[RFC 11/11] ext4: Add allocation criteria 1.5 (CR1_5)

Ojaswin Mujoo
2023-01-27T12:40:07Z

CR1_5 aims to optimize allocations which can't be satisfied in CR1. The
fact that we couldn't find a group in CR1 suggests that it would be
difficult to find a continuous extent to compleltely satisfy our
allocations. So before falling to the slower CR2, in CR1.5 we
proactively trim the the preallocations so we can find a group with
(free / fragments) big enough. This speeds up our allocation at the
cost of slightly reduced preallocation.

The patch also adds a new sysfs tunable:

* /sys/fs/ext4/<partition>/mb_cr1_5_max_trim_order

This controls how much CR1.5 can trim a request before falling to CR2.
For example, for a request of order 7 and max trim order 2, CR1.5 can
trim this upto order 5.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
fs/ext4/ext4.h | 7 +++-
fs/ext4/mballoc.c | 97 ++++++++++++++++++++++++++++++++++++++++++++---
fs/ext4/mballoc.h | 14 +++++++
fs/ext4/sysfs.c | 2 +
4 files changed, 113 insertions(+), 7 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index d8fa01e54e81..879aac5e39a9 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -132,7 +132,7 @@ enum SHIFT_DIRECTION {
* criteria the slower the allocation. We start at lower criterias and keep
* falling back to higher ones if we are not able to find any blocks.
*/
-#define EXT4_MB_NUM_CRS 4
+#define EXT4_MB_NUM_CRS 5

/*
* Flags used in mballoc's allocation_context flags field.
@@ -175,6 +175,9 @@ enum SHIFT_DIRECTION {
#define EXT4_MB_CR0_OPTIMIZED 0x8000
/* Avg fragment size rb tree lookup succeeded at least once for cr = 1 */
#define EXT4_MB_CR1_OPTIMIZED 0x00010000
+/* Avg fragment size rb tree lookup succeeded at least once for cr = 1.5 */
+#define EXT4_MB_CR1_5_OPTIMIZED 0x00020000
+
struct ext4_allocation_request {
/* target inode for block we're allocating */
struct inode *inode;
@@ -1627,6 +1630,7 @@ struct ext4_sb_info {
unsigned long s_mb_last_start;
unsigned int s_mb_prefetch;
unsigned int s_mb_prefetch_limit;
+ unsigned int s_mb_cr1_5_max_trim_order;

/* stats for buddy allocator */
atomic_t s_bal_reqs; /* number of reqs with len > 1 */
@@ -1641,6 +1645,7 @@ struct ext4_sb_info {
atomic_t s_bal_2orders; /* 2^order hits */
atomic_t s_bal_cr0_bad_suggestions;
atomic_t s_bal_cr1_bad_suggestions;
+ atomic_t s_bal_cr1_5_bad_suggestions;
atomic64_t s_bal_cX_groups_considered[EXT4_MB_NUM_CRS];
atomic64_t s_bal_cX_hits[EXT4_MB_NUM_CRS];
atomic64_t s_bal_cX_failed[EXT4_MB_NUM_CRS]; /* cX loop didn't find blocks */
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 1ce1174aea52..8e9032f94966 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -960,6 +960,67 @@ static void ext4_mb_choose_next_group_cr1(struct ext4_allocation_context *ac,
*group = grp->bb_group;
ac->ac_flags |= EXT4_MB_CR1_OPTIMIZED;
} else {
+ *new_cr = CR1_5;
+ }
+}
+
+/*
+ * We couldn't find a group in CR1 so try to find the highest free fragment
+ * order we have and proactively trim the goal request length to that order to
+ * find a suitable group faster.
+ *
+ * This optimizes allocation speed at the cost of slightly reduced
+ * preallocations. However, we make sure that we don't trim the request too
+ * much and fall to CR2 in that case.
+ */
+static void ext4_mb_choose_next_group_cr1_5(struct ext4_allocation_context *ac,
+ enum criteria *new_cr, ext4_group_t *group, ext4_group_t ngroups)
+{
+ struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
+ struct ext4_group_info *grp = NULL;
+ int i, order, min_order;
+
+ if (unlikely(ac->ac_flags & EXT4_MB_CR1_5_OPTIMIZED)) {
+ if (sbi->s_mb_stats)
+ atomic_inc(&sbi->s_bal_cr1_5_bad_suggestions);
+ }
+
+ /*
+ * mb_avg_fragment_size_order() returns order in a way that makes
+ * retrieving back the length using (1 << order) inaccurate. Hence, use
+ * fls() instead since we need to know the actual length while modifying
+ * goal length.
+ */
+ order = fls(ac->ac_g_ex.fe_len);
+ min_order = order - sbi->s_mb_cr1_5_max_trim_order;
+ if (min_order < 0)
+ min_order = 0;
+
+ for (i = order; i >= min_order; i--) {
+ if (ac->ac_o_ex.fe_len <= (1 << i)) {
+ /*
+ * Scale down goal len to make sure we find something
+ * in the free fragments list. Basically, reduce
+ * preallocations.
+ */
+ ac->ac_g_ex.fe_len = 1 << i;
+ } else {
+ break;
+ }
+
+ grp = ext4_mb_find_good_group_avg_frag_lists(ac,
+ mb_avg_fragment_size_order(ac->ac_sb,
+ ac->ac_g_ex.fe_len));
+ if (grp)
+ break;
+ }
+
+ if (grp) {
+ *group = grp->bb_group;
+ ac->ac_flags |= EXT4_MB_CR1_5_OPTIMIZED;
+ } else {
+ /* Reset goal length to original goal length before falling into CR2 */
+ ac->ac_g_ex.fe_len = ac->ac_orig_goal_len;
*new_cr = CR2;
}
}
@@ -1026,6 +1087,8 @@ static void ext4_mb_choose_next_group(struct ext4_allocation_context *ac,
ext4_mb_choose_next_group_cr0(ac, new_cr, group, ngroups);
} else if (*new_cr == CR1) {
ext4_mb_choose_next_group_cr1(ac, new_cr, group, ngroups);
+ } else if (*new_cr == CR1_5) {
+ ext4_mb_choose_next_group_cr1_5(ac, new_cr, group, ngroups);
} else {
/*
* TODO: For CR=2, we can arrange groups in an rb tree sorted by
@@ -2323,7 +2386,7 @@ void ext4_mb_complex_scan_group(struct ext4_allocation_context *ac,

if (ac->ac_criteria < CR2) {
/*
- * In CR1, we are sure that this group will
+ * In CR1 and CR1_5, we are sure that this group will
* have a large enough continuous free extent, so skip
* over the smaller free extents
*/
@@ -2453,6 +2516,7 @@ static bool ext4_mb_good_group(struct ext4_allocation_context *ac,

return true;
case CR1:
+ case CR1_5:
if ((free / fragments) >= ac->ac_g_ex.fe_len)
return true;
break;
@@ -2715,7 +2779,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
* spend a lot of time loading imperfect groups
*/
if ((prefetch_grp == group) &&
- (cr > CR1 ||
+ (cr > CR1_5 ||
prefetch_ios < sbi->s_mb_prefetch_limit)) {
nr = sbi->s_mb_prefetch;
if (ext4_has_feature_flex_bg(sb)) {
@@ -2755,8 +2819,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
ac->ac_groups_scanned++;
if (cr == CR0)
ext4_mb_simple_scan_group(ac, &e4b);
- else if (cr == CR1 && sbi->s_stripe &&
- !(ac->ac_g_ex.fe_len % sbi->s_stripe))
+ else if ((cr == CR1 || cr == CR1_5) && sbi->s_stripe &&
+ !(ac->ac_g_ex.fe_len % sbi->s_stripe))
ext4_mb_scan_aligned(ac, &e4b);
else
ext4_mb_complex_scan_group(ac, &e4b);
@@ -2770,6 +2834,11 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
/* Processed all groups and haven't found blocks */
if (sbi->s_mb_stats && i == ngroups)
atomic64_inc(&sbi->s_bal_cX_failed[cr]);
+
+ if (i == ngroups && ac->ac_criteria == CR1_5)
+ /* Reset goal length to original goal length before
+ * falling into CR2 */
+ ac->ac_g_ex.fe_len = ac->ac_orig_goal_len;
}

if (ac->ac_b_ex.fe_len > 0 && ac->ac_status != AC_STATUS_FOUND &&
@@ -2937,6 +3006,16 @@ int ext4_seq_mb_stats_show(struct seq_file *seq, void *offset)
seq_printf(seq, "\t\tbad_suggestions: %u\n",
atomic_read(&sbi->s_bal_cr1_bad_suggestions));

+ seq_puts(seq, "\tcr1.5_stats:\n");
+ seq_printf(seq, "\t\thits: %llu\n", atomic64_read(&sbi->s_bal_cX_hits[CR1_5]));
+ seq_printf(seq, "\t\tgroups_considered: %llu\n",
+ atomic64_read(&sbi->s_bal_cX_groups_considered[CR1_5]));
+ seq_printf(seq, "\t\textents_scanned: %u\n", atomic_read(&sbi->s_bal_cX_ex_scanned[CR1_5]));
+ seq_printf(seq, "\t\tuseless_loops: %llu\n",
+ atomic64_read(&sbi->s_bal_cX_failed[CR1_5]));
+ seq_printf(seq, "\t\tbad_suggestions: %u\n",
+ atomic_read(&sbi->s_bal_cr1_5_bad_suggestions));
+
seq_puts(seq, "\tcr2_stats:\n");
seq_printf(seq, "\t\thits: %llu\n", atomic64_read(&sbi->s_bal_cX_hits[CR2]));
seq_printf(seq, "\t\tgroups_considered: %llu\n",
@@ -3452,6 +3531,8 @@ int ext4_mb_init(struct super_block *sb)
sbi->s_mb_stream_request = MB_DEFAULT_STREAM_THRESHOLD;
sbi->s_mb_order2_reqs = MB_DEFAULT_ORDER2_REQS;
sbi->s_mb_max_inode_prealloc = MB_DEFAULT_MAX_INODE_PREALLOC;
+ sbi->s_mb_cr1_5_max_trim_order = MB_DEFAULT_CR1_5_TRIM_ORDER;
+
/*
* The default group preallocation is 512, which for 4k block
* sizes translates to 2 megabytes. However for bigalloc file
@@ -4218,6 +4299,7 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
* placement or satisfy big request as is */
ac->ac_g_ex.fe_logical = start;
ac->ac_g_ex.fe_len = EXT4_NUM_B2C(sbi, size);
+ ac->ac_orig_goal_len = ac->ac_g_ex.fe_len;

/* define goal start in order to merge */
if (ar->pright && (ar->lright == (start + size))) {
@@ -4258,8 +4340,10 @@ static void ext4_mb_collect_stats(struct ext4_allocation_context *ac)
if (ac->ac_g_ex.fe_start == ac->ac_b_ex.fe_start &&
ac->ac_g_ex.fe_group == ac->ac_b_ex.fe_group)
atomic_inc(&sbi->s_bal_goals);
- if (ac->ac_f_ex.fe_len == ac->ac_g_ex.fe_len)
+ /* did we allocate as much as normalizer originally wanted? */
+ if (ac->ac_f_ex.fe_len == ac->ac_orig_goal_len)
atomic_inc(&sbi->s_bal_len_goals);
+
if (ac->ac_found > sbi->s_mb_max_to_scan)
atomic_inc(&sbi->s_bal_breaks);
}
@@ -4652,7 +4736,7 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)

pa = ac->ac_pa;

- if (ac->ac_b_ex.fe_len < ac->ac_g_ex.fe_len) {
+ if (ac->ac_b_ex.fe_len < ac->ac_orig_goal_len) {
int winl;
int wins;
int win;
@@ -5281,6 +5365,7 @@ ext4_mb_initialize_context(struct ext4_allocation_context *ac,
ac->ac_o_ex.fe_start = block;
ac->ac_o_ex.fe_len = len;
ac->ac_g_ex = ac->ac_o_ex;
+ ac->ac_orig_goal_len = ac->ac_g_ex.fe_len;
ac->ac_flags = ar->flags;

/* we have to define context: we'll work with a file or
diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index 004b8d163cc9..c1b0bf2f6f4d 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h
@@ -90,6 +90,13 @@
*/
#define MB_DEFAULT_LINEAR_SCAN_THRESHOLD 16

+/*
+ * The maximum order upto which CR1.5 can trim a particular allocation request.
+ * Example, if we have an order 7 request and max trim order of 3, CR1.5 can
+ * trim this upto order 4.
+ */
+#define MB_DEFAULT_CR1_5_TRIM_ORDER 3
+
/*
* Number of valid buddy orders
*/
@@ -101,6 +108,7 @@
enum criteria {
CR0,
CR1,
+ CR1_5,
CR2,
CR3,
};
@@ -188,6 +196,12 @@ struct ext4_allocation_context {
/* copy of the best found extent taken before preallocation efforts */
struct ext4_free_extent ac_f_ex;

+ /*
+ * goal len can change in CR1.5, so save the original len. This is
+ * used while adjusting the PA window and for accounting.
+ */
+ ext4_grpblk_t ac_orig_goal_len;
+
__u32 ac_groups_considered;
__u32 ac_flags; /* allocation hints */
__u16 ac_groups_scanned;
diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
index d233c24ea342..5ba884a0246e 100644
--- a/fs/ext4/sysfs.c
+++ b/fs/ext4/sysfs.c
@@ -224,6 +224,7 @@ EXT4_RW_ATTR_SBI_UI(warning_ratelimit_interval_ms, s_warning_ratelimit_state.int
EXT4_RW_ATTR_SBI_UI(warning_ratelimit_burst, s_warning_ratelimit_state.burst);
EXT4_RW_ATTR_SBI_UI(msg_ratelimit_interval_ms, s_msg_ratelimit_state.interval);
EXT4_RW_ATTR_SBI_UI(msg_ratelimit_burst, s_msg_ratelimit_state.burst);
+EXT4_RW_ATTR_SBI_UI(mb_cr1_5_max_trim_order, s_mb_cr1_5_max_trim_order);
#ifdef CONFIG_EXT4_DEBUG
EXT4_RW_ATTR_SBI_UL(simulate_fail, s_simulate_fail);
#endif
@@ -275,6 +276,7 @@ static struct attribute *ext4_attrs[] = {
ATTR_LIST(warning_ratelimit_burst),
ATTR_LIST(msg_ratelimit_interval_ms),
ATTR_LIST(msg_ratelimit_burst),
+ ATTR_LIST(mb_cr1_5_max_trim_order),
ATTR_LIST(errors_count),
ATTR_LIST(warning_count),
ATTR_LIST(msg_count),
--
2.31.1