Re: [PATCH v12 1/3] bio: control bio max size

From: Damien Le Moal
Date: Fri Jun 04 2021 - 04:01:33 EST

On 2021/06/04 16:53, Changheun Lee wrote:
>> On 2021/06/04 14:22, Changheun Lee wrote:
>>> bio size can grow up to 4GB after muli-page bvec has been enabled.
>>> But sometimes large size of bio would lead to inefficient behaviors.
>>> Control of bio max size will be helpful to improve inefficiency.
>>>
>>> Below is a example for inefficient behaviours.
>>> 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.lee@xxxxxxxxxxx>
>>> ---
>>> block/bio.c | 17 ++++++++++++++---
>>> block/blk-settings.c | 19 +++++++++++++++++++
>>> include/linux/bio.h | 4 +++-
>>> include/linux/blkdev.h | 3 +++
>>> 4 files changed, 39 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/block/bio.c b/block/bio.c
>>> index 44205dfb6b60..73b673f1684e 100644
>>> --- a/block/bio.c
>>> +++ b/block/bio.c
>>> @@ -255,6 +255,13 @@ void bio_init(struct bio *bio, struct bio_vec *table,
>>> }
>>> EXPORT_SYMBOL(bio_init);
>>>
>>> +unsigned int bio_max_bytes(struct bio *bio)
>>> +{
>>> + struct block_device *bdev = bio->bi_bdev;
>>> +
>>> + return bdev ? bdev->bd_disk->queue->limits.max_bio_bytes : UINT_MAX;
>>> +}
>>
>> unsigned int bio_max_bytes(struct bio *bio)
>> {
>> struct block_device *bdev = bio->bi_bdev;
>>
>> if (!bdev)
>> return UINT_MAX;
>> return bdev->bd_disk->queue->limits.max_bio_bytes;
>> }
>>
>> is a lot more readable...
>> Also, I remember there was some problems with bd_disk possibly being null. Was
>> that fixed ?
>
> Thank you for review. But I'd like current style, and it's readable enough
> now I think. Null of bd_disk was just suspicion. bd_disk is not null if bdev
> is not null.
>
>>
>>> +
>>> /**
>>> * bio_reset - reinitialize a bio
>>> * @bio: bio to reset
>>> @@ -866,7 +873,7 @@ 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_bytes(bio) - len) {
>>> *same_page = false;
>>> return false;
>>> }
>>> @@ -995,6 +1002,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>>> {
>>> unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
>>> unsigned short entries_left = bio->bi_max_vecs - bio->bi_vcnt;
>>> + unsigned int bytes_left = bio_max_bytes(bio) - bio->bi_iter.bi_size;
>>> struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
>>> struct page **pages = (struct page **)bv;
>>> bool same_page = false;
>>> @@ -1010,7 +1018,8 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>>> BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
>>> pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
>>>
>>> - size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
>>> + size = iov_iter_get_pages(iter, pages, bytes_left, nr_pages,
>>> + &offset);
>>> if (unlikely(size <= 0))
>>> return size ? size : -EFAULT;
>>>
>>> @@ -1038,6 +1047,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
>>> {
>>> unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
>>> unsigned short entries_left = bio->bi_max_vecs - bio->bi_vcnt;
>>> + unsigned int bytes_left = bio_max_bytes(bio) - bio->bi_iter.bi_size;
>>> struct request_queue *q = bio->bi_bdev->bd_disk->queue;
>>> unsigned int max_append_sectors = queue_max_zone_append_sectors(q);
>>> struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
>>> @@ -1058,7 +1068,8 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
>>> BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
>>> pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
>>>
>>> - size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
>>> + size = iov_iter_get_pages(iter, pages, bytes_left, nr_pages,
>>> + &offset);
>>> if (unlikely(size <= 0))
>>> return size ? size : -EFAULT;
>>>
>>> diff --git a/block/blk-settings.c b/block/blk-settings.c
>>> index 902c40d67120..e270e31519a1 100644
>>> --- a/block/blk-settings.c
>>> +++ b/block/blk-settings.c
>>> @@ -32,6 +32,7 @@ EXPORT_SYMBOL_GPL(blk_queue_rq_timeout);
>>> */
>>> void blk_set_default_limits(struct queue_limits *lim)
>>> {
>>> + lim->max_bio_bytes = UINT_MAX;
>>> lim->max_segments = BLK_MAX_SEGMENTS;
>>> lim->max_discard_segments = 1;
>>> lim->max_integrity_segments = 0;
>>> @@ -100,6 +101,24 @@ void blk_queue_bounce_limit(struct request_queue *q, enum blk_bounce bounce)
>>> }
>>> EXPORT_SYMBOL(blk_queue_bounce_limit);
>>>
>>> +/**
>>> + * blk_queue_max_bio_bytes - set bio max size for queue
>>
>> blk_queue_max_bio_bytes - set max_bio_bytes queue limit
>>
>> And then you can drop the not very useful description.
>
> OK. I'll.
>
>>
>>> + * @q: the request queue for the device
>>> + * @bytes : bio max bytes to be set
>>> + *
>>> + * Description:
>>> + * Set proper bio max size to optimize queue operating.
>>> + **/
>>> +void blk_queue_max_bio_bytes(struct request_queue *q, unsigned int bytes)
>>> +{
>>> + struct queue_limits *limits = &q->limits;
>>> + unsigned int max_bio_bytes = round_up(bytes, PAGE_SIZE);
>>> +
>>> + limits->max_bio_bytes = max_t(unsigned int, max_bio_bytes,
>>> + BIO_MAX_VECS * PAGE_SIZE);
>>> +}
>>> +EXPORT_SYMBOL(blk_queue_max_bio_bytes);
>>
>> Setting of the stacked limits is still missing.
>
> max_bio_bytes for stacked device is just default(UINT_MAX) in this patch.
> Because blk_set_stacking_limits() call blk_set_default_limits().> I'll work continue for stacked device after this patchowork.

Why ? Without that added now, anybody using this performance fix will see no
benefits if a device mapper is used. The stacking limit should be super simple.
In blk_stack_limits(), just add:

t->max_bio_bytes = min(t->max_bio_bytes, b->max_bio_bytes);

>
>>
>>> +
>>> /**
>>> * blk_queue_max_hw_sectors - set max sectors for a request for this queue
>>> * @q: the request queue for the device
>>> diff --git a/include/linux/bio.h b/include/linux/bio.h
>>> index a0b4cfdf62a4..3959cc1a0652 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_bytes(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_bytes(bio) - len)
>>> return true;
>>>
>>> return false;
>>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>>> index 1255823b2bc0..861888501fc0 100644
>>> --- a/include/linux/blkdev.h
>>> +++ b/include/linux/blkdev.h
>>> @@ -326,6 +326,8 @@ enum blk_bounce {
>>> };
>>>
>>> struct queue_limits {
>>> + unsigned int max_bio_bytes;
>>> +
>>> enum blk_bounce bounce;
>>> unsigned long seg_boundary_mask;
>>> unsigned long virt_boundary_mask;
>>> @@ -1132,6 +1134,7 @@ extern void blk_abort_request(struct request *);
>>> * Access functions for manipulating queue properties
>>> */
>>> extern void blk_cleanup_queue(struct request_queue *);
>>> +extern void blk_queue_max_bio_bytes(struct request_queue *, unsigned int);
>>> void blk_queue_bounce_limit(struct request_queue *q, enum blk_bounce limit);
>>> extern void blk_queue_max_hw_sectors(struct request_queue *, unsigned int);
>>> extern void blk_queue_chunk_sectors(struct request_queue *, unsigned int);
>>>
>>
>>
>> --
>> Damien Le Moal
>> Western Digital Research
>
> Thank you,
> Changheun Lee
>


--
Damien Le Moal
Western Digital Research