Re: [PATCH V3 05/13] block: add req flag of REQ_POLL_CTX

[Ming Lei <ming.lei@redhat.com>]

On Wed, Mar 24, 2021 at 04:32:31PM +0100, Hannes Reinecke wrote:
> On 3/24/21 1:19 PM, Ming Lei wrote:
> > Add one req flag REQ_POLL_CTX which will be used in the following patch for
> > supporting bio based IO polling.
> > 
> > Exactly this flag can help us to do:
> > 
> > 1) request flag is cloned in bio_fast_clone(), so if we mark one FS bio
> > as REQ_POLL_CTX, all bios cloned from this FS bio will be marked as
> > REQ_POLL_CTX too.
> > 
> > 2) create per-task io polling context if the bio based queue supports
> > polling and the submitted bio is HIPRI. Per-task io poll context will be
> > created during submit_bio() before marking this HIPRI bio as REQ_POLL_CTX.
> > Then we can avoid to create such io polling context if one cloned bio with
> > REQ_POLL_CTX is submitted from another kernel context.
> > 
> > 3) for supporting bio based io polling, we need to poll IOs from all
> > underlying queues of the bio device, this way help us to recognize which
> > IO needs to polled in bio based style, which will be applied in
> > following patch.
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   block/blk-core.c          | 25 ++++++++++++++++++++++++-
> >   include/linux/blk_types.h |  4 ++++
> >   2 files changed, 28 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index 4671bbf31fd3..eb07d61cfdc2 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -840,11 +840,30 @@ static inline bool blk_queue_support_bio_poll(struct request_queue *q)
> >   static inline void blk_bio_poll_preprocess(struct request_queue *q,
> >   		struct bio *bio)
> >   {
> > +	bool mq;
> > +
> >   	if (!(bio->bi_opf & REQ_HIPRI))
> >   		return;
> > -	if (!blk_queue_poll(q) || (!queue_is_mq(q) && !blk_get_bio_poll_ctx()))
> > +	/*
> > +	 * Can't support bio based IO polling without per-task poll ctx
> > +	 *
> > +	 * We have created per-task io poll context, and mark this
> > +	 * bio as REQ_POLL_CTX, so: 1) if any cloned bio from this bio is
> > +	 * submitted from another kernel context, we won't create bio
> > +	 * poll context for it, and that bio can be completed by IRQ;
> > +	 * 2) If such bio is submitted from current context, we will
> > +	 * complete it via blk_poll(); 3) If driver knows that one
> > +	 * underlying bio allocated from driver is for FS bio, meantime
> > +	 * it is submitted in current context, driver can mark such bio
> > +	 * as REQ_HIPRI & REQ_POLL_CTX manually, so the bio can be completed
> > +	 * via blk_poll too.
> > +	 */
> > +	mq = queue_is_mq(q);
> > +	if (!blk_queue_poll(q) || (!mq && !blk_get_bio_poll_ctx()))
> >   		bio->bi_opf &= ~REQ_HIPRI;
> > +	else if (!mq)
> > +		bio->bi_opf |= REQ_POLL_CTX;
> >   }
> >   static noinline_for_stack bool submit_bio_checks(struct bio *bio)
> > @@ -894,8 +913,12 @@ static noinline_for_stack bool submit_bio_checks(struct bio *bio)
> >   	/*
> >   	 * Create per-task io poll ctx if bio polling supported and HIPRI
> >   	 * set.
> > +	 *
> > +	 * If REQ_POLL_CTX isn't set for this HIPRI bio, we think it originated
> > +	 * from FS and allocate io polling context.
> >   	 */
> >   	blk_create_io_context(q, blk_queue_support_bio_poll(q) &&
> > +			!(bio->bi_opf & REQ_POLL_CTX) &&
> >   			(bio->bi_opf & REQ_HIPRI));
> >   	blk_bio_poll_preprocess(q, bio);
> > diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> > index db026b6ec15a..99160d588c2d 100644
> > --- a/include/linux/blk_types.h
> > +++ b/include/linux/blk_types.h
> > @@ -394,6 +394,9 @@ enum req_flag_bits {
> >   	__REQ_HIPRI,
> > +	/* for marking IOs originated from same FS bio in same context */
> > +	__REQ_POLL_CTX,
> > +
> >   	/* for driver use */
> >   	__REQ_DRV,
> >   	__REQ_SWAP,		/* swapping request. */
> > @@ -418,6 +421,7 @@ enum req_flag_bits {
> >   #define REQ_NOUNMAP		(1ULL << __REQ_NOUNMAP)
> >   #define REQ_HIPRI		(1ULL << __REQ_HIPRI)
> > +#define REQ_POLL_CTX			(1ULL << __REQ_POLL_CTX)
> >   #define REQ_DRV			(1ULL << __REQ_DRV)
> >   #define REQ_SWAP		(1ULL << __REQ_SWAP)
> > 
> What happens to split bios?
> Will they be tracked similarly to cloned bios?
> If so, shouldn't you document that here, too?

split bios are simply cloned bios, please see bio_split().


thanks,
Ming