[PATCH 3/5] ext4: call ext4_mb_mark_free_simple in mb_mark_used to clear bits

[Kemeng Shi]

Function ext4_mb_mark_free_simple could search order for bit clearing in
O(1) cost while mb_mark_used will search order in O(distance from chunk
order to target order) and introduce unnecessary bit flips.

Consider we have 4 continuous free bits and going to mark bit 0-2 inuse.
initial state of buddy bitmap:
order 2 |           0           |
order 1 |     1     |     1     |
order 0 |  1  |  1  |  1  |  1  |

mark whole chunk inuse
order 2 |           1           |
order 1 |     1     |     1     |
order 0 |  1  |  1  |  1  |  1  |

split chunk to order 1
order 2 |           1           |
order 1 |     0     |     0     |
order 0 |  1  |  1  |  1  |  1  |

set the first bit in order 1 to mark bit 0-1 inuse
set the second bit in order 1 for split
order 2 |           1           |
order 1 |     1     |     1     |
order 0 |  1  |  1  |  1  |  1  |

step 3: split the second bit in order 1 to order 0
order 2 |           1           |
order 1 |     1     |     1     |
order 0 |  1  |  1  |  0  |  0  |

step 4: set the third bit in order 0 to mark bit 2 inuse.
order 2 |           1           |
order 1 |     1     |     1     |
order 0 |  1  |  1  |  1  |  0  |
There are two unnecessary splits and three unnecessary bit flips.

With ext4_mb_mark_free_simple, we will clear the 4th bit in order 0
with O(1) search and no extra bit flip.

The cost estimated by test_mb_mark_used_cost is as following:
Before (three runs of test):
    # test_mb_mark_used_cost: costed jiffies 311
    # test_mb_mark_used_cost: costed jiffies 304
    # test_mb_mark_used_cost: costed jiffies 305
    # test_mb_mark_used_cost: costed jiffies 323
    # test_mb_mark_used_cost: costed jiffies 317
    # test_mb_mark_used_cost: costed jiffies 317
After (three runs of test):
    # test_mb_mark_used_cost: costed jiffies 166
    # test_mb_mark_used_cost: costed jiffies 152
    # test_mb_mark_used_cost: costed jiffies 159
    # test_mb_mark_used_cost: costed jiffies 138
    # test_mb_mark_used_cost: costed jiffies 149

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 fs/ext4/mballoc.c | 37 ++++++++++++++++++++-----------------
 1 file changed, 20 insertions(+), 17 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a61fc52956b2..62d468379722 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2040,13 +2040,12 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
 	int ord;
 	int mlen = 0;
 	int max = 0;
-	int cur;
 	int start = ex->fe_start;
 	int len = ex->fe_len;
 	unsigned ret = 0;
 	int len0 = len;
 	void *buddy;
-	bool split = false;
+	int ord_start, ord_end;
 
 	BUG_ON(start + len > (e4b->bd_sb->s_blocksize << 3));
 	BUG_ON(e4b->bd_group != ex->fe_group);
@@ -2071,16 +2070,12 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
 
 	/* let's maintain buddy itself */
 	while (len) {
-		if (!split)
-			ord = mb_find_order_for_block(e4b, start);
+		ord = mb_find_order_for_block(e4b, start);
 
 		if (((start >> ord) << ord) == start && len >= (1 << ord)) {
 			/* the whole chunk may be allocated at once! */
 			mlen = 1 << ord;
-			if (!split)
-				buddy = mb_find_buddy(e4b, ord, &max);
-			else
-				split = false;
+			buddy = mb_find_buddy(e4b, ord, &max);
 			BUG_ON((start >> ord) >= max);
 			mb_set_bit(start >> ord, buddy);
 			e4b->bd_info->bb_counters[ord]--;
@@ -2094,20 +2089,28 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
 		if (ret == 0)
 			ret = len | (ord << 16);
 
-		/* we have to split large buddy */
 		BUG_ON(ord <= 0);
 		buddy = mb_find_buddy(e4b, ord, &max);
 		mb_set_bit(start >> ord, buddy);
 		e4b->bd_info->bb_counters[ord]--;
 
-		ord--;
-		cur = (start >> ord) & ~1U;
-		buddy = mb_find_buddy(e4b, ord, &max);
-		mb_clear_bit(cur, buddy);
-		mb_clear_bit(cur + 1, buddy);
-		e4b->bd_info->bb_counters[ord]++;
-		e4b->bd_info->bb_counters[ord]++;
-		split = true;
+		ord_start = (start >> ord) << ord;
+		ord_end = ord_start + (1 << ord);
+		if (start > ord_start)
+			ext4_mb_mark_free_simple(e4b->bd_sb, e4b->bd_buddy,
+						 ord_start, start - ord_start,
+						 e4b->bd_info);
+
+		if (start + len < ord_end) {
+			ext4_mb_mark_free_simple(e4b->bd_sb, e4b->bd_buddy,
+						 start + len,
+						 ord_end - (start + len),
+						 e4b->bd_info);
+			break;
+		}
+
+		len = start + len - ord_end;
+		start = ord_end;
 	}
 	mb_set_largest_free_order(e4b->bd_sb, e4b->bd_info);
 
-- 
2.30.0