Re: [PATCH v2] bio: limit bio max size
From: Damien Le Moal
Date: Thu Jan 21 2021 - 05:21:17 EST
On Thu, 2021-01-21 at 18:36 +0900, 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/
>
> Thank you for advice. :)
>
> >
> > >
> > > 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.
>
> I have a qustion.
> Is adding new variable in bio not possible?
It is possible, but since it is a critical kernel resource used a lot, keeping
it as small as possible for performance reasons is strongly desired. So if
there is a coding scheme that can avoid increasing struct bio size, it should
be explored first and discarded only with very good reasons.
> Additional check for every page merge like as below is inefficient I think.
For the general case of devices that do not care about limiting the bio size
(like now), this will add one boolean evaluation (queue flag test). That's it.
For your case, sure you now have 2 boolean evals instead of one. But that must
be put in perspective with the cost of increasing the bio size.
>
> bool __bio_try_merge_page(struct bio *bio, struct page *page,
> unsigned int len, unsigned int off, bool *same_page)
> {
> ...
> if (page_is_mergeable(bv, page, len, off, same_page)) {
> if (bio->bi_iter.bi_size > UINT_MAX - len) {
> *same_page = false;
> return false;
> }
>
> + if (blk_queue_limit_bio_max_size(bio) &&
> + (bio->bi_iter.bi_size >
> blk_queue_get_bio_max_size(bio) - len)) {
> + *same_page = false;
> + return false;
> + }
>
> bv->bv_len += len;
> bio->bi_iter.bi_size += len;
> return true;
> }
> ...
> }
>
>
> static inline bool bio_full(struct bio *bio, unsigned len)
> {
> ...
> if (bio->bi_iter.bi_size > UINT_MAX - len)
> return true;
>
> + if (blk_queue_limit_bio_max_size(bio) &&
> + (bio->bi_iter.bi_size > blk_queue_get_bio_max_size(bio) - len))
> + return true;
> ...
> }
>
>
> Page merge is CPU-bound job as you said.
> How about below with adding of bi_max_size in bio?
I am not a fan of adding a bio field for using it only in one place.
This is only my opinion. I will let others comment about this, but personnally
I would rather do something like this:
#define blk_queue_limit_bio_merge_size(q) \
test_bit(QUEUE_FLAG_LIMIT_MERGE, &(q)->queue_flags)
static inline unsigned int bio_max_merge_size(struct bio *bio)
{
struct request_queue *q = bio->bi_disk->queue;
if (blk_queue_limit_bio_merge_size(q))
return blk_queue_get_max_sectors(q, bio_op(bio))
<< SECTOR_SHIFT;
return UINT_MAX;
}
and use that helper in __bio_try_merge_page(), e.g.:
if (bio->bi_iter.bi_size > bio_max_merge_size(bio) - len) {
*same_page = false;
return false;
}
No need to change the bio struct.
If you measure performance with and without this change on nullblk, you can
verify if it has any impact for regular devices. And for your use case, that
should give you the same performance.
>
> bool __bio_try_merge_page(struct bio *bio, struct page *page,
> unsigned int len, unsigned int off, bool *same_page)
> {
> ...
> 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;
> }
>
> bv->bv_len += len;
> bio->bi_iter.bi_size += len;
> return true;
> }
> ...
> }
>
>
> static inline bool bio_full(struct bio *bio, unsigned len)
> {
> ...
> - if (bio->bi_iter.bi_size > UINT_MAX - len)
> + if (bio->bi_iter.bi_size > bio->bi_max_size - len)
> return true;
> ...
> }
>
> +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;
> + if (blk_queue_limit_bio_max_size(bio))
> + bio->bi_max_size = blk_queue_get_bio_max_size(bio);
> +
> + bio_associate_blkg(bio);
> +}
> +EXPORT_SYMBOL(bio_set_dev);
>
> > --
> > Damien Le Moal
> > Western Digital Research
>
> ---
> Changheun Lee
> Samsung Electronics
--
Damien Le Moal
Western Digital