> > + while ((bio = __bio_grp_list_pop(&grp->list))) {
> > + if (bio_flagged(bio, BIO_DONE)) {
> > +
>
> Remove empty newline? ^
>
> > + /* now recover original data */
> > + bio->bi_poll = grp->grp_data;
> > +
> > + /* clear BIO_END_BY_POLL and end me really */
> > + bio_clear_flag(bio, BIO_END_BY_POLL);
> > + bio_endio(bio);
> > + } else {
> > + __bio_grp_list_add(&bl, bio);
> > + }
> > + }
> > + __bio_grp_list_merge(&grp->list, &bl);
> > + }
> > + return ret;
> > +}
> > +
> > +static int __blk_bio_poll_io(struct request_queue *q,
> > + struct blk_bio_poll_ctx *submit_ctx,
> > + struct blk_bio_poll_ctx *poll_ctx)
> > +{
> > + /*
> > + * Move IO submission result from submission queue in submission
> > + * context to poll queue of poll context.
> > + */
> > + spin_lock(&submit_ctx->sq_lock);
> > + bio_grp_list_move(poll_ctx->pq, submit_ctx->sq);
> > + spin_unlock(&submit_ctx->sq_lock);
> > +
> > + return blk_bio_poll_and_end_io(q, poll_ctx);
> > +}
> > +
> > +static int blk_bio_poll_io(struct request_queue *q,
> > + struct io_context *submit_ioc,
> > + struct io_context *poll_ioc)
> > +{
> > + struct blk_bio_poll_ctx *submit_ctx = submit_ioc->data;
> > + struct blk_bio_poll_ctx *poll_ctx = poll_ioc->data;
> > + int ret;
> > +
> > + if (unlikely(atomic_read(&poll_ioc->nr_tasks) > 1)) {
> > + mutex_lock(&poll_ctx->pq_lock);
> > + ret = __blk_bio_poll_io(q, submit_ctx, poll_ctx);
> > + mutex_unlock(&poll_ctx->pq_lock);
> > + } else {
> > + ret = __blk_bio_poll_io(q, submit_ctx, poll_ctx);
> > + }
> > + return ret;
> > +}
> > +
> > +static bool blk_bio_ioc_valid(struct task_struct *t)
> > +{
> > + if (!t)
> > + return false;
> > +
> > + if (!t->io_context)
> > + return false;
> > +
> > + if (!t->io_context->data)
> > + return false;
> > +
> > + return true;
> > +}
> > +
> > +static int __blk_bio_poll(struct request_queue *q, blk_qc_t cookie)
> > +{
> > + struct io_context *poll_ioc = current->io_context;
> > + pid_t pid;
> > + struct task_struct *submit_task;
> > + int ret;
> > +
> > + pid = (pid_t)cookie;
> > +
> > + /* io poll often share io submission context */
> > + if (likely(current->pid == pid && blk_bio_ioc_valid(current)))
> > + return blk_bio_poll_io(q, poll_ioc, poll_ioc);
> > +
> > + submit_task = find_get_task_by_vpid(pid);
> > + if (likely(blk_bio_ioc_valid(submit_task)))
> > + ret = blk_bio_poll_io(q, submit_task->io_context,
> > + poll_ioc);
>
> Style nit, but think it fine to put "poll_ioc);" on previous line.
> Otherwise, best to add braces.
>
> > + else
> > + ret = 0;
> > +
> > + put_task_struct(submit_task);
> > +
> > + return ret;
> > +}
> > +
> > static int blk_bio_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
> > {
> > + long state;
> > +
> > + /* no need to poll */
> > + if (cookie == 0)
> > + return 0;
> > +
> > /*
> > * Create poll queue for storing poll bio and its cookie from
> > * submission queue
> > */
> > blk_create_io_context(q, true);
> >
> > + state = current->state;
> > + do {
> > + int ret;
> > +
> > + ret = __blk_bio_poll(q, cookie);
> > + if (ret > 0) {
> > + __set_current_state(TASK_RUNNING);
> > + return ret;
> > + }
> > +
> > + if (signal_pending_state(state, current))
> > + __set_current_state(TASK_RUNNING);
> > +
> > + if (current->state == TASK_RUNNING)
> > + return 1;
> > + if (ret < 0 || !spin)
> > + break;
> > + cpu_relax();
> > + } while (!need_resched());
> > +
> > + __set_current_state(TASK_RUNNING);
> > return 0;
> > }
> >
> > @@ -3893,7 +4064,7 @@ int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
> > struct blk_mq_hw_ctx *hctx;
> > long state;
> >
> > - if (!blk_qc_t_valid(cookie) || !blk_queue_poll(q))
> > + if (!blk_queue_poll(q) || (queue_is_mq(q) && !blk_qc_t_valid(cookie)))
> > return 0;
> >
> > if (current->plug)
> > diff --git a/block/blk.h b/block/blk.h
> > index ae58a706327e..05b9f5eafdd1 100644
> > --- a/block/blk.h
> > +++ b/block/blk.h
> > @@ -403,4 +403,13 @@ static inline void blk_create_io_context(struct request_queue *q,
> > bio_poll_ctx_alloc(ioc);
> > }
> >
> > +BIO_LIST_HELPERS(__bio_grp_list, poll);
>
> Why the leading double underscore?
> Especially given the following 2 helpers don't have leading double underscore?
>
> Whichever you decide, just looking for consistency.
The double underscore helpers are base helpers to build helper without
double underscore.
Thanks,
Ming
