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

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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




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