[PATCH v2 16/16] ext4: ensure global ordered traversal across all free groups xarrays

Baokun Li
2025-06-23T07:47:10Z

Although we now perform ordered traversal within an xarray, this is
currently limited to a single xarray, traversing right then left. However,
we have multiple such xarrays, which prevents us from guaranteeing a
linear-like traversal where all groups on the right are visited before all
groups on the left.

Therefore, this change modifies the traversal to first iterate through
all right groups across all xarrays, and then all left groups across all
xarrays. This achieves a linear-like effect, mitigating contention
between block allocation and block freeing paths.

Performance test data follows:

CPU: Kunpeng 920 | P80 | P1 |
Memory: 512GB |-------------------------|-------------------------|
Disk: 960GB SSD | base | patched | base | patched |
-------------------|-------|-----------------|-------|-----------------|
mb_optimize_scan=0 | 20976 | 20619 (-1.7%) | 319396| 299238 (-6.3%) |
mb_optimize_scan=1 | 14580 | 20119 (+37.9%) | 319237| 315268 (-1.2%) |

CPU: AMD 9654 * 2 | P96 | P1 |
Memory: 1536GB |-------------------------|-------------------------|
Disk: 960GB SSD | base | patched | base | patched |
-------------------|-------|-----------------|-------|-----------------|
mb_optimize_scan=0 | 51713 | 51983 (+0.5%) | 206655| 207033 (0.18%) |
mb_optimize_scan=1 | 35527 | 48486 (+36.4%) | 212574| 202415 (+4.7%) |

Signed-off-by: Baokun Li <libaokun1@huawei.com>
---
fs/ext4/mballoc.c | 69 ++++++++++++++++++++++++++++++++---------------
1 file changed, 47 insertions(+), 22 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index d8372a649a0c..d26a0e8e3f7e 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -877,22 +877,20 @@ mb_update_avg_fragment_size(struct super_block *sb, struct ext4_group_info *grp)
grp->bb_group, new, ret);
}

-static int ext4_mb_scan_groups_xarray(struct ext4_allocation_context *ac,
- struct xarray *xa, ext4_group_t start)
+static int ext4_mb_scan_groups_xa_range(struct ext4_allocation_context *ac,
+ struct xarray *xa,
+ ext4_group_t start, ext4_group_t end)
{
struct super_block *sb = ac->ac_sb;
struct ext4_sb_info *sbi = EXT4_SB(sb);
enum criteria cr = ac->ac_criteria;
ext4_group_t ngroups = ext4_get_groups_count(sb);
unsigned long group = start;
- ext4_group_t end;
struct ext4_group_info *grp;

- if (WARN_ON_ONCE(start >= ngroups))
+ if (WARN_ON_ONCE(end >= ngroups || start > end))
return 0;
- end = ngroups - 1;

-wrap_around:
xa_for_each_range(xa, group, grp, start, end) {
int err;

@@ -906,28 +904,23 @@ static int ext4_mb_scan_groups_xarray(struct ext4_allocation_context *ac,
cond_resched();
}

- if (start) {
- end = start - 1;
- start = 0;
- goto wrap_around;
- }
-
return 0;
}

/*
* Find a suitable group of given order from the largest free orders xarray.
*/
-static int
-ext4_mb_scan_groups_largest_free_order(struct ext4_allocation_context *ac,
- int order, ext4_group_t start)
+static inline int
+ext4_mb_scan_groups_largest_free_order_range(struct ext4_allocation_context *ac,
+ int order, ext4_group_t start,
+ ext4_group_t end)
{
struct xarray *xa = &EXT4_SB(ac->ac_sb)->s_mb_largest_free_orders[order];

if (xa_empty(xa))
return 0;

- return ext4_mb_scan_groups_xarray(ac, xa, start);
+ return ext4_mb_scan_groups_xa_range(ac, xa, start, end - 1);
}

/*
@@ -940,13 +933,23 @@ static int ext4_mb_scan_groups_p2_aligned(struct ext4_allocation_context *ac,
struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
int i;
int ret = 0;
+ ext4_group_t start, end;

ac->ac_flags |= EXT4_MB_CR_POWER2_ALIGNED_OPTIMIZED;
+ start = group;
+ end = ext4_get_groups_count(ac->ac_sb);
+wrap_around:
for (i = ac->ac_2order; i < MB_NUM_ORDERS(ac->ac_sb); i++) {
- ret = ext4_mb_scan_groups_largest_free_order(ac, i, group);
+ ret = ext4_mb_scan_groups_largest_free_order_range(ac, i,
+ start, end);
if (ret || ac->ac_status != AC_STATUS_CONTINUE)
goto out;
}
+ if (start) {
+ end = start;
+ start = 0;
+ goto wrap_around;
+ }

if (sbi->s_mb_stats)
atomic64_inc(&sbi->s_bal_cX_failed[ac->ac_criteria]);
@@ -961,15 +964,17 @@ static int ext4_mb_scan_groups_p2_aligned(struct ext4_allocation_context *ac,
/*
* Find a suitable group of given order from the average fragments xarray.
*/
-static int ext4_mb_scan_groups_avg_frag_order(struct ext4_allocation_context *ac,
- int order, ext4_group_t start)
+static int
+ext4_mb_scan_groups_avg_frag_order_range(struct ext4_allocation_context *ac,
+ int order, ext4_group_t start,
+ ext4_group_t end)
{
struct xarray *xa = &EXT4_SB(ac->ac_sb)->s_mb_avg_fragment_size[order];

if (xa_empty(xa))
return 0;

- return ext4_mb_scan_groups_xarray(ac, xa, start);
+ return ext4_mb_scan_groups_xa_range(ac, xa, start, end - 1);
}

/*
@@ -981,14 +986,24 @@ static int ext4_mb_scan_groups_goal_fast(struct ext4_allocation_context *ac,
{
struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
int i, ret = 0;
+ ext4_group_t start, end;

ac->ac_flags |= EXT4_MB_CR_GOAL_LEN_FAST_OPTIMIZED;
+ start = group;
+ end = ext4_get_groups_count(ac->ac_sb);
+wrap_around:
i = mb_avg_fragment_size_order(ac->ac_sb, ac->ac_g_ex.fe_len);
for (; i < MB_NUM_ORDERS(ac->ac_sb); i++) {
- ret = ext4_mb_scan_groups_avg_frag_order(ac, i, group);
+ ret = ext4_mb_scan_groups_avg_frag_order_range(ac, i,
+ start, end);
if (ret || ac->ac_status != AC_STATUS_CONTINUE)
goto out;
}
+ if (start) {
+ end = start;
+ start = 0;
+ goto wrap_around;
+ }

if (sbi->s_mb_stats)
atomic64_inc(&sbi->s_bal_cX_failed[ac->ac_criteria]);
@@ -1025,6 +1040,7 @@ static int ext4_mb_scan_groups_best_avail(struct ext4_allocation_context *ac,
struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
int i, order, min_order;
unsigned long num_stripe_clusters = 0;
+ ext4_group_t start, end;

/*
* mb_avg_fragment_size_order() returns order in a way that makes
@@ -1057,6 +1073,9 @@ static int ext4_mb_scan_groups_best_avail(struct ext4_allocation_context *ac,
min_order = fls(ac->ac_o_ex.fe_len);

ac->ac_flags |= EXT4_MB_CR_BEST_AVAIL_LEN_OPTIMIZED;
+ start = group;
+ end = ext4_get_groups_count(ac->ac_sb);
+wrap_around:
for (i = order; i >= min_order; i--) {
int frag_order;
/*
@@ -1079,10 +1098,16 @@ static int ext4_mb_scan_groups_best_avail(struct ext4_allocation_context *ac,
frag_order = mb_avg_fragment_size_order(ac->ac_sb,
ac->ac_g_ex.fe_len);

- ret = ext4_mb_scan_groups_avg_frag_order(ac, frag_order, group);
+ ret = ext4_mb_scan_groups_avg_frag_order_range(ac, frag_order,
+ start, end);
if (ret || ac->ac_status != AC_STATUS_CONTINUE)
goto out;
}
+ if (start) {
+ end = start;
+ start = 0;
+ goto wrap_around;
+ }

/* Reset goal length to original goal length before falling into CR_GOAL_LEN_SLOW */
ac->ac_g_ex.fe_len = ac->ac_orig_goal_len;
--
2.46.1