ocfs2: improve write IO performance when fragmentation is high

The group_search function ocfs2_cluster_group_search() should
bypass groups with insufficient space to avoid unnecessary
searches.

This patch is particularly useful when ocfs2 is handling huge
number small files, and volume fragmentation is very high.
In this case, ocfs2 is busy with looking up available la window
from //global_bitmap.

This patch introduces a new member in the Group Description (gd)
struct called 'bg_contig_free_bits', representing the max
contigous free bits in this gd. When ocfs2 allocates a new
la window from //global_bitmap, 'bg_contig_free_bits' helps
expedite the search process.

Let's image below path.

1. la state (->local_alloc_state) is set THROTTLED or DISABLED.

2. when user delete a large file and trigger
   ocfs2_local_alloc_seen_free_bits set osb->local_alloc_state
   unconditionally.

3. a write IOs thread run and trigger the worst performance path

```
ocfs2_reserve_clusters_with_limit
 ocfs2_reserve_local_alloc_bits
  ocfs2_local_alloc_slide_window //[1]
   + ocfs2_local_alloc_reserve_for_window //[2]
   + ocfs2_local_alloc_new_window //[3]
      ocfs2_recalc_la_window
```

[1]:
will be called when la window bits used up.

[2]:
under la state is ENABLED, and this func only check global_bitmap
free bits, it will succeed in general.

[3]:
will use the default la window size to search clusters then fail.
ocfs2_recalc_la_window attempts other la window sizes.
the timing complexity is O(n^4), resulting in a significant time
cost for scanning global bitmap. This leads to a dramatic slowdown
in write I/Os (e.g., user space 'dd').

i.e.
an ocfs2 partition size: 1.45TB, cluster size: 4KB,
la window default size: 106MB.
The partition is fragmentation by creating & deleting huge mount of
small files.

before this patch, the timing of [3] should be
(the number got from real world):
- la window size change order (size: MB):
  106, 53, 26.5, 13, 6.5, 3.25, 1.6, 0.8
  only 0.8MB succeed, 0.8MB also triggers la window to disable.
  ocfs2_local_alloc_new_window retries 8 times, first 7 times totally
  runs in worst case.
- group chain number: 242
  ocfs2_claim_suballoc_bits calls for-loop 242 times
- each chain has 49 block group
  ocfs2_search_chain calls while-loop 49 times
- each bg has 32256 blocks
  ocfs2_block_group_find_clear_bits calls while-loop for 32256 bits.
  for ocfs2_find_next_zero_bit uses ffz() to find zero bit, let's use
  (32256/64) (this is not worst value) for timing calucation.

the loop times: 7*242*49*(32256/64) = 41835024 (~42 million times)

In the worst case, user space writes 1MB data will trigger 42M scanning
times.

under this patch, the timing is '7*242*49 = 83006', reduced by three
orders of magnitude.

Link: https://lkml.kernel.org/r/20240328125203.20892-2-heming.zhao@suse.com
Signed-off-by: Heming Zhao <heming.zhao@suse.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Jun Piao <piaojun@huawei.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Mark Fasheh <mark@fasheh.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 92 insertions, 8 deletions

diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
index 961998415308..7e30e9d79890 100644
--- a/fs/ocfs2/suballoc.c
+++ b/fs/ocfs2/suballoc.c
@@ -50,6 +50,10 @@ struct ocfs2_suballoc_result {
 	u64		sr_blkno;	/* The first allocated block */
 	unsigned int	sr_bit_offset;	/* The bit in the bg */
 	unsigned int	sr_bits;	/* How many bits we claimed */
+	unsigned int	sr_max_contig_bits; /* The length for contiguous
+					     * free bits, only available
+					     * for cluster group
+					     */
 };
 
 static u64 ocfs2_group_from_res(struct ocfs2_suballoc_result *res)
@@ -1272,6 +1276,26 @@ static int ocfs2_test_bg_bit_allocatable(struct buffer_head *bg_bh,
 	return ret;
 }
 
+u16 ocfs2_find_max_contig_free_bits(void *bitmap,
+			 u16 total_bits, u16 start)
+{
+	u16 offset, free_bits;
+	u16 contig_bits = 0;
+
+	while (start < total_bits) {
+		offset = ocfs2_find_next_zero_bit(bitmap, total_bits, start);
+		if (offset == total_bits)
+			break;
+
+		start = ocfs2_find_next_bit(bitmap, total_bits, offset);
+		free_bits = start - offset;
+		if (contig_bits < free_bits)
+			contig_bits = free_bits;
+	}
+
+	return contig_bits;
+}
+
 static int ocfs2_block_group_find_clear_bits(struct ocfs2_super *osb,
 					     struct buffer_head *bg_bh,
 					     unsigned int bits_wanted,
@@ -1280,6 +1304,7 @@ static int ocfs2_block_group_find_clear_bits(struct ocfs2_super *osb,
 {
 	void *bitmap;
 	u16 best_offset, best_size;
+	u16 prev_best_size = 0;
 	int offset, start, found, status = 0;
 	struct ocfs2_group_desc *bg = (struct ocfs2_group_desc *) bg_bh->b_data;
 
@@ -1306,6 +1331,7 @@ static int ocfs2_block_group_find_clear_bits(struct ocfs2_super *osb,
 			/* got a zero after some ones */
 			found = 1;
 			start = offset + 1;
+			prev_best_size = best_size;
 		}
 		if (found > best_size) {
 			best_size = found;
@@ -1318,6 +1344,8 @@ static int ocfs2_block_group_find_clear_bits(struct ocfs2_super *osb,
 		}
 	}
 
+	/* best_size will be allocated, we save prev_best_size */
+	res->sr_max_contig_bits = prev_best_size;
 	if (best_size) {
 		res->sr_bit_offset = best_offset;
 		res->sr_bits = best_size;
@@ -1335,11 +1363,15 @@ int ocfs2_block_group_set_bits(handle_t *handle,
 					     struct ocfs2_group_desc *bg,
 					     struct buffer_head *group_bh,
 					     unsigned int bit_off,
-					     unsigned int num_bits)
+					     unsigned int num_bits,
+					     unsigned int max_contig_bits,
+					     int fastpath)
 {
 	int status;
 	void *bitmap = bg->bg_bitmap;
 	int journal_type = OCFS2_JOURNAL_ACCESS_WRITE;
+	unsigned int start = bit_off + num_bits;
+	u16 contig_bits;
 
 	/* All callers get the descriptor via
 	 * ocfs2_read_group_descriptor().  Any corruption is a code bug. */
@@ -1371,6 +1403,28 @@ int ocfs2_block_group_set_bits(handle_t *handle,
 	while(num_bits--)
 		ocfs2_set_bit(bit_off++, bitmap);
 
+	/*
+	 * this is optimize path, caller set old contig value
+	 * in max_contig_bits to bypass finding action.
+	 */
+	if (fastpath) {
+		bg->bg_contig_free_bits = cpu_to_le16(max_contig_bits);
+	} else if (ocfs2_is_cluster_bitmap(alloc_inode)) {
+		/*
+		 * Usually, the block group bitmap allocates only 1 bit
+		 * at a time, while the cluster group allocates n bits
+		 * each time. Therefore, we only save the contig bits for
+		 * the cluster group.
+		 */
+		contig_bits = ocfs2_find_max_contig_free_bits(bitmap,
+				    le16_to_cpu(bg->bg_bits), start);
+		if (contig_bits > max_contig_bits)
+			max_contig_bits = contig_bits;
+		bg->bg_contig_free_bits = cpu_to_le16(max_contig_bits);
+	} else {
+		bg->bg_contig_free_bits = 0;
+	}
+
 	ocfs2_journal_dirty(handle, group_bh);
 
 bail:
@@ -1484,7 +1538,12 @@ static int ocfs2_cluster_group_search(struct inode *inode,
 
 	BUG_ON(!ocfs2_is_cluster_bitmap(inode));
 
-	if (gd->bg_free_bits_count) {
+	if (le16_to_cpu(gd->bg_contig_free_bits) &&
+	    le16_to_cpu(gd->bg_contig_free_bits) < bits_wanted)
+		return -ENOSPC;
+
+	/* ->bg_contig_free_bits may un-initialized, so compare again */
+	if (le16_to_cpu(gd->bg_free_bits_count) >= bits_wanted) {
 		max_bits = le16_to_cpu(gd->bg_bits);
 
 		/* Tail groups in cluster bitmaps which aren't cpg
@@ -1553,7 +1612,7 @@ static int ocfs2_block_group_search(struct inode *inode,
 	BUG_ON(min_bits != 1);
 	BUG_ON(ocfs2_is_cluster_bitmap(inode));
 
-	if (bg->bg_free_bits_count) {
+	if (le16_to_cpu(bg->bg_free_bits_count) >= bits_wanted) {
 		ret = ocfs2_block_group_find_clear_bits(OCFS2_SB(inode->i_sb),
 							group_bh, bits_wanted,
 							le16_to_cpu(bg->bg_bits),
@@ -1713,7 +1772,8 @@ static int ocfs2_search_one_group(struct ocfs2_alloc_context *ac,
 	}
 
 	ret = ocfs2_block_group_set_bits(handle, alloc_inode, gd, group_bh,
-					 res->sr_bit_offset, res->sr_bits);
+					 res->sr_bit_offset, res->sr_bits,
+					 res->sr_max_contig_bits, 0);
 	if (ret < 0) {
 		ocfs2_rollback_alloc_dinode_counts(alloc_inode, ac->ac_bh,
 					       res->sr_bits,
@@ -1847,7 +1907,9 @@ static int ocfs2_search_chain(struct ocfs2_alloc_context *ac,
 					    bg,
 					    group_bh,
 					    res->sr_bit_offset,
-					    res->sr_bits);
+					    res->sr_bits,
+					    res->sr_max_contig_bits,
+					    0);
 	if (status < 0) {
 		ocfs2_rollback_alloc_dinode_counts(alloc_inode,
 					ac->ac_bh, res->sr_bits, chain);
@@ -2161,7 +2223,9 @@ int ocfs2_claim_new_inode_at_loc(handle_t *handle,
 					 bg,
 					 bg_bh,
 					 res->sr_bit_offset,
-					 res->sr_bits);
+					 res->sr_bits,
+					 res->sr_max_contig_bits,
+					 0);
 	if (ret < 0) {
 		ocfs2_rollback_alloc_dinode_counts(ac->ac_inode,
 					       ac->ac_bh, res->sr_bits, chain);
@@ -2380,11 +2444,13 @@ static int ocfs2_block_group_clear_bits(handle_t *handle,
 					struct buffer_head *group_bh,
 					unsigned int bit_off,
 					unsigned int num_bits,
+					unsigned int max_contig_bits,
 					void (*undo_fn)(unsigned int bit,
 							unsigned long *bmap))
 {
 	int status;
 	unsigned int tmp;
+	u16 contig_bits;
 	struct ocfs2_group_desc *undo_bg = NULL;
 	struct journal_head *jh;
 
@@ -2431,6 +2497,20 @@ static int ocfs2_block_group_clear_bits(handle_t *handle,
 				   num_bits);
 	}
 
+	/*
+	 * TODO: even 'num_bits == 1' (the worst case, release 1 cluster),
+	 * we still need to rescan whole bitmap.
+	 */
+	if (ocfs2_is_cluster_bitmap(alloc_inode)) {
+		contig_bits = ocfs2_find_max_contig_free_bits(bg->bg_bitmap,
+				    le16_to_cpu(bg->bg_bits), 0);
+		if (contig_bits > max_contig_bits)
+			max_contig_bits = contig_bits;
+		bg->bg_contig_free_bits = cpu_to_le16(max_contig_bits);
+	} else {
+		bg->bg_contig_free_bits = 0;
+	}
+
 	if (undo_fn)
 		spin_unlock(&jh->b_state_lock);
 
@@ -2457,6 +2537,7 @@ static int _ocfs2_free_suballoc_bits(handle_t *handle,
 	struct ocfs2_chain_list *cl = &fe->id2.i_chain;
 	struct buffer_head *group_bh = NULL;
 	struct ocfs2_group_desc *group;
+	__le16 old_bg_contig_free_bits = 0;
 
 	/* The alloc_bh comes from ocfs2_free_dinode() or
 	 * ocfs2_free_clusters().  The callers have all locked the
@@ -2481,9 +2562,11 @@ static int _ocfs2_free_suballoc_bits(handle_t *handle,
 
 	BUG_ON((count + start_bit) > le16_to_cpu(group->bg_bits));
 
+	if (ocfs2_is_cluster_bitmap(alloc_inode))
+		old_bg_contig_free_bits = group->bg_contig_free_bits;
 	status = ocfs2_block_group_clear_bits(handle, alloc_inode,
 					      group, group_bh,
-					      start_bit, count, undo_fn);
+					      start_bit, count, 0, undo_fn);
 	if (status < 0) {
 		mlog_errno(status);
 		goto bail;
@@ -2494,7 +2577,8 @@ static int _ocfs2_free_suballoc_bits(handle_t *handle,
 	if (status < 0) {
 		mlog_errno(status);
 		ocfs2_block_group_set_bits(handle, alloc_inode, group, group_bh,
-				start_bit, count);
+				start_bit, count,
+				le16_to_cpu(old_bg_contig_free_bits), 1);
 		goto bail;
 	}