On 3/16/21 3:17 PM, Ming Lei wrote:
> On Tue, Mar 16, 2021 at 02:46:08PM +0800, JeffleXu wrote:
>> It is a giant progress to gather all split bios that need to be polled
>> in a per-task queue. Still some comments below.
>>
>>
>> On 3/16/21 11:15 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, store associated underlying bios into the
>>> 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 DM or 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.
>>>
>>> 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 | 74 +++++++++++++++++-
>>> block/blk-mq.c | 156 +++++++++++++++++++++++++++++++++++++-
>>> include/linux/blk_types.h | 3 +
>>> 4 files changed, 235 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/block/bio.c b/block/bio.c
>>> index a1c4d2900c7a..bcf5eca0e8e3 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 a082bbc856fb..970b23fa2e6e 100644
>>> --- a/block/blk-core.c
>>> +++ b/block/blk-core.c
>>> @@ -854,6 +854,40 @@ 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_data data = {
>>> + .bio = bio,
>>> + };
>>> + struct blk_bio_poll_ctx *pc = ioc->data;
>>> + unsigned int queued;
>>> +
>>> + /* lock is required if there is more than one writer */
>>> + if (unlikely(atomic_read(&ioc->nr_tasks) > 1)) {
>>> + spin_lock(&pc->lock);
>>> + queued = kfifo_put(&pc->sq, data);
>>> + spin_unlock(&pc->lock);
>>> + } else {
>>> + queued = kfifo_put(&pc->sq, data);
>>> + }
>>> +
>>> + /*
>>> + * Now the bio is added per-task fifo, mark it as END_BY_POLL,
>>> + * so we can save cookie into this bio after submit_bio().
>>> + */
>>> + if (queued)
>>> + bio_set_flag(bio, BIO_END_BY_POLL);
>>> + else
>>> + bio->bi_opf &= ~(REQ_HIPRI | REQ_TAG);
>>> +
>>> + return queued;
>>> +}
>>
>> The size of kfifo is limited, and it seems that once the sq of kfifio is
>> full, REQ_HIPRI flag is cleared and the corresponding bio is actually
>> enqueued into the default hw queue, which is IRQ driven.
>
> Yeah, this patch starts with 64 queue depth, and we can increase it to
> 128, which should cover most of cases.
>
>>
>>
>>> +
>>> +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;
>>> @@ -1008,7 +1042,7 @@ 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;
>>> @@ -1031,7 +1065,16 @@ 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
>>> @@ -1057,6 +1100,33 @@ 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;
>>> + int entries = kfifo_len(&pc->sq);
>>> +
>>> + __submit_bio_noacct_int(bio, ioc);
>>> +
>>> + /* bio submissions queued to per-task poll context */
>>> + if (kfifo_len(&pc->sq) > entries)
>>> + 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..4e6f1467d303 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -3865,14 +3865,168 @@ 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)
>>> +{
>>> + struct blk_bio_poll_data *poll_data = &poll_ctx->pq[0];
>>> + int ret = 0;
>>> + int i;
>>> +
>>> + for (i = 0; i < BLK_BIO_POLL_PQ_SZ; i++) {
>>> + struct bio *bio = poll_data[i].bio;
>>> +
>>> + if (!bio)
>>> + continue;
>>> +
>>> + ret += blk_mq_poll_io(bio);
>>> + if (bio_flagged(bio, BIO_DONE)) {
>>> + poll_data[i].bio = NULL;
>>> +
>>> + /* clear BIO_END_BY_POLL and end me really */
>>> + bio_clear_flag(bio, BIO_END_BY_POLL);
>>> + bio_endio(bio);
>>> + }
>>> + }
>>> + return ret;
>>> +}
>>
>> When there are multiple threads polling, saying thread A and thread B,
>> then there's one bio which should be polled by thread A (the pid is
>> passed to thread A), while it's actually completed by thread B. In this
>> case, when the bio is completed by thread B, the bio is not really
>> completed and one extra blk_poll() still needs to be called.
>
> When this happens, the dm bio can't be completed, and the associated
> kiocb can't be completed too, io_uring or other poll code context will
> keep calling blk_poll() by passing thread A's pid until this dm bio is
> done, since the dm bio is submitted from thread A.
>
This will affect the multi-thread polling performance. I tested
dm-stripe, in which every bio will be split and enqueued into all
underlying devices, and thus amplify the interference between multiple
threads.
Test Result:
IOPS: 332k (IRQ) -> 363k (iopoll), aka ~10% performance gain
Test Environment:
nvme.poll_queues = 3
BLK_BIO_POLL_SQ_SZ = 128
dmsetup create testdev --table "0 629145600 striped 3 8 /dev/nvme0n1 0
/dev/nvme1n1 0 /dev/nvme4n1 0"
```
$cat fio.conf
[global]
name=iouring-sqpoll-iopoll-1
ioengine=io_uring
iodepth=128
numjobs=1
thread
rw=randread
direct=1
hipri=1
runtime=10
time_based
group_reporting
randrepeat=0
filename=/dev/mapper/testdev
bs=12k
[job-1]
cpus_allowed=14
[job-2]
cpus_allowed=16
[job-3]
cpus_allowed=84
```
--
Thanks,
Jeffle
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