2020-04-28 05:20:54

by Alex Zhuravlev

[permalink] [raw]
Subject: [PATCH 1/2] ext4: mballoc to prefetch groups ahead of scanning

Hi,

Here is a refreshed patch to improve bitmap loading.
This should significantly improve bitmap loading, especially for flex groups as it tries
to load all bitmaps within a flex.group instead of one by one synchronously.

Prefetching is done in 8 * flex_bg groups, so it should be 8 read-ahead
reads for a single allocating thread. At the end of allocation the
thread waits for read-ahead completion and initializes buddy information
so that read-aheads are not lost in case of memory pressure.

At cr=0 the number of prefetching IOs is limited per allocation context
to prevent a situation when mballoc loads thousands of bitmaps looking
for a perfect group and ignoring groups with good chunks.

Together with the patch "ext4: limit scanning of uninitialized groups"
the mount time of a 1PB filesystem is reduced significantly:

0% full 50%-full unpatched patched
mount time 33s 9279s 563s

Lustre-bug-id: https://jira.whamcloud.com/browse/LU-12988
Signed-off-by: Alex Zhuravlev <[email protected]>
Reviewed-by: Andreas Dilger <[email protected]>
---
fs/ext4/balloc.c | 12 +++++-
fs/ext4/ext4.h | 5 ++-
fs/ext4/mballoc.c | 106 +++++++++++++++++++++++++++++++++++++++++++++-
fs/ext4/mballoc.h | 2 +
fs/ext4/sysfs.c | 4 ++
5 files changed, 125 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index a32e5f7b5385..dc6cc8c7b0f8 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -413,7 +413,8 @@ static int ext4_validate_block_bitmap(struct super_block *sb,
* Return buffer_head on success or an ERR_PTR in case of failure.
*/
struct buffer_head *
-ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
+ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group,
+ int ignore_locked)
{
struct ext4_group_desc *desc;
struct ext4_sb_info *sbi = EXT4_SB(sb);
@@ -444,6 +445,13 @@ ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
if (bitmap_uptodate(bh))
goto verify;

+ if (ignore_locked && buffer_locked(bh)) {
+ /* buffer under IO already, do not wait
+ * if called for prefetching */
+ put_bh(bh);
+ return NULL;
+ }
+
lock_buffer(bh);
if (bitmap_uptodate(bh)) {
unlock_buffer(bh);
@@ -534,7 +542,7 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
struct buffer_head *bh;
int err;

- bh = ext4_read_block_bitmap_nowait(sb, block_group);
+ bh = ext4_read_block_bitmap_nowait(sb, block_group, 0);
if (IS_ERR(bh))
return bh;
err = ext4_wait_block_bitmap(sb, block_group, bh);
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 91eb4381cae5..1a4afaecc967 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1483,6 +1483,8 @@ struct ext4_sb_info {
/* where last allocation was done - for stream allocation */
unsigned long s_mb_last_group;
unsigned long s_mb_last_start;
+ unsigned int s_mb_prefetch;
+ unsigned int s_mb_prefetch_limit;

/* stats for buddy allocator */
atomic_t s_bal_reqs; /* number of reqs with len > 1 */
@@ -2420,7 +2422,8 @@ extern struct ext4_group_desc * ext4_get_group_desc(struct super_block * sb,
extern int ext4_should_retry_alloc(struct super_block *sb, int *retries);

extern struct buffer_head *ext4_read_block_bitmap_nowait(struct super_block *sb,
- ext4_group_t block_group);
+ ext4_group_t block_group,
+ int ignore_locked);
extern int ext4_wait_block_bitmap(struct super_block *sb,
ext4_group_t block_group,
struct buffer_head *bh);
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 30d5d97548c4..e84c298e739b 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -861,7 +861,7 @@ static int ext4_mb_init_cache(struct page *page, char *incore, gfp_t gfp)
bh[i] = NULL;
continue;
}
- bh[i] = ext4_read_block_bitmap_nowait(sb, group);
+ bh[i] = ext4_read_block_bitmap_nowait(sb, group, 0);
if (IS_ERR(bh[i])) {
err = PTR_ERR(bh[i]);
bh[i] = NULL;
@@ -2104,6 +2104,87 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
return 0;
}

+/*
+ * each allocation context (i.e. a thread doing allocation) has own
+ * sliding prefetch window of @s_mb_prefetch size which starts at the
+ * very first goal and moves ahead of scaning.
+ * a side effect is that subsequent allocations will likely find
+ * the bitmaps in cache or at least in-flight.
+ */
+static void
+ext4_mb_prefetch(struct ext4_allocation_context *ac,
+ ext4_group_t start)
+{
+ struct super_block *sb = ac->ac_sb;
+ ext4_group_t ngroups = ext4_get_groups_count(sb);
+ struct ext4_sb_info *sbi = EXT4_SB(sb);
+ struct ext4_group_info *grp;
+ ext4_group_t group = start;
+ struct buffer_head *bh;
+ int nr;
+
+ /* limit prefetching at cr=0, otherwise mballoc can
+ * spend a lot of time loading imperfect groups */
+ if (ac->ac_criteria < 2 && ac->ac_prefetch_ios >= sbi->s_mb_prefetch_limit)
+ return;
+
+ /* batch prefetching to get few READs in flight */
+ nr = ac->ac_prefetch - group;
+ if (ac->ac_prefetch < group)
+ /* wrapped to the first groups */
+ nr += ngroups;
+ if (nr > 0)
+ return;
+ BUG_ON(nr < 0);
+
+ nr = sbi->s_mb_prefetch;
+ if (ext4_has_feature_flex_bg(sb)) {
+ /* align to flex_bg to get more bitmas with a single IO */
+ nr = (group / sbi->s_mb_prefetch) * sbi->s_mb_prefetch;
+ nr = nr + sbi->s_mb_prefetch - group;
+ }
+ while (nr-- > 0) {
+ grp = ext4_get_group_info(sb, group);
+ /* ignore empty groups - those will be skipped
+ * during the scanning as well */
+ if (grp->bb_free > 0 && EXT4_MB_GRP_NEED_INIT(grp)) {
+ bh = ext4_read_block_bitmap_nowait(sb, group, 1);
+ if (bh && !IS_ERR(bh)) {
+ if (!buffer_uptodate(bh))
+ ac->ac_prefetch_ios++;
+ brelse(bh);
+ }
+ }
+ if (++group >= ngroups)
+ group = 0;
+ }
+ ac->ac_prefetch = group;
+}
+
+static void
+ext4_mb_prefetch_fini(struct ext4_allocation_context *ac)
+{
+ struct ext4_group_info *grp;
+ ext4_group_t group;
+ int nr, rc;
+
+ /* initialize last window of prefetched groups */
+ nr = ac->ac_prefetch_ios;
+ if (nr > EXT4_SB(ac->ac_sb)->s_mb_prefetch)
+ nr = EXT4_SB(ac->ac_sb)->s_mb_prefetch;
+ group = ac->ac_prefetch;
+ while (nr-- > 0) {
+ grp = ext4_get_group_info(ac->ac_sb, group);
+ if (grp->bb_free > 0 && EXT4_MB_GRP_NEED_INIT(grp)) {
+ rc = ext4_mb_init_group(ac->ac_sb, group, GFP_NOFS);
+ if (rc)
+ break;
+ }
+ if (group-- == 0)
+ group = ext4_get_groups_count(ac->ac_sb) - 1;
+ }
+}
+
static noinline_for_stack int
ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
{
@@ -2177,6 +2258,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
* from the goal value specified
*/
group = ac->ac_g_ex.fe_group;
+ ac->ac_prefetch = group;

for (i = 0; i < ngroups; group++, i++) {
int ret = 0;
@@ -2188,6 +2270,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
if (group >= ngroups)
group = 0;

+ ext4_mb_prefetch(ac, group);
+
/* This now checks without needing the buddy page */
ret = ext4_mb_good_group(ac, group, cr);
if (ret <= 0) {
@@ -2260,6 +2344,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
out:
if (!err && ac->ac_status != AC_STATUS_FOUND && first_err)
err = first_err;
+ /* use prefetched bitmaps to init buddy so that read info is not lost */
+ ext4_mb_prefetch_fini(ac);
return err;
}

@@ -2776,6 +2862,24 @@ int ext4_mb_release(struct super_block *sb)
atomic_read(&sbi->s_mb_preallocated),
atomic_read(&sbi->s_mb_discarded));
}
+ if (ext4_has_feature_flex_bg(sb)) {
+ /* a single flex group is supposed to be read by a single IO */
+ sbi->s_mb_prefetch = 1 << sbi->s_es->s_log_groups_per_flex;
+ sbi->s_mb_prefetch *= 8; /* 8 prefetch IOs in flight at most */
+ } else {
+ sbi->s_mb_prefetch = 32;
+ }
+ if (sbi->s_mb_prefetch > ext4_get_groups_count(sb))
+ sbi->s_mb_prefetch = ext4_get_groups_count(sb);
+ /* now many real IOs to prefetch within a single allocation at cr=0
+ * given cr=0 is an CPU-related optimization we shouldn't try to
+ * load too many groups, at some point we should start to use what
+ * we've got in memory.
+ * with an average random access time 5ms, it'd take a second to get
+ * 200 groups (* N with flex_bg), so let's make this limit 4 */
+ sbi->s_mb_prefetch_limit = sbi->s_mb_prefetch * 4;
+ if (sbi->s_mb_prefetch_limit > ext4_get_groups_count(sb))
+ sbi->s_mb_prefetch_limit = ext4_get_groups_count(sb);

free_percpu(sbi->s_locality_groups);

diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index 88c98f17e3d9..c96a2bd81f72 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h
@@ -175,6 +175,8 @@ struct ext4_allocation_context {
struct page *ac_buddy_page;
struct ext4_prealloc_space *ac_pa;
struct ext4_locality_group *ac_lg;
+ ext4_group_t ac_prefetch;
+ int ac_prefetch_ios; /* number of initialied prefetch IO */
};

#define AC_STATUS_CONTINUE 1
diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
index 04bfaf63752c..5f443f9d54b8 100644
--- a/fs/ext4/sysfs.c
+++ b/fs/ext4/sysfs.c
@@ -240,6 +240,8 @@ EXT4_RO_ATTR_ES_STRING(last_error_func, s_last_error_func, 32);
EXT4_ATTR(first_error_time, 0444, first_error_time);
EXT4_ATTR(last_error_time, 0444, last_error_time);
EXT4_ATTR(journal_task, 0444, journal_task);
+EXT4_RW_ATTR_SBI_UI(mb_prefetch, s_mb_prefetch);
+EXT4_RW_ATTR_SBI_UI(mb_prefetch_limit, s_mb_prefetch_limit);

static unsigned int old_bump_val = 128;
EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
@@ -283,6 +285,8 @@ static struct attribute *ext4_attrs[] = {
#ifdef CONFIG_EXT4_DEBUG
ATTR_LIST(simulate_fail),
#endif
+ ATTR_LIST(mb_prefetch),
+ ATTR_LIST(mb_prefetch_limit),
NULL,
};
ATTRIBUTE_GROUPS(ext4);
--