[dm-devel] [RFC PATCH 08/11] block: use per-task poll context to implement bio based io poll

[Ming Lei]

On Wed, Mar 17, 2021 at 11:53:12AM +0800, JeffleXu wrote:
> 
> 
> On 3/17/21 10:54 AM, Ming Lei wrote:
> > On Tue, Mar 16, 2021 at 04:52:36PM +0800, JeffleXu wrote:
> >>
> >>
> >> 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 at redhat.com>
> >>>>> ---
> >>>>>  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.
> >>
> >> It seems that the queue depth of kfifo will affect the performance as I
> >> did a fast test.
> >>
> >>
> >>
> >> Test Result:
> >>
> >> BLK_BIO_POLL_SQ_SZ | iodepth | IOPS
> >> ------------------ | ------- | ----
> >> 64                 | 128     | 301k (IRQ) -> 340k (iopoll)
> >> 64                 | 16      | 304k (IRQ) -> 392k (iopoll)
> >> 128                | 128     | 204k (IRQ) -> 317k (iopoll)
> >> 256                | 128     | 241k (IRQ) -> 391k (iopoll)
> >>
> >> It seems that BLK_BIO_POLL_SQ_SZ need to be increased accordingly when
> >> iodepth is quite large. But I don't know why the performance in IRQ mode
> >> decreases when BLK_BIO_POLL_SQ_SZ is increased.
> > 
> > This patchset is supposed to not affect IRQ mode because HIPRI isn't set
> > at IRQ mode. Or you mean '--hipri' & io_uring is setup but setting
> > nvme.poll_queues as 0 at your 'IRQ' mode test?
> > 
> > Thanks for starting to run performance test, and so far I just run test
> > in KVM, not start performance test yet.
> > 
> 
> 'IRQ' means 'hipri=0' of fio configuration.

'hipri=0' isn't supposed to be affected by this patchset.


thanks,
Ming