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

[Ming Lei]

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.



thanks,
Ming