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 think you're trying to say that the access pattern here is 98400 ->
98408 -> 98384, which is not sequential?
> 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@xxxxxxxxxxxxx>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> cc: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
> ---
> 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...
> 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?
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?
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?
--D
> /*
> * Move the caller beyond our range so that it keeps making progress.
> * For that, we have to include any leading non-uptodate ranges, but
> @@ -459,9 +486,43 @@ static loff_t iomap_read_folio_iter(const struct iomap_iter *iter,
> return ret;
> }
>
> + if (ctx->bio) {
> + submit_bio(ctx->bio);
> + WARN_ON_ONCE(!ctx->cur_folio_in_bio);
> + ctx->bio = NULL;
> + }
> + if (offset_in_folio(folio, iter->pos + done) == 0 &&
> + !ctx->cur_folio_in_bio) {
> + folio_unlock(ctx->cur_folio);
> + }
> +
> return done;
> }
>
> +static void iomap_handle_read_error(struct iomap_readpage_ctx *ctx,
> + struct iomap_iter *iter)
> +{
> + struct folio *folio = ctx->cur_folio;
> + struct iomap_folio_state *ifs;
> + unsigned long flags;
> + bool rbp_finished = false;
> + size_t rbp_adjust = folio_size(folio) - offset_in_folio(folio,
> + iter->pos);
> + ifs = folio->private;
> + if (!ifs || !ifs->read_bytes_pending)
> + goto unlock;
> +
> + spin_lock_irqsave(&ifs->state_lock, flags);
> + ifs->read_bytes_pending -= rbp_adjust;
> + if (!ifs->read_bytes_pending)
> + rbp_finished = true;
> + spin_unlock_irqrestore(&ifs->state_lock, flags);
> +
> +unlock:
> + if (rbp_finished || !ctx->cur_folio_in_bio)
> + folio_unlock(folio);
> +}
> +
> int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops)
> {
> struct iomap_iter iter = {
> @@ -479,15 +540,9 @@ int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops)
> while ((ret = iomap_iter(&iter, ops)) > 0)
> iter.processed = iomap_read_folio_iter(&iter, &ctx);
>
> - if (ret < 0)
> + if (ret < 0) {
> folio_set_error(folio);
> -
> - if (ctx.bio) {
> - submit_bio(ctx.bio);
> - WARN_ON_ONCE(!ctx.cur_folio_in_bio);
> - } else {
> - WARN_ON_ONCE(ctx.cur_folio_in_bio);
> - folio_unlock(folio);
> + iomap_handle_read_error(&ctx, &iter);
> }
>
> /*
> @@ -506,12 +561,6 @@ static loff_t iomap_readahead_iter(const struct iomap_iter *iter,
> loff_t done, ret;
>
> for (done = 0; done < length; done += ret) {
> - if (ctx->cur_folio &&
> - offset_in_folio(ctx->cur_folio, iter->pos + done) == 0) {
> - if (!ctx->cur_folio_in_bio)
> - folio_unlock(ctx->cur_folio);
> - ctx->cur_folio = NULL;
> - }
> if (!ctx->cur_folio) {
> ctx->cur_folio = readahead_folio(ctx->rac);
> ctx->cur_folio_in_bio = false;
> @@ -519,6 +568,17 @@ static loff_t iomap_readahead_iter(const struct iomap_iter *iter,
> ret = iomap_readpage_iter(iter, ctx, done);
> if (ret <= 0)
> return ret;
> + if (ctx->cur_folio && offset_in_folio(ctx->cur_folio,
> + iter->pos + done + ret) == 0) {
> + if (!ctx->cur_folio_in_bio)
> + folio_unlock(ctx->cur_folio);
> + ctx->cur_folio = NULL;
> + }
> + }
> +
> + if (ctx->bio) {
> + submit_bio(ctx->bio);
> + ctx->bio = NULL;
> }
>
> return done;
> @@ -549,18 +609,16 @@ void iomap_readahead(struct readahead_control *rac, const struct iomap_ops *ops)
> struct iomap_readpage_ctx ctx = {
> .rac = rac,
> };
> + int ret = 0;
>
> trace_iomap_readahead(rac->mapping->host, readahead_count(rac));
>
> - while (iomap_iter(&iter, ops) > 0)
> + while ((ret = iomap_iter(&iter, ops)) > 0)
> iter.processed = iomap_readahead_iter(&iter, &ctx);
>
> - if (ctx.bio)
> - submit_bio(ctx.bio);
> - if (ctx.cur_folio) {
> - if (!ctx.cur_folio_in_bio)
> - folio_unlock(ctx.cur_folio);
> - }
> + if (ret < 0 && ctx.cur_folio)
> + iomap_handle_read_error(&ctx, &iter);
> +
> }
> EXPORT_SYMBOL_GPL(iomap_readahead);
>
> --
> 2.44.0
>
>
