[dm-devel] [RFC PATCH V2 09/13] block: use per-task poll context to implement bio based io poll
JeffleXu
jefflexu at linux.alibaba.com
Sat Mar 20 05:56:13 UTC 2021
On 3/19/21 9:46 PM, Ming Lei wrote:
> On Fri, Mar 19, 2021 at 05:38:38PM +0800, JeffleXu wrote:
>> I'm thinking how this mechanism could work with *original* bio-based
>> devices that don't ne built upon mq devices, such as nvdimm. This
>
> non-mq device needs driver to implement io polling by itself, block
> layer can't help it, and that can't be this patchset's job.
>
>> mechanism (also including my original design) mainly focuses on virtual
>> devices that built upon mq devices, i.e., md/dm.
>>
>> As the original bio-based devices wants to support IO polling in the
>> future, then they should be somehow distingushed from md/dm.
>>
>>
>> On 3/19/21 12:48 AM, Ming Lei 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 at 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;
>>> +}
>>
>> Not sure why it's checked in reverse order (starting from 'nr_grps - 1').
>
> Then for bio group list in submission side, only first .nr_grps groups
> includes bios.
>
>>
>>
>>> +
>>> 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);
>>> + }
>>
>> If input @ioc is NULL, it will still return cookie (returned by
>> __submit_bio()), which will call into blk_bio_poll(), which is not
>> expected. (It can pass blk_queue_poll() check in blk_poll(), e.g., dm
>> device itself is marked as QUEUE_FLAG_POLL, but ->io_context of IO
>> submitting process failed to allocate the io_context, and thus calls
>> __submit_bio_noacct_int() with @ioc is NULL).
>
> Good catch, looks the following change is needed:
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 778d25a7e76c..dba12ba0fa48 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1211,7 +1211,8 @@ static inline blk_qc_t __submit_bio_noacct(struct bio *bio)
> if (ioc && ioc->data && (bio->bi_opf & REQ_HIPRI))
> return __submit_bio_noacct_poll(bio, ioc);
>
> - return __submit_bio_noacct_int(bio, NULL);
> + __submit_bio_noacct_int(bio, NULL);
> + return 0;
> }
Looks good as far as now no original bio-based device supporting IO polling.
>
> static blk_qc_t __submit_bio_noacct_mq(struct 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)) {
>>> +
>>> + /* 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);
>>
>> Why mutex is used to protect pq here rather than spinlock? Where will
>
> spinlock should be fine.
>
>> the polling routine go to sleep?
>
> The current blk_poll() can go to sleep really.
I know that hybrid polling can go to sleep. Except that, it seems no
other place can sleep?
>
>>
>> Besides, how to protect the concurrent bio_list operation to sq between
>> producer (submission routine) and consumer (polling routine)? As far as
>
> submit_ctx->sq_lock is held in __blk_bio_poll_io() for moving bios
> from sq to pq, see __blk_bio_poll_io().
>
>> I understand, pc->sq_lock is used to prevent concurrent access from
>> multiple submission processes, while pc->pq_lock is used to prevent
>> concurrent access from multiple polling processes.
>
> Usually poll context needn't any lock except for shared io context,
> because blk_bio_poll_ctx->pq is only accessed in poll context, and
> it is still per-task.
>
>>
>>
>>> + 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);
>>
>> What if the process to which the returned cookie refers has exited?
>> find_get_task_by_vpid() will return NULL, thus blk_poll() won't help
>> reap anything, while maybe there are still bios in the poll context,
>> waiting to be reaped.
>>
>> Maybe we need to flush the poll context when a task detaches from the
>> io_context.
>
> Yeah, I know that issue, and just not address it in RFC stage.
>
> It can be handled in the following way:
>
> 1) drain all bios in submission context until all bios are completed before
> it exits since the code won't sleep.
>
> OR
>
> 2) schedule wq for completing all submitted bios.
>
> If poll context exits, and there is still bios not completed, not sure
> if it can happen because the current in-tree code has the same issue.
>
>>
>>
>>> + if (likely(blk_bio_ioc_valid(submit_task)))
>>> + ret = blk_bio_poll_io(q, submit_task->io_context,
>>> + poll_ioc);
>>
>> poll_ioc may be invalid in this case, since the previous
>> blk_create_io_context() in blk_bio_poll() may fail.
>
> Yeah, it can be addressed by above patch, 0 will be returned
> for this case.
>
Not really. I mean in the case of 'current->pid != pid', poll_ioc, i.e.,
current->io_context could be invalid, i.e., current->io_context is NULL,
or current->io_context->data is NULL. Maybe it should be fixed by
+ if (likely(blk_bio_ioc_valid(submit_task) && blk_bio_ioc_valid(current)))
+ ret = blk_bio_poll_io(q, submit_task->io_context,
+ poll_ioc);
Can't understand why the above patch you mentioned could fix this.
--
Thanks,
Jeffle
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