> 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....@samsung.com> > > --- > > 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