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().
> +
> /**
> * 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.
> +
> /**
> * 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
