Re: [RESEND PATCH v5 1/2] bio: limit bio max size

From: Changheun Lee
Date: Tue Apr 06 2021 - 20:33:53 EST

> On Tue, Apr 06, 2021 at 10:31:28AM +0900, Changheun Lee wrote:
> > > bio size can grow up to 4GB when muli-page bvec is enabled.
> > > but sometimes it would lead to inefficient behaviors.
> > > in case of large chunk direct I/O, - 32MB chunk read in user space -
> > > all pages for 32MB would be merged to a bio structure if the pages
> > > physical addresses are contiguous. it makes some delay to submit
> > > until merge complete. bio max size should be limited to a proper size.
> > >
> > > When 32MB chunk read with direct I/O option is coming from userspace,
> > > kernel behavior is below now in do_direct_IO() loop. it's timeline.
> > >
> > > | bio merge for 32MB. total 8,192 pages are merged.
> > > | total elapsed time is over 2ms.
> > > |------------------ ... ----------------------->|
> > > | 8,192 pages merged a bio.
> > > | at this time, first bio submit is done.
> > > | 1 bio is split to 32 read request and issue.
> > > |--------------->
> > > |--------------->
> > > |--------------->
> > > ......
> > > |--------------->
> > > |--------------->|
> > > total 19ms elapsed to complete 32MB read done from device. |
> > >
> > > If bio max size is limited with 1MB, behavior is changed below.
> > >
> > > | bio merge for 1MB. 256 pages are merged for each bio.
> > > | total 32 bio will be made.
> > > | total elapsed time is over 2ms. it's same.
> > > | but, first bio submit timing is fast. about 100us.
> > > |--->|--->|--->|---> ... -->|--->|--->|--->|--->|
> > > | 256 pages merged a bio.
> > > | at this time, first bio submit is done.
> > > | and 1 read request is issued for 1 bio.
> > > |--------------->
> > > |--------------->
> > > |--------------->
> > > ......
> > > |--------------->
> > > |--------------->|
> > > total 17ms elapsed to complete 32MB read done from device. |
> > >
> > > As a result, read request issue timing is faster if bio max size is limited.
> > > Current kernel behavior with multipage bvec, super large bio can be created.
> > > And it lead to delay first I/O request issue.
> > >
> > > Signed-off-by: Changheun Lee <nanich.lee@xxxxxxxxxxx>
> > > ---
> > > block/bio.c | 13 ++++++++++++-
> > > include/linux/bio.h | 2 +-
> > > include/linux/blkdev.h | 3 +++
> > > 3 files changed, 16 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/block/bio.c b/block/bio.c
> > > index 1f2cc1fbe283..c528e1f944c7 100644
> > > --- a/block/bio.c
> > > +++ b/block/bio.c
> > > @@ -287,6 +287,17 @@ void bio_init(struct bio *bio, struct bio_vec *table,
> > > }
> > > EXPORT_SYMBOL(bio_init);
> > >
> > > +unsigned int bio_max_size(struct bio *bio)
> > > +{
> > > + struct request_queue *q = bio->bi_disk->queue;
> > > +
> > > + if (blk_queue_limit_bio_size(q))
> > > + return blk_queue_get_max_sectors(q, bio_op(bio))
> > > + << SECTOR_SHIFT;
> > > +
> > > + return UINT_MAX;
> > > +}
> > > +
> > > /**
> > > * bio_reset - reinitialize a bio
> > > * @bio: bio to reset
> > > @@ -877,7 +888,7 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
> > > struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
> > >
> > > if (page_is_mergeable(bv, page, len, off, same_page)) {
> > > - if (bio->bi_iter.bi_size > UINT_MAX - len) {
> > > + if (bio->bi_iter.bi_size > bio_max_size(bio) - len) {
> > > *same_page = false;
> > > return false;
> > > }
> > > diff --git a/include/linux/bio.h b/include/linux/bio.h
> > > index 1edda614f7ce..13b6f6562a5b 100644
> > > --- a/include/linux/bio.h
> > > +++ b/include/linux/bio.h
> > > @@ -113,7 +113,7 @@ static inline bool bio_full(struct bio *bio, unsigned len)
> > > if (bio->bi_vcnt >= bio->bi_max_vecs)
> > > return true;
> > >
> > > - if (bio->bi_iter.bi_size > UINT_MAX - len)
> > > + if (bio->bi_iter.bi_size > bio_max_size(bio) - len)
> > > return true;
> > >
> > > return false;
> > > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > > index f94ee3089e01..3aeab9e7e97b 100644
> > > --- a/include/linux/blkdev.h
> > > +++ b/include/linux/blkdev.h
> > > @@ -621,6 +621,7 @@ struct request_queue {
> > > #define QUEUE_FLAG_RQ_ALLOC_TIME 27 /* record rq->alloc_time_ns */
> > > #define QUEUE_FLAG_HCTX_ACTIVE 28 /* at least one blk-mq hctx is active */
> > > #define QUEUE_FLAG_NOWAIT 29 /* device supports NOWAIT */
> > > +#define QUEUE_FLAG_LIMIT_BIO_SIZE 30 /* limit bio size */
> > >
> > > #define QUEUE_FLAG_MQ_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
> > > (1 << QUEUE_FLAG_SAME_COMP) | \
> > > @@ -667,6 +668,8 @@ bool blk_queue_flag_test_and_set(unsigned int flag, struct request_queue *q);
> > > #define blk_queue_fua(q) test_bit(QUEUE_FLAG_FUA, &(q)->queue_flags)
> > > #define blk_queue_registered(q) test_bit(QUEUE_FLAG_REGISTERED, &(q)->queue_flags)
> > > #define blk_queue_nowait(q) test_bit(QUEUE_FLAG_NOWAIT, &(q)->queue_flags)
> > > +#define blk_queue_limit_bio_size(q) \
> > > + test_bit(QUEUE_FLAG_LIMIT_BIO_SIZE, &(q)->queue_flags)
> > >
> > > extern void blk_set_pm_only(struct request_queue *q);
> > > extern void blk_clear_pm_only(struct request_queue *q);
> > > --
> > > 2.28.0
> > >
> >
> > Please feedback to me if more modification is needed to apply. :)
>
> You are adding code that tests for a value to be set, yet you never set
> it in this code so why is it needed at all?

This patch is a solution for some inefficient case of multipage bvec like
as current DIO scenario. So it's not set as a default.
It will be set when bio size limitation is needed in runtime.

>
> thanks,
>
> greg k-h

---
Changheun Lee