[dm-devel] [PATCH V3 09/13] block: use per-task poll context to implement bio based io polling
JeffleXu
jefflexu at linux.alibaba.com
Thu Mar 25 09:18:41 UTC 2021
On 3/25/21 4:05 PM, Ming Lei wrote:
> 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 at redhat.com>
>>> ---
>>> 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.
I mean if the following is adequate?
if (ioc && queue_is_mq(q) &&(bio->bi_opf & REQ_HIPRI)) {
queued = blk_bio_poll_prep_submit(ioc, bio);
ret = __submit_bio(bio);
if (queued)
bio_set_private_data(bio, ret);
}
Only REQ_HIPRI is checked here, thus bios with REQ_POLL_CTX but without
REQ_HIPRI needn't be enqueued into poll_context->sq.
For the original bio (with REQ_HIPRI | REQ_POLL_CTX), it's enqueued into
poll_context->sq, while the following split bios (only with
REQ_POLL_CTX, REQ_POLL_CTX is cleared by commit cc29e1bf0d63 ("block:
disable iopoll for split bio")) needn't be enqueued into
poll_context->sq. Since these bios (only with REQ_POLL_CTX) are not
enqueued, you won't touch them anymore and you needn't care their lifetime.
Your original code works, and my optimization here could make the code
be more clear and faster. However I'm not sure the effect of the
optimization, since the scenario of commit cc29e1bf0d63 ("block: disable
iopoll for split bio") should be rare.
>
>>
>>
>>> + 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.
Just like what I said, when the process, to which the returned cookie
refers, has exited, while the corresponding io_context exist there for a
long time (since it's shared by other processes and thus the refcount is
greater than zero), the bios in io_context->sq have no chance be reaped,
if the following two conditions are required at the same time
1) the process, to which the returned cookie refers, has exited
2) the io_context is shared by other processes, while these processes
dont's submit HIPRI IO, thus blk_poll() won't be called for this io_context.
Though the case may be rare.
>
>>
>>
>>> 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
>
--
Thanks,
Jeffle
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