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....@samsung.com> --- block/bio.c | 13 ++++++++++++- block/blk-settings.c | 17 +++++++++++++++++ include/linux/bio.h | 4 +++- include/linux/blkdev.h | 4 ++++ 4 files changed, 36 insertions(+), 2 deletions(-) diff --git a/block/bio.c b/block/bio.c index 50e579088aca..4bad97f1d37b 100644 --- a/block/bio.c +++ b/block/bio.c @@ -255,6 +255,17 @@ 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_bdev->bd_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 @@ -866,7 +877,7 @@ 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/block/blk-settings.c b/block/blk-settings.c index b4aa2f37fab6..1d94b97cea4f 100644 --- a/block/blk-settings.c +++ b/block/blk-settings.c @@ -928,6 +928,23 @@ void blk_queue_set_zoned(struct gendisk *disk, enum blk_zoned_model model) } EXPORT_SYMBOL_GPL(blk_queue_set_zoned); +/** + * blk_queue_set_limit_bio_size - set limit bio size flag + * @q: the request queue for the device + * @limit: limit bio size on(true), or off + * + * bio max size will be limited to queue max sectors size, + * if limit is true. + */ +void blk_queue_set_limit_bio_size(struct request_queue *q, bool limit) +{ + if (limit) + blk_queue_flag_set(QUEUE_FLAG_LIMIT_BIO_SIZE, q); + else + blk_queue_flag_clear(QUEUE_FLAG_LIMIT_BIO_SIZE, q); +} +EXPORT_SYMBOL_GPL(blk_queue_set_limit_bio_size); + static int __init blk_settings_init(void) { blk_max_low_pfn = max_low_pfn - 1; diff --git a/include/linux/bio.h b/include/linux/bio.h index d0246c92a6e8..e5add63da3af 100644 --- a/include/linux/bio.h +++ b/include/linux/bio.h @@ -106,6 +106,8 @@ static inline void *bio_data(struct bio *bio) return NULL; } +extern unsigned int bio_max_size(struct bio *bio); + /** * bio_full - check if the bio is full * @bio: bio to check @@ -119,7 +121,7 @@ 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 158aefae1030..c69a6ed7a189 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -617,6 +617,7 @@ 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) | \ @@ -663,6 +664,8 @@ 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); @@ -1183,6 +1186,7 @@ extern void blk_queue_required_elevator_features(struct request_queue *q, unsigned int features); extern bool blk_queue_can_use_dma_map_merging(struct request_queue *q, struct device *dev); +extern void blk_queue_set_limit_bio_size(struct request_queue *q, bool limit); /* * Number of physical segments as sent to the device. -- 2.29.0