Received-SPF: pass (google.com: domain of linux-ext4+bounces-2209-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Fri, 26 Apr 2024 22:47:19 +0530
Message-Id: <87a5lgyrfk.fsf@gmail.com>
From: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>, Ojaswin Mujoo <ojaswin@linux.ibm.com>, Jan Kara <jack@suse.cz>
Subject: Re: [RFCv3 7/7] iomap: Optimize data access patterns for filesystems with indirect mappings
In-Reply-To: <20240426162417.GN360919@frogsfrogsfrogs>
Precedence: bulk

"Darrick J. Wong" <djwong@kernel.org> writes:

> On Thu, Apr 25, 2024 at 06:58:51PM +0530, Ritesh Harjani (IBM) wrote:
>> This patch optimizes the data access patterns for filesystems with
>> indirect block mapping by implementing BH_Boundary handling within
>> iomap.
>> 
>> Currently the bios for reads within iomap are only submitted at
>> 2 places -
>> 1. If we cannot merge the new req. with previous bio, only then we
>>    submit the previous bio.
>> 2. Submit the bio at the end of the entire read processing.
>> 
>> This means for filesystems with indirect block mapping, we call into
>> ->iomap_begin() again w/o submitting the previous bios. That causes
>> unoptimized data access patterns for blocks which are of BH_Boundary type.
>> 
>> For e.g. consider the file mapping
>> logical block(4k) 		physical block(4k)
>> 0-11 				1000-1011
>> 12-15 				1013-1016
>> 
>> In above physical block 1012 is an indirect metadata block which has the
>> mapping information for next set of indirect blocks (1013-1016).
>> With iomap buffered reads for reading 1st 16 logical blocks of a file
>> (0-15), we get below I/O pattern
>> 	- submit a bio for 1012
>> 	- complete the bio for 1012
>> 	- submit a bio for 1000-1011
>> 	- submit a bio for 1013-1016
>> 	- complete the bios for 1000-1011
>> 	- complete the bios for 1013-1016
>> 
>> So as we can see, above is an non-optimal I/O access pattern and also we
>> get 3 bio completions instead of 2.
>> 
>> This patch changes this behavior by doing submit_bio() if there are any
>> bios already processed, before calling ->iomap_begin() again.
>> That means if there are any blocks which are already processed, gets
>> submitted for I/O earlier and then within ->iomap_begin(), if we get a
>> request for reading an indirect metadata block, then block layer can merge
>> those bios with the already submitted read request to reduce the no. of bio
>> completions.
>> 
>> Now, for bs < ps or for large folios, this patch requires proper handling
>> of "ifs->read_bytes_pending". In that we first set ifs->read_bytes_pending
>> to folio_size. Then handle all the cases where we need to subtract
>> ifs->read_bytes_pending either during the submission side
>> (if we don't need to submit any I/O - for e.g. for uptodate sub blocks),
>> or during an I/O error, or at the completion of an I/O.
>> 
>> Here is the ftrace output of iomap and block layer with ext2 iomap
>> conversion patches -
>> 
>> root# filefrag -b512 -v /mnt1/test/f1
>> Filesystem type is: ef53
>> Filesystem cylinder groups approximately 32
>> File size of /mnt1/test/f1 is 65536 (128 blocks of 512 bytes)
>>  ext:     logical_offset:        physical_offset: length:   expected: flags:
>>    0:        0..      95:      98304..     98399:     96:             merged
>>    1:       96..     127:      98408..     98439:     32:      98400: last,merged,eof
>> /mnt1/test/f1: 2 extents found
>> 
>> root# #This reads 4 blocks starting from lblk 10, 11, 12, 13
>> root# xfs_io -c "pread -b$((4*4096)) $((10*4096)) $((4*4096))" /mnt1/test/f1
>> 
>> w/o this patch - (indirect block is submitted before and does not get merged, resulting in 3 bios completion)
>>       xfs_io-907     [002] .....   185.608791: iomap_readahead: dev 8:16 ino 0xc nr_pages 4
>>       xfs_io-907     [002] .....   185.608819: iomap_iter: dev 8:16 ino 0xc pos 0xa000 length 0x4000 processed 0 flags  (0x0) ops 0xffffffff82242160 caller iomap_readahead+0x9d/0x2c0
>>       xfs_io-907     [002] .....   185.608823: iomap_iter_dstmap: dev 8:16 ino 0xc bdev 8:16 addr 0x300a000 offset 0xa000 length 0x2000 type MAPPED flags MERGED
>>       xfs_io-907     [002] .....   185.608831: iomap_iter: dev 8:16 ino 0xc pos 0xa000 length 0x2000 processed 8192 flags  (0x0) ops 0xffffffff82242160 caller iomap_readahead+0x1e1/0x2c0
>>       xfs_io-907     [002] .....   185.608859: block_bio_queue: 8,16 R 98400 + 8 [xfs_io]
>>       xfs_io-907     [002] .....   185.608865: block_getrq: 8,16 R 98400 + 8 [xfs_io]
>>       xfs_io-907     [002] .....   185.608867: block_io_start: 8,16 R 4096 () 98400 + 8 [xfs_io]
>>       xfs_io-907     [002] .....   185.608869: block_plug: [xfs_io]
>>       xfs_io-907     [002] .....   185.608872: block_unplug: [xfs_io] 1
>>       xfs_io-907     [002] .....   185.608874: block_rq_insert: 8,16 R 4096 () 98400 + 8 [xfs_io]
>> kworker/2:1H-198     [002] .....   185.608908: block_rq_issue: 8,16 R 4096 () 98400 + 8 [kworker/2:1H]
>>       <idle>-0       [002] d.h2.   185.609579: block_rq_complete: 8,16 R () 98400 + 8 [0]
>>       <idle>-0       [002] dNh2.   185.609631: block_io_done: 8,16 R 0 () 98400 + 0 [swapper/2]
>>       xfs_io-907     [002] .....   185.609694: iomap_iter_dstmap: dev 8:16 ino 0xc bdev 8:16 addr 0x300d000 offset 0xc000 length 0x2000 type MAPPED flags MERGED
>>       xfs_io-907     [002] .....   185.609704: block_bio_queue: 8,16 RA 98384 + 16 [xfs_io]
>>       xfs_io-907     [002] .....   185.609718: block_getrq: 8,16 RA 98384 + 16 [xfs_io]
>>       xfs_io-907     [002] .....   185.609721: block_io_start: 8,16 RA 8192 () 98384 + 16 [xfs_io]
>>       xfs_io-907     [002] .....   185.609726: block_plug: [xfs_io]
>>       xfs_io-907     [002] .....   185.609735: iomap_iter: dev 8:16 ino 0xc pos 0xc000 length 0x2000 processed 8192 flags  (0x0) ops 0xffffffff82242160 caller iomap_readahead+0x1e1/0x2c0
>>       xfs_io-907     [002] .....   185.609736: block_bio_queue: 8,16 RA 98408 + 16 [xfs_io]
>>       xfs_io-907     [002] .....   185.609740: block_getrq: 8,16 RA 98408 + 16 [xfs_io]
>>       xfs_io-907     [002] .....   185.609741: block_io_start: 8,16 RA 8192 () 98408 + 16 [xfs_io]
>>       xfs_io-907     [002] .....   185.609756: block_rq_issue: 8,16 RA 8192 () 98408 + 16 [xfs_io]
>>       xfs_io-907     [002] .....   185.609769: block_rq_issue: 8,16 RA 8192 () 98384 + 16 [xfs_io]
>>       <idle>-0       [002] d.H2.   185.610280: block_rq_complete: 8,16 RA () 98408 + 16 [0]
>>       <idle>-0       [002] d.H2.   185.610289: block_io_done: 8,16 RA 0 () 98408 + 0 [swapper/2]
>>       <idle>-0       [002] d.H2.   185.610292: block_rq_complete: 8,16 RA () 98384 + 16 [0]
>>       <idle>-0       [002] dNH2.   185.610301: block_io_done: 8,16 RA 0 () 98384 + 0 [swapper/2]
>
> Could this be shortened to ... the iomap calls and
> block_bio_queue/backmerge?  It's a bit difficult to see the point you're
> getting at with all the other noise.

I will remove this log and move it to cover letter and will just extend
the simple example I considered before in this commit message,
to show the difference with and w/o patch.

>
> I think you're trying to say that the access pattern here is 98400 ->
> 98408 -> 98384, which is not sequential?
>

it's (98400,8 ==> metadata block) -> (98384,16 == lblk 10 & 11) -> (98408,16 ==> lblk 12 & 13)
... w/o the patch

>> v/s with the patch - (optimzed I/O access pattern and bio gets merged resulting in only 2 bios completion)
>>       xfs_io-944     [005] .....    99.926187: iomap_readahead: dev 8:16 ino 0xc nr_pages 4
>>       xfs_io-944     [005] .....    99.926208: iomap_iter: dev 8:16 ino 0xc pos 0xa000 length 0x4000 processed 0 flags  (0x0) ops 0xffffffff82242160 caller iomap_readahead+0x9d/0x2c0
>>       xfs_io-944     [005] .....    99.926211: iomap_iter_dstmap: dev 8:16 ino 0xc bdev 8:16 addr 0x300a000 offset 0xa000 length 0x2000 type MAPPED flags MERGED
>>       xfs_io-944     [005] .....    99.926222: block_bio_queue: 8,16 RA 98384 + 16 [xfs_io]
>>       xfs_io-944     [005] .....    99.926232: block_getrq: 8,16 RA 98384 + 16 [xfs_io]
>>       xfs_io-944     [005] .....    99.926233: block_io_start: 8,16 RA 8192 () 98384 + 16 [xfs_io]
>>       xfs_io-944     [005] .....    99.926234: block_plug: [xfs_io]
>>       xfs_io-944     [005] .....    99.926235: iomap_iter: dev 8:16 ino 0xc pos 0xa000 length 0x2000 processed 8192 flags  (0x0) ops 0xffffffff82242160 caller iomap_readahead+0x1f9/0x2c0
>>       xfs_io-944     [005] .....    99.926261: block_bio_queue: 8,16 R 98400 + 8 [xfs_io]
>>       xfs_io-944     [005] .....    99.926266: block_bio_backmerge: 8,16 R 98400 + 8 [xfs_io]
>>       xfs_io-944     [005] .....    99.926271: block_unplug: [xfs_io] 1
>>       xfs_io-944     [005] .....    99.926272: block_rq_insert: 8,16 RA 12288 () 98384 + 24 [xfs_io]
>> kworker/5:1H-234     [005] .....    99.926314: block_rq_issue: 8,16 RA 12288 () 98384 + 24 [kworker/5:1H]
>>       <idle>-0       [005] d.h2.    99.926905: block_rq_complete: 8,16 RA () 98384 + 24 [0]
>>       <idle>-0       [005] dNh2.    99.926931: block_io_done: 8,16 RA 0 () 98384 + 0 [swapper/5]
>>       xfs_io-944     [005] .....    99.926971: iomap_iter_dstmap: dev 8:16 ino 0xc bdev 8:16 addr 0x300d000 offset 0xc000 length 0x2000 type MAPPED flags MERGED
>>       xfs_io-944     [005] .....    99.926981: block_bio_queue: 8,16 RA 98408 + 16 [xfs_io]
>>       xfs_io-944     [005] .....    99.926989: block_getrq: 8,16 RA 98408 + 16 [xfs_io]
>>       xfs_io-944     [005] .....    99.926989: block_io_start: 8,16 RA 8192 () 98408 + 16 [xfs_io]
>>       xfs_io-944     [005] .....    99.926991: block_plug: [xfs_io]
>>       xfs_io-944     [005] .....    99.926993: iomap_iter: dev 8:16 ino 0xc pos 0xc000 length 0x2000 processed 8192 flags  (0x0) ops 0xffffffff82242160 caller iomap_readahead+0x1f9/0x2c0
>>       xfs_io-944     [005] .....    99.927001: block_rq_issue: 8,16 RA 8192 () 98408 + 16 [xfs_io]
>>       <idle>-0       [005] d.h2.    99.927397: block_rq_complete: 8,16 RA () 98408 + 16 [0]
>>       <idle>-0       [005] dNh2.    99.927414: block_io_done: 8,16 RA 0 () 98408 + 0 [swapper/5]
>> 
>> Suggested-by: Matthew Wilcox <willy@infradead.org>
>> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> cc: Ojaswin Mujoo <ojaswin@linux.ibm.com>
>> ---
>>  fs/iomap/buffered-io.c | 112 +++++++++++++++++++++++++++++++----------
>>  1 file changed, 85 insertions(+), 27 deletions(-)
>> 
>> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
>> index 0a4269095ae2..a1d50086a3f5 100644
>> --- a/fs/iomap/buffered-io.c
>> +++ b/fs/iomap/buffered-io.c
>> @@ -30,7 +30,7 @@ typedef int (*iomap_punch_t)(struct inode *inode, loff_t offset, loff_t length);
>>   */
>>  struct iomap_folio_state {
>>  	spinlock_t		state_lock;
>> -	unsigned int		read_bytes_pending;
>> +	size_t			read_bytes_pending;
>>  	atomic_t		write_bytes_pending;
>> 
>>  	/*
>> @@ -380,6 +380,7 @@ static loff_t iomap_readpage_iter(const struct iomap_iter *iter,
>>  	loff_t orig_pos = pos;
>>  	size_t poff, plen;
>>  	sector_t sector;
>> +	bool rbp_finished = false;
>
> What is "rbp"?  My assembly programmer brain says x64 frame pointer, but
> that's clearly wrong here.  Maybe I'm confused...
>

rbp == read_bytes_pending ;)

>>  	if (iomap->type == IOMAP_INLINE)
>>  		return iomap_read_inline_data(iter, folio);
>> @@ -387,21 +388,39 @@ static loff_t iomap_readpage_iter(const struct iomap_iter *iter,
>>  	/* zero post-eof blocks as the page may be mapped */
>>  	ifs = ifs_alloc(iter->inode, folio, iter->flags);
>>  	iomap_adjust_read_range(iter->inode, folio, &pos, length, &poff, &plen);
>> +
>> +	if (ifs) {
>> +		loff_t to_read = min_t(loff_t, iter->len - offset,
>> +			folio_size(folio) - offset_in_folio(folio, orig_pos));
>> +		size_t padjust;
>> +
>> +		spin_lock_irq(&ifs->state_lock);
>> +		if (!ifs->read_bytes_pending)
>> +			ifs->read_bytes_pending = to_read;
>> +		padjust = pos - orig_pos;
>> +		ifs->read_bytes_pending -= padjust;
>> +		if (!ifs->read_bytes_pending)
>> +			rbp_finished = true;
>> +		spin_unlock_irq(&ifs->state_lock);
>> +	}
>> +
>>  	if (plen == 0)
>>  		goto done;
>> 
>>  	if (iomap_block_needs_zeroing(iter, pos)) {
>> +		if (ifs) {
>> +			spin_lock_irq(&ifs->state_lock);
>> +			ifs->read_bytes_pending -= plen;
>> +			if (!ifs->read_bytes_pending)
>> +				rbp_finished = true;
>> +			spin_unlock_irq(&ifs->state_lock);
>> +		}
>>  		folio_zero_range(folio, poff, plen);
>>  		iomap_set_range_uptodate(folio, poff, plen);
>>  		goto done;
>>  	}
>> 
>>  	ctx->cur_folio_in_bio = true;
>> -	if (ifs) {
>> -		spin_lock_irq(&ifs->state_lock);
>> -		ifs->read_bytes_pending += plen;
>> -		spin_unlock_irq(&ifs->state_lock);
>> -	}
>> 
>>  	sector = iomap_sector(iomap, pos);
>>  	if (!ctx->bio ||
>> @@ -435,6 +454,14 @@ static loff_t iomap_readpage_iter(const struct iomap_iter *iter,
>>  	}
>> 
>>  done:
>> +	/*
>> +	 * If there is no bio prepared and if rbp is finished and
>> +	 * this was the last offset within this folio then mark
>> +	 * cur_folio_in_bio to false.
>> +	 */
>> +	if (!ctx->bio && rbp_finished &&
>> +			offset_in_folio(folio, pos + plen) == 0)
>> +		ctx->cur_folio_in_bio = false;
>
> ...yes, I think I am confused.  When would ctx->bio be NULL but
> cur_folio_in_bio is true?

Previously we had the bio submitted and so we make it null, but we still
have ctx->cur_folio & ctx->cur_folio_in_bio to true, since we haven't
completely processed the folio.

>
> I /think/ what you're doing here is using read_bytes_pending to figure
> out if you've processed the folio up to the end of the mapping?  But
> then you submit the bio unconditionally below for each readpage_iter
> call?
>

yes, that's right.

> Why not add an IOMAP_BOUNDARY flag that means "I will have to do some IO
> if you call ->iomap_begin again"?  Then if we get to this point in
> readpage_iter with a ctx->bio, we can submit the bio, clear
> cur_folio_in_bio, and return?  And then you don't need this machinery?

TBH, I initially didn't think the approach taken in the patch would
require such careful handling of r_b_p. It was because of all of this
corner cases when we don't need to read the update blocks and/or in case
of an error we need to ensure we reduce r_b_p carefully so that we could
unlock the folio and when extent spans beyond i_size.

So it's all about how do we know if we could unlock the folio and that it's
corresponding blocks/mapping has been all processed or submitted for
I/O. 

Assume we have a folio which spans over multiple extents. In such a
case, 
-> we process a bio for 1st extent, 
-> then we go back to iomap_iter() to get new extent mapping, 
-> We now increment the r_b_p with this new plen to be processed. 
-> We then submit the previous bio, since this new mapping couldn't be
merged due to discontinuous extents. 
So by first incrementing the r_b_p before doing submit_bio(), we don't
unlock the folio at bio completion.

Maybe, it would be helpful if we have an easy mechanism to keep some state
from the time of submit_bio() till the bio completion to know that the
corresponding folio is still being processed and it shouldn't be
unlocked.
 -> This currently is what we are doing by making r_b_p to the value of
 folio_size() and then carefully reducing r_b_p for all the cases I
 mentioned above.

Let me think if adding a IOMAP_BH_BOUNDARY flag could be helpful or not.
Say if we have a pagesize of 64k that means all first 16 blocks belongs
to same page. So even with IOMAP_BH_BOUNDARY flag the problem that still
remains is that, even if we submit the bio at block 11 (bh_boundary
block), how will the bio completion side know that the folio is not
completely processed and so we shouldn't unlock the folio?


-ritesh