On Thu, Mar 25, 2021 at 02:34:18PM +0800, JeffleXu wrote:
>
>
> On 3/24/21 8:19 PM, Ming Lei wrote:
> > Currently bio based IO polling needs to poll all hw queue blindly, this
> > way is very inefficient, and one big reason is that we can't pass any
> > bio submission result to blk_poll().
> >
> > In IO submission context, track associated underlying bios by per-task
> > submission queue and store returned 'cookie' 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, then we can find bios from the per-task io pull context of
> > submission context. Moving bios 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 flags of BIO_DONE and
> > BIO_END_BY_POLL for such purpose.
> >
> > In was found in Jeffle Xu's test that kfifo doesn't scale well for a
> > submission queue as queue depth is increased, so a new mechanism for
> > tracking bios is needed. 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 ending bio really, 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 16 or less 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.
> >
> > Also when the submission task is exiting, drain pending IOs in the context
> > until all are done.
> >
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > ---
> > block/bio.c | 5 +
> > block/blk-core.c | 154 ++++++++++++++++++++++++-
> > block/blk-ioc.c | 2 +
> > block/blk-mq.c | 234 +++++++++++++++++++++++++++++++++++++-
> > block/blk.h | 10 ++
> > include/linux/blk_types.h | 18 ++-
> > 6 files changed, 419 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 eb07d61cfdc2..95f7e36c8759 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -805,6 +805,81 @@ 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->nr_grps; i++) {
> > + grp = &list->head[i];
> > + if (grp->grp_data == grp_data)
> > + return i;
> > + }
> > +
> > + if (i < list->max_nr_grps) {
> > + grp = &list->head[i];
> > + bio_list_init(&grp->list);
> > + 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;
> > + dst->nr_grps++;
> > + }
> > + __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 +941,45 @@ static inline void blk_bio_poll_preprocess(struct request_queue *q,
> > bio->bi_opf |= REQ_POLL_CTX;
> > }
> >
> > +static inline void blk_bio_poll_mark_queued(struct bio *bio, bool queued)
> > +{
> > + /*
> > + * The bio has been added to per-task poll queue, mark it as
> > + * END_BY_POLL, so that this bio is always completed from
> > + * blk_poll() which is provided with cookied from this bio's
> > + * submission.
> > + */
> > + if (!queued)
> > + bio->bi_opf &= ~(REQ_HIPRI | REQ_POLL_CTX);
> > + else
> > + bio_set_flag(bio, BIO_END_BY_POLL);
> > +}
> > +
> > +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
> > + * submission, so simply don't 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);
> > + blk_bio_poll_mark_queued(bio, queued);
> > + spin_unlock(&pc->sq_lock);
> > + } else {
> > + queued = false;
> > + blk_bio_poll_mark_queued(bio, false);
> > + }
> > +
> > + return queued;
> > +}
> > +
> > static noinline_for_stack bool submit_bio_checks(struct bio *bio)
> > {
> > struct block_device *bdev = bio->bi_bdev;
> > @@ -1018,7 +1132,7 @@ 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_ctx(struct bio *bio, struct io_context *ioc)
> > {
> > struct bio_list bio_list_on_stack[2];
> > blk_qc_t ret = BLK_QC_T_NONE;
> > @@ -1041,7 +1155,16 @@ 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_POLL_CTX))) {
>
>
> I can see no sense to enqueue the bio into the context->sq when
> REQ_HIPRI is cleared while REQ_POLL_CTX is set for the bio.
> BLK_QC_T_NONE is returned in this case. This is possible since commit
> cc29e1bf0d63 ("block: disable iopoll for split bio").
bio has to be enqueued before submission, and once it is enqueued, it has
to be ended by blk_poll(), this way actually simplifies polled bio lifetime
a lot, no matter if this bio is really completed via poll or irq.
When submit_bio() is returning BLK_QC_T_NONE, this bio may have been
completed already, and we shouldn't touch that bio any more, otherwise
things can become quite complicated.
>
>
> > + bool queued = blk_bio_poll_prep_submit(ioc, bio);
> > +
> > + ret = __submit_bio(bio);
> > + if (queued)
> > + bio_set_private_data(bio, ret);
> > + } else {
> > + ret = __submit_bio(bio);
> > + }
> >
> > /*
> > * Sort new bios into those for a lower level and those for the
> > @@ -1067,6 +1190,33 @@ 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_ctx(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 BLK_QC_T_BIO_NONE;
> > +}
> > +
> > +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);
> > +
> > + __submit_bio_noacct_ctx(bio, NULL);
> > +
> > + return BLK_QC_T_BIO_NONE;
> > +}
> > +
> > static blk_qc_t __submit_bio_noacct_mq(struct bio *bio)
> > {
> > struct bio_list bio_list[2] = { };
> > diff --git a/block/blk-ioc.c b/block/blk-ioc.c
> > index 5574c398eff6..b9a512f066f8 100644
> > --- a/block/blk-ioc.c
> > +++ b/block/blk-ioc.c
> > @@ -19,6 +19,8 @@ static struct kmem_cache *iocontext_cachep;
> >
> > static inline void free_io_context(struct io_context *ioc)
> > {
> > + blk_bio_poll_io_drain(ioc);
> > +
>
> There may be a time window between the IO submission process detaches
> the io_context and the io_context's refcount finally decreased to zero,
> when there'are multiple processes sharing one io_context. I don't know
> if it is possible that the other process sharing the io_context won't
> submit any IO, in which case the bios remained in the io_context won't
> be reaped for a long time.
>
> If the above case is possible, then is it possible to drain the sq once
> the process detaches the io_context?
free_io_context() is called after the ioc's refcount drops to zero, so
any process sharing this ioc has to be exited.
>
>
> > kfree(ioc->data);
> > kmem_cache_free(iocontext_cachep, ioc);
> > }
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 03f59915fe2c..76a90da83d9c 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -3865,14 +3865,246 @@ static inline int blk_mq_poll_hctx(struct request_queue *q,
> > return ret;
> > }
> >
> > +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_private_data(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 bio_grp_list *grps)
> > +{
> > + 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 < grps->nr_grps; i++) {
> > + struct bio_grp_list_data *grp = &grps->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 < grps->nr_grps; i++) {
> > + struct bio_grp_list_data *grp = &grps->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)) {
> > + /* 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 void blk_bio_poll_pack_groups(struct bio_grp_list *grps)
> > +{
> > + int i, j, k = 0;
> > + int cnt = 0;
> > +
> > + for (i = grps->nr_grps - 1; i >= 0; i--) {
> > + struct bio_grp_list_data *grp = &grps->head[i];
> > + struct bio_grp_list_data *hole = NULL;
> > +
> > + if (bio_grp_list_grp_empty(grp)) {
> > + cnt++;
> > + continue;
> > + }
> > +
>
> > + for (j = k; j < i; j++) {
> > + hole = &grps->head[j];
> > + if (bio_grp_list_grp_empty(hole))
> > + break;
> > + }
>
> Shoule be
>
> > + for (j = k; j < i; j++) {
> > + tmp = &grps->head[j];
> > + if (bio_grp_list_grp_empty(tmp)) {
> > + hole = tmp;
> > + break;
> > + }
> > + }
Good catch!
>
> > + if (hole == NULL)
> > + break;
> > + *hole = *grp;
> > + cnt++;
> > + k = j;
> > + }
> > +
> > + grps->nr_grps -= cnt;
> > +}
> > +
> > +#define MAX_BIO_GRPS_ON_STACK 8
> > +struct bio_grp_list_stack {
> > + unsigned int max_nr_grps, nr_grps;
> > + struct bio_grp_list_data head[MAX_BIO_GRPS_ON_STACK];
> > +};
> > +
> > +static int blk_bio_poll_io(struct io_context *submit_ioc,
> > + struct io_context *poll_ioc)
> > +
> > +{
> > + struct bio_grp_list_stack _bio_grps = {
> > + .max_nr_grps = ARRAY_SIZE(_bio_grps.head),
> > + .nr_grps = 0
> > + };
> > + struct bio_grp_list *bio_grps = (struct bio_grp_list *)&_bio_grps;
> > + struct blk_bio_poll_ctx *submit_ctx = submit_ioc->data;
> > + struct blk_bio_poll_ctx *poll_ctx = poll_ioc ?
> > + poll_ioc->data : NULL;
> > + int ret = 0;
> > +
> > + /*
> > + * 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(bio_grps, submit_ctx->sq);
> > + spin_unlock(&submit_ctx->sq_lock);
> > +
> > + /* merge new bios first, then start to poll bios from pq */
> > + if (poll_ctx) {
> > + spin_lock(&poll_ctx->pq_lock);
> > + bio_grp_list_move(poll_ctx->pq, bio_grps);
> > + bio_grp_list_move(bio_grps, poll_ctx->pq);
>
> What's the purpose of this two-step merge? Is that for new bios (from
> sq) is at the tail of the bio_list, and thus old bios (from pq) is
> polled first?
Yeah, so we can poll old bios first. Also the following bio polling can
cover new bios just from submission context too.
>
> > + spin_unlock(&poll_ctx->pq_lock);
> > + }
> > +
> > + do {
> > + ret += blk_bio_poll_and_end_io(bio_grps);
> > + blk_bio_poll_pack_groups(bio_grps);
> > +
> > + if (bio_grps->nr_grps) {
> > + /*
> > + * move back, and keep polling until all can be
> > + * held in either poll queue or submission queue.
> > + */
> > + if (poll_ctx) {
> > + spin_lock(&poll_ctx->pq_lock);
> > + bio_grp_list_move(poll_ctx->pq, bio_grps);
> > + spin_unlock(&poll_ctx->pq_lock);
> > + } else {
> > + spin_lock(&submit_ctx->sq_lock);
> > + bio_grp_list_move(submit_ctx->sq, bio_grps);
> > + spin_unlock(&submit_ctx->sq_lock);
> > + }
> > + }
> > + } while (bio_grps->nr_grps > 0);
> > +
> > + return ret;
> > +}
> > +
> > +void blk_bio_poll_io_drain(struct io_context *submit_ioc)
> > +{
> > + struct blk_bio_poll_ctx *submit_ctx = submit_ioc->data;
> > +
> > + if (!submit_ctx)
> > + return;
> > +
> > + while (submit_ctx->sq->nr_grps > 0) {
> > + blk_bio_poll_io(submit_ioc, NULL);
> > + cpu_relax();
> > + }
> > +}
> > +
> > +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(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(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(submit_task->io_context, poll_ioc);
> > + else
> > + ret = 0;
> > +
> > + put_task_struct(submit_task);
>
> put_task_struct() is not needed when @submit_task is NULL.
Good catch, usually submit_task shouldn't be NULL, but it can be exited
already.
Thanks,
Ming
