Re: [PATCH v2] bio: limit bio max size.

From: Damien Le Moal
Date: Wed Jan 20 2021 - 22:21:33 EST

On 2021/01/21 10:13, Changheun Lee wrote:

Please drop the "." at the end of the patch title.

> 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 memory address is
> continued phsycally. it makes some delay to submit until merge complete.

s/if memory address is continued phsycally/if the pages physical addresses are
contiguous/

> bio max size should be limited as a proper size.

s/as/to/

>
> 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 | 13 +++----------
> include/linux/blk_types.h | 1 +
> 3 files changed, 20 insertions(+), 11 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 1f2cc1fbe283..027503c2e2e7 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -284,9 +284,24 @@ void bio_init(struct bio *bio, struct bio_vec *table,
>
> bio->bi_io_vec = table;
> bio->bi_max_vecs = max_vecs;
> + bio->bi_max_size = UINT_MAX;
> }
> EXPORT_SYMBOL(bio_init);
>
> +void bio_set_dev(struct bio *bio, struct block_device *bdev)
> +{
> + if (bio->bi_disk != bdev->bd_disk)
> + bio_clear_flag(bio, BIO_THROTTLED);
> +
> + bio->bi_disk = bdev->bd_disk;
> + bio->bi_partno = bdev->bd_partno;
> + bio->bi_max_size = blk_queue_get_max_sectors(bio->bi_disk->queue,
> + bio_op(bio)) << SECTOR_SHIFT;
> +
> + bio_associate_blkg(bio);
> +}
> +EXPORT_SYMBOL(bio_set_dev);
> +
> /**
> * 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->bi_max_size - len)
> *same_page = false;
> return false;
> }
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 1edda614f7ce..b9803e80c259 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->bi_max_size - len)
> return true;
>
> return false;
> @@ -482,20 +482,13 @@ extern struct bio_vec *bvec_alloc(gfp_t, int, unsigned long *, mempool_t *);
> extern void bvec_free(mempool_t *, struct bio_vec *, unsigned int);
> extern unsigned int bvec_nr_vecs(unsigned short idx);
> extern const char *bio_devname(struct bio *bio, char *buffer);
> -
> -#define bio_set_dev(bio, bdev) \
> -do { \
> - if ((bio)->bi_disk != (bdev)->bd_disk) \
> - bio_clear_flag(bio, BIO_THROTTLED);\
> - (bio)->bi_disk = (bdev)->bd_disk; \
> - (bio)->bi_partno = (bdev)->bd_partno; \
> - bio_associate_blkg(bio); \
> -} while (0)
> +extern void bio_set_dev(struct bio *bio, struct block_device *bdev);
>
> #define bio_copy_dev(dst, src) \
> do { \
> (dst)->bi_disk = (src)->bi_disk; \
> (dst)->bi_partno = (src)->bi_partno; \
> + (dst)->bi_max_size = (src)->bi_max_size;\
> bio_clone_blkg_association(dst, src); \
> } while (0)
>
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 866f74261b3b..e5dd5b7d8fc1 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -270,6 +270,7 @@ struct bio {
> */
>
> unsigned short bi_max_vecs; /* max bvl_vecs we can hold */
> + unsigned int bi_max_size; /* max data size we can hold */
>
> atomic_t __bi_cnt; /* pin count */

This modification comes at the cost of increasing the bio structure size to
simply tell the block layer "do not delay BIO splitting"...

I think there is a much simpler approach. What about:

1) Use a request queue flag to indicate "limit BIO size"
2) modify __bio_try_merge_page() to look at that flag to disallow page merging
if the bio size exceeds blk_queue_get_max_sectors(), or more ideally a version
of it that takes into account the bio start sector.
3) Set the "limit bio size" queue flag in the driver of the device that benefit
from this change. Eventually, that could also be controlled through sysfs.

With such change, you will get the same result without having to increase the
BIO structure size.


--
Damien Le Moal
Western Digital Research