Re: [PATCH v3 1/2] bio: limit bio max size
From: Changheun Lee
Date: Wed Jan 27 2021 - 02:28:05 EST
> On 2021/01/26 18:37, 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. 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 | 17 ++++++++++++++++-
> > include/linux/bio.h | 4 +++-
> > include/linux/blkdev.h | 3 +++
> > 3 files changed, 22 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/bio.c b/block/bio.c
> > index 1f2cc1fbe283..ec0281889045 100644
> > --- a/block/bio.c
> > +++ b/block/bio.c
> > @@ -287,6 +287,21 @@ 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;
> > +
> > + if (!bio->bi_disk)
> > + return UINT_MAX;
> > +
> > + q = bio->bi_disk->queue;
> > + if (!blk_queue_limit_bio_size(q))
> > + return UINT_MAX;
> > +
> > + return blk_queue_get_max_sectors(q, bio_op(bio)) << SECTOR_SHIFT;
> > +}
> > +EXPORT_SYMBOL(bio_max_size);
>
> Why export this ? This is only used in this file and in bio.h in bio_full().
OK. I'll remove it.
>
> > +
> > /**
> > * bio_reset - reinitialize a bio
> > * @bio: bio to reset
> > @@ -877,7 +892,7 @@ 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..cdb134ca7bf5 100644
> > --- a/include/linux/bio.h
> > +++ b/include/linux/bio.h
> > @@ -100,6 +100,8 @@ static inline void *bio_data(struct bio *bio)
> > return NULL;
> > }
> >
> > +extern unsigned int bio_max_size(struct bio *);
>
> No need for extern.
It's just for compile warning in my test environment.
I'll remove it too. But I think compile warning could be in the other
.c file which includes bio.h. Is it OK?
>
> > +
> > /**
> > * bio_full - check if the bio is full
> > * @bio: bio to check
> > @@ -113,7 +115,7 @@ 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);
> >
>
>
> --
> Damien Le Moal
> Western Digital Research
--
Changheun Lee
Samsung Electronics