I ran some experiments this morning and here are the results:
1) Config 1:
- add change to skip CR0 / CR1 optimizations and fall back to
linear search when all groups are not loaded
- prefetch block bitmaps mount option turned off
- first_write_latency: 6m 56s
- number_of_groups_loaded_at_the_end_of_first_write: 30726 out of
491520 total
2) Config 2:
- add change to skip CR0 / CR1 optimizations and fall back to
linear search when all groups are not loaded
- prefetch block bitmaps mount option turned on
- first_write_latency:4m42s
- number_of_groups_loaded_at_the_end_of_first_write: 490497 out
of 491520 total
3) Config 3:
- don't fallback to linear search
- prefetch block bitmaps turned off
- first_write_latency: 0.13s
- number_of_groups_loaded_at_the_end_of_first_write: 7 out of 491520 total
4) Config 4:
- don't fallback to linear search
- prefetch block bitmaps turned on
- first_write_latency: 0.15s
- number_of_groups_loaded_at_the_end_of_first_write: 48384 out of
491520 total
Based on the above numbers, I think we should go for config 4, which
means we should have prefetch_block_bitmaps turned on when
mb_optimize_scan is on. What do you think about turning
prefetch_block_bitmaps mount option on by default?
- Harshad
On Mon, Mar 29, 2021 at 9:10 PM Andreas Dilger <adilger@xxxxxxxxx> wrote:
>
> On Mar 29, 2021, at 17:03, harshad shirwadkar <harshadshirwadkar@xxxxxxxxx> wrote:
> >
> > Another problem that I found in the patch was that when all groups are
> > not loaded, the optimization concludes that CR0 and CR1 cannot be used
> > for allocation (since these structures aren't populated). The problem
> > goes away if we also mount with prefetch_block_bitmaps mount option.
> > But I think if all the groups are not loaded, we should at least fall
> > back to linear search at CR1. Maybe skipping CR0 is okay.
>
> Alex's patch to skip cr0/1 if no groups are loaded is OK, as long as there is
> some group prefetch is triggered in the background, IMHO, and is there
> for later allocations.
>
> That avoids lots of scanning at mount time if the filesystem is mostly full,
> and the first allocation is not critical as long as this triggers group reads.
>
> Do you have any idea what the performance impact of this is (eg.
> latency to first write)? Ideally there would be some group prefetch done
> right at mount time (itable zero thread?) so that if writes are not
> done *right* after mount there would not be any noticeable latency.
>
> Cheers, Andreas
>
> >> On Mon, Mar 29, 2021 at 12:32 PM harshad shirwadkar
> >> <harshadshirwadkar@xxxxxxxxx> wrote:
> >>
> >> Okay sounds good. I'll do that!
> >>
> >>> On Mon, Mar 29, 2021 at 12:14 PM Andreas Dilger <adilger@xxxxxxxxx> wrote:
> >>>
> >>> On Mar 24, 2021, at 5:19 PM, Harshad Shirwadkar <harshadshirwadkar@xxxxxxxxx> wrote:
> >>>>
> >>>> Instead of traversing through groups linearly, scan groups in specific
> >>>> orders at cr 0 and cr 1. At cr 0, we want to find groups that have the
> >>>> largest free order >= the order of the request. So, with this patch,
> >>>> we maintain lists for each possible order and insert each group into a
> >>>> list based on the largest free order in its buddy bitmap. During cr 0
> >>>> allocation, we traverse these lists in the increasing order of largest
> >>>> free orders. This allows us to find a group with the best available cr
> >>>> 0 match in constant time. If nothing can be found, we fallback to cr 1
> >>>> immediately.
> >>>>
> >>>> At CR1, the story is slightly different. We want to traverse in the
> >>>> order of increasing average fragment size. For CR1, we maintain a rb
> >>>> tree of groupinfos which is sorted by average fragment size. Instead
> >>>> of traversing linearly, at CR1, we traverse in the order of increasing
> >>>> average fragment size, starting at the most optimal group. This brings
> >>>> down cr 1 search complexity to log(num groups).
> >>>>
> >>>> For cr >= 2, we just perform the linear search as before. Also, in
> >>>> case of lock contention, we intermittently fallback to linear search
> >>>> even in CR 0 and CR 1 cases. This allows us to proceed during the
> >>>> allocation path even in case of high contention.
> >>>
> >>> If you are refreshing this patch anyway, could you rename "mb_linear_limit"
> >>> to "mb_linear_groups" or "mb_max_linear_groups" or similar? It otherwise
> >>> isn't clear what units the "limit" is in (could be MB/GB or something else).
> >>>
> >>> Cheers, Andreas
> >>>
> >>>> There is an opportunity to do optimization at CR2 too. That's because
> >>>> at CR2 we only consider groups where bb_free counter (number of free
> >>>> blocks) is greater than the request extent size. That's left as future
> >>>> work.
> >>>>
> >>>> All the changes introduced in this patch are protected under a new
> >>>> mount option "mb_optimize_scan".
> >>>>
> >>>> With this patchset, following experiment was performed:
> >>>>
> >>>> Created a highly fragmented disk of size 65TB. The disk had no
> >>>> contiguous 2M regions. Following command was run consecutively for 3
> >>>> times:
> >>>>
> >>>> time dd if=/dev/urandom of=file bs=2M count=10
> >>>>
> >>>> Here are the results with and without cr 0/1 optimizations introduced
> >>>> in this patch:
> >>>>
> >>>> |---------+------------------------------+---------------------------|
> >>>> | | Without CR 0/1 Optimizations | With CR 0/1 Optimizations |
> >>>> |---------+------------------------------+---------------------------|
> >>>> | 1st run | 5m1.871s | 2m47.642s |
> >>>> | 2nd run | 2m28.390s | 0m0.611s |
> >>>> | 3rd run | 2m26.530s | 0m1.255s |
> >>>> |---------+------------------------------+---------------------------|
> >>>>
> >>>> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@xxxxxxxxx>
> >>>> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> >>>> Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> >>>> Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>
> >>>> ---
> >>>> fs/ext4/ext4.h | 19 ++-
> >>>> fs/ext4/mballoc.c | 381 ++++++++++++++++++++++++++++++++++++++++++++--
> >>>> fs/ext4/mballoc.h | 17 ++-
> >>>> fs/ext4/super.c | 28 +++-
> >>>> fs/ext4/sysfs.c | 2 +
> >>>> 5 files changed, 432 insertions(+), 15 deletions(-)
> >>>>
> >>>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> >>>> index 85eeeba3bca3..5930c8cb5159 100644
> >>>> --- a/fs/ext4/ext4.h
> >>>> +++ b/fs/ext4/ext4.h
> >>>> @@ -162,7 +162,10 @@ enum SHIFT_DIRECTION {
> >>>> #define EXT4_MB_USE_RESERVED 0x2000
> >>>> /* Do strict check for free blocks while retrying block allocation */
> >>>> #define EXT4_MB_STRICT_CHECK 0x4000
> >>>> -
> >>>> +/* Large fragment size list lookup succeeded at least once for cr = 0 */
> >>>> +#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
> >>>> struct ext4_allocation_request {
> >>>> /* target inode for block we're allocating */
> >>>> struct inode *inode;
> >>>> @@ -1247,7 +1250,9 @@ struct ext4_inode_info {
> >>>> #define EXT4_MOUNT2_JOURNAL_FAST_COMMIT 0x00000010 /* Journal fast commit */
> >>>> #define EXT4_MOUNT2_DAX_NEVER 0x00000020 /* Do not allow Direct Access */
> >>>> #define EXT4_MOUNT2_DAX_INODE 0x00000040 /* For printing options only */
> >>>> -
> >>>> +#define EXT4_MOUNT2_MB_OPTIMIZE_SCAN 0x00000080 /* Optimize group
> >>>> + * scanning in mballoc
> >>>> + */
> >>>>
> >>>> #define clear_opt(sb, opt) EXT4_SB(sb)->s_mount_opt &= \
> >>>> ~EXT4_MOUNT_##opt
> >>>> @@ -1527,9 +1532,14 @@ struct ext4_sb_info {
> >>>> unsigned int s_mb_free_pending;
> >>>> struct list_head s_freed_data_list; /* List of blocks to be freed
> >>>> after commit completed */
> >>>> + struct rb_root s_mb_avg_fragment_size_root;
> >>>> + rwlock_t s_mb_rb_lock;
> >>>> + struct list_head *s_mb_largest_free_orders;
> >>>> + rwlock_t *s_mb_largest_free_orders_locks;
> >>>>
> >>>> /* tunables */
> >>>> unsigned long s_stripe;
> >>>> + unsigned int s_mb_linear_limit;
> >>>> unsigned int s_mb_stream_request;
> >>>> unsigned int s_mb_max_to_scan;
> >>>> unsigned int s_mb_min_to_scan;
> >>>> @@ -1553,6 +1563,8 @@ struct ext4_sb_info {
> >>>> atomic_t s_bal_goals; /* goal hits */
> >>>> atomic_t s_bal_breaks; /* too long searches */
> >>>> atomic_t s_bal_2orders; /* 2^order hits */
> >>>> + atomic_t s_bal_cr0_bad_suggestions;
> >>>> + atomic_t s_bal_cr1_bad_suggestions;
> >>>> atomic64_t s_bal_cX_groups_considered[4];
> >>>> atomic64_t s_bal_cX_hits[4];
> >>>> atomic64_t s_bal_cX_failed[4]; /* cX loop didn't find blocks */
> >>>> @@ -3309,11 +3321,14 @@ struct ext4_group_info {
> >>>> ext4_grpblk_t bb_free; /* total free blocks */
> >>>> ext4_grpblk_t bb_fragments; /* nr of freespace fragments */
> >>>> ext4_grpblk_t bb_largest_free_order;/* order of largest frag in BG */
> >>>> + ext4_group_t bb_group; /* Group number */
> >>>> struct list_head bb_prealloc_list;
> >>>> #ifdef DOUBLE_CHECK
> >>>> void *bb_bitmap;
> >>>> #endif
> >>>> struct rw_semaphore alloc_sem;
> >>>> + struct rb_node bb_avg_fragment_size_rb;
> >>>> + struct list_head bb_largest_free_order_node;
> >>>> ext4_grpblk_t bb_counters[]; /* Nr of free power-of-two-block
> >>>> * regions, index is order.
> >>>> * bb_counters[3] = 5 means
> >>>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> >>>> index 15127d815461..cbf9a89c0ef5 100644
> >>>> --- a/fs/ext4/mballoc.c
> >>>> +++ b/fs/ext4/mballoc.c
> >>>> @@ -127,11 +127,50 @@
> >>>> * smallest multiple of the stripe value (sbi->s_stripe) which is
> >>>> * greater than the default mb_group_prealloc.
> >>>> *
> >>>> + * If "mb_optimize_scan" mount option is set, we maintain in memory group info
> >>>> + * structures in two data structures:
> >>>> + *
> >>>> + * 1) Array of largest free order lists (sbi->s_mb_largest_free_orders)
> >>>> + *
> >>>> + * Locking: sbi->s_mb_largest_free_orders_locks(array of rw locks)
> >>>> + *
> >>>> + * This is an array of lists where the index in the array represents the
> >>>> + * largest free order in the buddy bitmap of the participating group infos of
> >>>> + * that list. So, there are exactly MB_NUM_ORDERS(sb) (which means total
> >>>> + * number of buddy bitmap orders possible) number of lists. Group-infos are
> >>>> + * placed in appropriate lists.
> >>>> + *
> >>>> + * 2) Average fragment size rb tree (sbi->s_mb_avg_fragment_size_root)
> >>>> + *
> >>>> + * Locking: sbi->s_mb_rb_lock (rwlock)
> >>>> + *
> >>>> + * This is a red black tree consisting of group infos and the tree is sorted
> >>>> + * by average fragment sizes (which is calculated as ext4_group_info->bb_free
> >>>> + * / ext4_group_info->bb_fragments).
> >>>> + *
> >>>> + * When "mb_optimize_scan" mount option is set, mballoc consults the above data
> >>>> + * structures to decide the order in which groups are to be traversed for
> >>>> + * fulfilling an allocation request.
> >>>> + *
> >>>> + * At CR = 0, we look for groups which have the largest_free_order >= the order
> >>>> + * of the request. We directly look at the largest free order list in the data
> >>>> + * structure (1) above where largest_free_order = order of the request. If that
> >>>> + * list is empty, we look at remaining list in the increasing order of
> >>>> + * largest_free_order. This allows us to perform CR = 0 lookup in O(1) time.
> >>>> + *
> >>>> + * At CR = 1, we only consider groups where average fragment size > request
> >>>> + * size. So, we lookup a group which has average fragment size just above or
> >>>> + * equal to request size using our rb tree (data structure 2) in O(log N) time.
> >>>> + *
> >>>> + * If "mb_optimize_scan" mount option is not set, mballoc traverses groups in
> >>>> + * linear order which requires O(N) search time for each CR 0 and CR 1 phase.
> >>>> + *
> >>>> * The regular allocator (using the buddy cache) supports a few tunables.
> >>>> *
> >>>> * /sys/fs/ext4/<partition>/mb_min_to_scan
> >>>> * /sys/fs/ext4/<partition>/mb_max_to_scan
> >>>> * /sys/fs/ext4/<partition>/mb_order2_req
> >>>> + * /sys/fs/ext4/<partition>/mb_linear_limit
> >>>> *
> >>>> * The regular allocator uses buddy scan only if the request len is power of
> >>>> * 2 blocks and the order of allocation is >= sbi->s_mb_order2_reqs. The
> >>>> @@ -149,6 +188,16 @@
> >>>> * can be used for allocation. ext4_mb_good_group explains how the groups are
> >>>> * checked.
> >>>> *
> >>>> + * When "mb_optimize_scan" is turned on, as mentioned above, the groups may not
> >>>> + * get traversed linearly. That may result in subsequent allocations being not
> >>>> + * close to each other. And so, the underlying device may get filled up in a
> >>>> + * non-linear fashion. While that may not matter on non-rotational devices, for
> >>>> + * rotational devices that may result in higher seek times. "mb_linear_limit"
> >>>> + * tells mballoc how many groups mballoc should search linearly before
> >>>> + * performing consulting above data structures for more efficient lookups. For
> >>>> + * non rotational devices, this value defaults to 0 and for rotational devices
> >>>> + * this is set to MB_DEFAULT_LINEAR_LIMIT.
> >>>> + *
> >>>> * Both the prealloc space are getting populated as above. So for the first
> >>>> * request we will hit the buddy cache which will result in this prealloc
> >>>> * space getting filled. The prealloc space is then later used for the
> >>>> @@ -299,6 +348,8 @@
> >>>> * - bitlock on a group (group)
> >>>> * - object (inode/locality) (object)
> >>>> * - per-pa lock (pa)
> >>>> + * - cr0 lists lock (cr0)
> >>>> + * - cr1 tree lock (cr1)
> >>>> *
> >>>> * Paths:
> >>>> * - new pa
> >>>> @@ -328,6 +379,9 @@
> >>>> * group
> >>>> * object
> >>>> *
> >>>> + * - allocation path (ext4_mb_regular_allocator)
> >>>> + * group
> >>>> + * cr0/cr1
> >>>> */
> >>>> static struct kmem_cache *ext4_pspace_cachep;
> >>>> static struct kmem_cache *ext4_ac_cachep;
> >>>> @@ -351,6 +405,9 @@ static void ext4_mb_generate_from_freelist(struct super_block *sb, void *bitmap,
> >>>> ext4_group_t group);
> >>>> static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac);
> >>>>
> >>>> +static bool ext4_mb_good_group(struct ext4_allocation_context *ac,
> >>>> + ext4_group_t group, int cr);
> >>>> +
> >>>> /*
> >>>> * The algorithm using this percpu seq counter goes below:
> >>>> * 1. We sample the percpu discard_pa_seq counter before trying for block
> >>>> @@ -744,6 +801,251 @@ static void ext4_mb_mark_free_simple(struct super_block *sb,
> >>>> }
> >>>> }
> >>>>
> >>>> +static void ext4_mb_rb_insert(struct rb_root *root, struct rb_node *new,
> >>>> + int (*cmp)(struct rb_node *, struct rb_node *))
> >>>> +{
> >>>> + struct rb_node **iter = &root->rb_node, *parent = NULL;
> >>>> +
> >>>> + while (*iter) {
> >>>> + parent = *iter;
> >>>> + if (cmp(new, *iter) > 0)
> >>>> + iter = &((*iter)->rb_left);
> >>>> + else
> >>>> + iter = &((*iter)->rb_right);
> >>>> + }
> >>>> +
> >>>> + rb_link_node(new, parent, iter);
> >>>> + rb_insert_color(new, root);
> >>>> +}
> >>>> +
> >>>> +static int
> >>>> +ext4_mb_avg_fragment_size_cmp(struct rb_node *rb1, struct rb_node *rb2)
> >>>> +{
> >>>> + struct ext4_group_info *grp1 = rb_entry(rb1,
> >>>> + struct ext4_group_info,
> >>>> + bb_avg_fragment_size_rb);
> >>>> + struct ext4_group_info *grp2 = rb_entry(rb2,
> >>>> + struct ext4_group_info,
> >>>> + bb_avg_fragment_size_rb);
> >>>> + int num_frags_1, num_frags_2;
> >>>> +
> >>>> + num_frags_1 = grp1->bb_fragments ?
> >>>> + grp1->bb_free / grp1->bb_fragments : 0;
> >>>> + num_frags_2 = grp2->bb_fragments ?
> >>>> + grp2->bb_free / grp2->bb_fragments : 0;
> >>>> +
> >>>> + return (num_frags_2 - num_frags_1);
> >>>> +}
> >>>> +
> >>>> +/*
> >>>> + * Reinsert grpinfo into the avg_fragment_size tree with new average
> >>>> + * fragment size.
> >>>> + */
> >>>> +static void
> >>>> +mb_update_avg_fragment_size(struct super_block *sb, struct ext4_group_info *grp)
> >>>> +{
> >>>> + struct ext4_sb_info *sbi = EXT4_SB(sb);
> >>>> +
> >>>> + if (!test_opt2(sb, MB_OPTIMIZE_SCAN) || grp->bb_free == 0)
> >>>> + return;
> >>>> +
> >>>> + write_lock(&sbi->s_mb_rb_lock);
> >>>> + if (!RB_EMPTY_NODE(&grp->bb_avg_fragment_size_rb)) {
> >>>> + rb_erase(&grp->bb_avg_fragment_size_rb,
> >>>> + &sbi->s_mb_avg_fragment_size_root);
> >>>> + RB_CLEAR_NODE(&grp->bb_avg_fragment_size_rb);
> >>>> + }
> >>>> +
> >>>> + ext4_mb_rb_insert(&sbi->s_mb_avg_fragment_size_root,
> >>>> + &grp->bb_avg_fragment_size_rb,
> >>>> + ext4_mb_avg_fragment_size_cmp);
> >>>> + write_unlock(&sbi->s_mb_rb_lock);
> >>>> +}
> >>>> +
> >>>> +/*
> >>>> + * Choose next group by traversing largest_free_order lists. Return 0 if next
> >>>> + * group was selected optimally. Return 1 if next group was not selected
> >>>> + * optimally. Updates *new_cr if cr level needs an update.
> >>>> + */
> >>>> +static int ext4_mb_choose_next_group_cr0(struct ext4_allocation_context *ac,
> >>>> + int *new_cr, ext4_group_t *group, ext4_group_t ngroups)
> >>>> +{
> >>>> + struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> >>>> + struct ext4_group_info *iter, *grp;
> >>>> + int i;
> >>>> +
> >>>> + if (ac->ac_status == AC_STATUS_FOUND)
> >>>> + return 1;
> >>>> +
> >>>> + if (unlikely(sbi->s_mb_stats && ac->ac_flags & EXT4_MB_CR0_OPTIMIZED))
> >>>> + atomic_inc(&sbi->s_bal_cr0_bad_suggestions);
> >>>> +
> >>>> + grp = NULL;
> >>>> + for (i = ac->ac_2order; i < MB_NUM_ORDERS(ac->ac_sb); i++) {
> >>>> + if (list_empty(&sbi->s_mb_largest_free_orders[i]))
> >>>> + continue;
> >>>> + read_lock(&sbi->s_mb_largest_free_orders_locks[i]);
> >>>> + if (list_empty(&sbi->s_mb_largest_free_orders[i])) {
> >>>> + read_unlock(&sbi->s_mb_largest_free_orders_locks[i]);
> >>>> + continue;
> >>>> + }
> >>>> + grp = NULL;
> >>>> + list_for_each_entry(iter, &sbi->s_mb_largest_free_orders[i],
> >>>> + bb_largest_free_order_node) {
> >>>> + if (sbi->s_mb_stats)
> >>>> + atomic64_inc(&sbi->s_bal_cX_groups_considered[0]);
> >>>> + if (likely(ext4_mb_good_group(ac, iter->bb_group, 0))) {
> >>>> + grp = iter;
> >>>> + break;
> >>>> + }
> >>>> + }
> >>>> + read_unlock(&sbi->s_mb_largest_free_orders_locks[i]);
> >>>> + if (grp)
> >>>> + break;
> >>>> + }
> >>>> +
> >>>> + if (!grp) {
> >>>> + /* Increment cr and search again */
> >>>> + *new_cr = 1;
> >>>> + } else {
> >>>> + *group = grp->bb_group;
> >>>> + ac->ac_last_optimal_group = *group;
> >>>> + ac->ac_flags |= EXT4_MB_CR0_OPTIMIZED;
> >>>> + }
> >>>> + return 0;
> >>>> +}
> >>>> +
> >>>> +/*
> >>>> + * Choose next group by traversing average fragment size tree. Return 0 if next
> >>>> + * group was selected optimally. Return 1 if next group could not selected
> >>>> + * optimally (due to lock contention). Updates *new_cr if cr lvel needs an
> >>>> + * update.
> >>>> + */
> >>>> +static int ext4_mb_choose_next_group_cr1(struct ext4_allocation_context *ac,
> >>>> + int *new_cr, ext4_group_t *group, ext4_group_t ngroups)
> >>>> +{
> >>>> + struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> >>>> + int avg_fragment_size, best_so_far;
> >>>> + struct rb_node *node, *found;
> >>>> + struct ext4_group_info *grp;
> >>>> +
> >>>> + /*
> >>>> + * If there is contention on the lock, instead of waiting for the lock
> >>>> + * to become available, just continue searching lineraly. We'll resume
> >>>> + * our rb tree search later starting at ac->ac_last_optimal_group.
> >>>> + */
> >>>> + if (!read_trylock(&sbi->s_mb_rb_lock))
> >>>> + return 1;
> >>>> +
> >>>> + if (unlikely(ac->ac_flags & EXT4_MB_CR1_OPTIMIZED)) {
> >>>> + if (sbi->s_mb_stats)
> >>>> + atomic_inc(&sbi->s_bal_cr1_bad_suggestions);
> >>>> + /* We have found something at CR 1 in the past */
> >>>> + grp = ext4_get_group_info(ac->ac_sb, ac->ac_last_optimal_group);
> >>>> + for (found = rb_next(&grp->bb_avg_fragment_size_rb); found != NULL;
> >>>> + found = rb_next(found)) {
> >>>> + grp = rb_entry(found, struct ext4_group_info,
> >>>> + bb_avg_fragment_size_rb);
> >>>> + if (sbi->s_mb_stats)
> >>>> + atomic64_inc(&sbi->s_bal_cX_groups_considered[1]);
> >>>> + if (likely(ext4_mb_good_group(ac, grp->bb_group, 1)))
> >>>> + break;
> >>>> + }
> >>>> +
> >>>> + goto done;
> >>>> + }
> >>>> +
> >>>> + node = sbi->s_mb_avg_fragment_size_root.rb_node;
> >>>> + best_so_far = 0;
> >>>> + found = NULL;
> >>>> +
> >>>> + while (node) {
> >>>> + grp = rb_entry(node, struct ext4_group_info,
> >>>> + bb_avg_fragment_size_rb);
> >>>> + avg_fragment_size = 0;
> >>>> + /*
> >>>> + * Perform this check without locking, we'll lock later to confirm.
> >>>> + */
> >>>> + if (ext4_mb_good_group(ac, grp->bb_group, 1)) {
> >>>> + avg_fragment_size = grp->bb_fragments ?
> >>>> + grp->bb_free / grp->bb_fragments : 0;
> >>>> + if (!best_so_far || avg_fragment_size < best_so_far) {
> >>>> + best_so_far = avg_fragment_size;
> >>>> + found = node;
> >>>> + }
> >>>> + }
> >>>> + if (avg_fragment_size > ac->ac_g_ex.fe_len)
> >>>> + node = node->rb_right;
> >>>> + else
> >>>> + node = node->rb_left;
> >>>> + }
> >>>> +
> >>>> +done:
> >>>> + if (found) {
> >>>> + grp = rb_entry(found, struct ext4_group_info,
> >>>> + bb_avg_fragment_size_rb);
> >>>> + *group = grp->bb_group;
> >>>> + ac->ac_flags |= EXT4_MB_CR1_OPTIMIZED;
> >>>> + } else {
> >>>> + *new_cr = 2;
> >>>> + }
> >>>> +
> >>>> + read_unlock(&sbi->s_mb_rb_lock);
> >>>> + ac->ac_last_optimal_group = *group;
> >>>> + return 0;
> >>>> +}
> >>>> +
> >>>> +/*
> >>>> + * ext4_mb_choose_next_group: choose next group for allocation.
> >>>> + *
> >>>> + * @ac Allocation Context
> >>>> + * @new_cr This is an output parameter. If the there is no good group
> >>>> + * available at current CR level, this field is updated to indicate
> >>>> + * the new cr level that should be used.
> >>>> + * @group This is an input / output parameter. As an input it indicates the
> >>>> + * last group used for allocation. As output, this field indicates
> >>>> + * the next group that should be used.
> >>>> + * @ngroups Total number of groups
> >>>> + */
> >>>> +static void ext4_mb_choose_next_group(struct ext4_allocation_context *ac,
> >>>> + int *new_cr, ext4_group_t *group, ext4_group_t ngroups)
> >>>> +{
> >>>> + int ret;
> >>>> +
> >>>> + *new_cr = ac->ac_criteria;
> >>>> +
> >>>> + if (!test_opt2(ac->ac_sb, MB_OPTIMIZE_SCAN) ||
> >>>> + *new_cr >= 2 ||
> >>>> + !ext4_test_inode_flag(ac->ac_inode, EXT4_INODE_EXTENTS))
> >>>> + goto inc_and_return;
> >>>> +
> >>>> + if (ac->ac_groups_linear_remaining) {
> >>>> + ac->ac_groups_linear_remaining--;
> >>>> + goto inc_and_return;
> >>>> + }
> >>>> +
> >>>> + if (*new_cr == 0) {
> >>>> + ret = ext4_mb_choose_next_group_cr0(ac, new_cr, group, ngroups);
> >>>> + if (ret)
> >>>> + goto inc_and_return;
> >>>> + }
> >>>> + if (*new_cr == 1) {
> >>>> + ret = ext4_mb_choose_next_group_cr1(ac, new_cr, group, ngroups);
> >>>> + if (ret)
> >>>> + goto inc_and_return;
> >>>> + }
> >>>> + return;
> >>>> +
> >>>> +inc_and_return:
> >>>> + /*
> >>>> + * Artificially restricted ngroups for non-extent
> >>>> + * files makes group > ngroups possible on first loop.
> >>>> + */
> >>>> + *group = *group + 1;
> >>>> + if (*group >= ngroups)
> >>>> + *group = 0;
> >>>> +}
> >>>> +
> >>>> /*
> >>>> * Cache the order of the largest free extent we have available in this block
> >>>> * group.
> >>>> @@ -751,18 +1053,33 @@ static void ext4_mb_mark_free_simple(struct super_block *sb,
> >>>> static void
> >>>> mb_set_largest_free_order(struct super_block *sb, struct ext4_group_info *grp)
> >>>> {
> >>>> + struct ext4_sb_info *sbi = EXT4_SB(sb);
> >>>> int i;
> >>>> - int bits;
> >>>>
> >>>> + if (test_opt2(sb, MB_OPTIMIZE_SCAN) && grp->bb_largest_free_order >= 0) {
> >>>> + write_lock(&sbi->s_mb_largest_free_orders_locks[
> >>>> + grp->bb_largest_free_order]);
> >>>> + list_del_init(&grp->bb_largest_free_order_node);
> >>>> + write_unlock(&sbi->s_mb_largest_free_orders_locks[
> >>>> + grp->bb_largest_free_order]);
> >>>> + }
> >>>> grp->bb_largest_free_order = -1; /* uninit */
> >>>>
> >>>> - bits = MB_NUM_ORDERS(sb) - 1;
> >>>> - for (i = bits; i >= 0; i--) {
> >>>> + for (i = MB_NUM_ORDERS(sb) - 1; i >= 0; i--) {
> >>>> if (grp->bb_counters[i] > 0) {
> >>>> grp->bb_largest_free_order = i;
> >>>> break;
> >>>> }
> >>>> }
> >>>> + if (test_opt2(sb, MB_OPTIMIZE_SCAN) &&
> >>>> + grp->bb_largest_free_order >= 0 && grp->bb_free) {
> >>>> + write_lock(&sbi->s_mb_largest_free_orders_locks[
> >>>> + grp->bb_largest_free_order]);
> >>>> + list_add_tail(&grp->bb_largest_free_order_node,
> >>>> + &sbi->s_mb_largest_free_orders[grp->bb_largest_free_order]);
> >>>> + write_unlock(&sbi->s_mb_largest_free_orders_locks[
> >>>> + grp->bb_largest_free_order]);
> >>>> + }
> >>>> }
> >>>>
> >>>> static noinline_for_stack
> >>>> @@ -818,6 +1135,7 @@ void ext4_mb_generate_buddy(struct super_block *sb,
> >>>> period = get_cycles() - period;
> >>>> atomic_inc(&sbi->s_mb_buddies_generated);
> >>>> atomic64_add(period, &sbi->s_mb_generation_time);
> >>>> + mb_update_avg_fragment_size(sb, grp);
> >>>> }
> >>>>
> >>>> /* The buddy information is attached the buddy cache inode
> >>>> @@ -1517,6 +1835,7 @@ static void mb_free_blocks(struct inode *inode, struct ext4_buddy *e4b,
> >>>>
> >>>> done:
> >>>> mb_set_largest_free_order(sb, e4b->bd_info);
> >>>> + mb_update_avg_fragment_size(sb, e4b->bd_info);
> >>>> mb_check_buddy(e4b);
> >>>> }
> >>>>
> >>>> @@ -1653,6 +1972,7 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
> >>>> }
> >>>> mb_set_largest_free_order(e4b->bd_sb, e4b->bd_info);
> >>>>
> >>>> + mb_update_avg_fragment_size(e4b->bd_sb, e4b->bd_info);
> >>>> ext4_set_bits(e4b->bd_bitmap, ex->fe_start, len0);
> >>>> mb_check_buddy(e4b);
> >>>>
> >>>> @@ -2347,17 +2667,21 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> >>>> * from the goal value specified
> >>>> */
> >>>> group = ac->ac_g_ex.fe_group;
> >>>> + ac->ac_last_optimal_group = group;
> >>>> + ac->ac_groups_linear_remaining = sbi->s_mb_linear_limit;
> >>>> prefetch_grp = group;
> >>>>
> >>>> - for (i = 0; i < ngroups; group++, i++) {
> >>>> - int ret = 0;
> >>>> + for (i = 0; i < ngroups; i++) {
> >>>> + int ret = 0, new_cr;
> >>>> +
> >>>> cond_resched();
> >>>> - /*
> >>>> - * Artificially restricted ngroups for non-extent
> >>>> - * files makes group > ngroups possible on first loop.
> >>>> - */
> >>>> - if (group >= ngroups)
> >>>> - group = 0;
> >>>> +
> >>>> + ext4_mb_choose_next_group(ac, &new_cr, &group, ngroups);
> >>>> +
> >>>> + if (new_cr != cr) {
> >>>> + cr = new_cr;
> >>>> + goto repeat;
> >>>> + }
> >>>>
> >>>> /*
> >>>> * Batch reads of the block allocation bitmaps
> >>>> @@ -2578,6 +2902,8 @@ int ext4_seq_mb_stats_show(struct seq_file *seq, void *offset)
> >>>> atomic64_read(&sbi->s_bal_cX_groups_considered[0]));
> >>>> seq_printf(seq, "\t\tuseless_loops: %llu\n",
> >>>> atomic64_read(&sbi->s_bal_cX_failed[0]));
> >>>> + seq_printf(seq, "\t\tbad_suggestions: %u\n",
> >>>> + atomic_read(&sbi->s_bal_cr0_bad_suggestions));
> >>>>
> >>>> seq_puts(seq, "\tcr1_stats:\n");
> >>>> seq_printf(seq, "\t\thits: %llu\n", atomic64_read(&sbi->s_bal_cX_hits[1]));
> >>>> @@ -2585,6 +2911,8 @@ int ext4_seq_mb_stats_show(struct seq_file *seq, void *offset)
> >>>> atomic64_read(&sbi->s_bal_cX_groups_considered[1]));
> >>>> seq_printf(seq, "\t\tuseless_loops: %llu\n",
> >>>> atomic64_read(&sbi->s_bal_cX_failed[1]));
> >>>> + seq_printf(seq, "\t\tbad_suggestions: %u\n",
> >>>> + atomic_read(&sbi->s_bal_cr1_bad_suggestions));
> >>>>
> >>>> seq_puts(seq, "\tcr2_stats:\n");
> >>>> seq_printf(seq, "\t\thits: %llu\n", atomic64_read(&sbi->s_bal_cX_hits[2]));
> >>>> @@ -2719,7 +3047,10 @@ int ext4_mb_add_groupinfo(struct super_block *sb, ext4_group_t group,
> >>>> INIT_LIST_HEAD(&meta_group_info[i]->bb_prealloc_list);
> >>>> init_rwsem(&meta_group_info[i]->alloc_sem);
> >>>> meta_group_info[i]->bb_free_root = RB_ROOT;
> >>>> + INIT_LIST_HEAD(&meta_group_info[i]->bb_largest_free_order_node);
> >>>> + RB_CLEAR_NODE(&meta_group_info[i]->bb_avg_fragment_size_rb);
> >>>> meta_group_info[i]->bb_largest_free_order = -1; /* uninit */
> >>>> + meta_group_info[i]->bb_group = group;
> >>>>
> >>>> mb_group_bb_bitmap_alloc(sb, meta_group_info[i], group);
> >>>> return 0;
> >>>> @@ -2909,6 +3240,26 @@ int ext4_mb_init(struct super_block *sb)
> >>>> i++;
> >>>> } while (i < MB_NUM_ORDERS(sb));
> >>>>
> >>>> + sbi->s_mb_avg_fragment_size_root = RB_ROOT;
> >>>> + sbi->s_mb_largest_free_orders =
> >>>> + kmalloc_array(MB_NUM_ORDERS(sb), sizeof(struct list_head),
> >>>> + GFP_KERNEL);
> >>>> + if (!sbi->s_mb_largest_free_orders) {
> >>>> + ret = -ENOMEM;
> >>>> + goto out;
> >>>> + }
> >>>> + sbi->s_mb_largest_free_orders_locks =
> >>>> + kmalloc_array(MB_NUM_ORDERS(sb), sizeof(rwlock_t),
> >>>> + GFP_KERNEL);
> >>>> + if (!sbi->s_mb_largest_free_orders_locks) {
> >>>> + ret = -ENOMEM;
> >>>> + goto out;
> >>>> + }
> >>>> + for (i = 0; i < MB_NUM_ORDERS(sb); i++) {
> >>>> + INIT_LIST_HEAD(&sbi->s_mb_largest_free_orders[i]);
> >>>> + rwlock_init(&sbi->s_mb_largest_free_orders_locks[i]);
> >>>> + }
> >>>> + rwlock_init(&sbi->s_mb_rb_lock);
> >>>>
> >>>> spin_lock_init(&sbi->s_md_lock);
> >>>> sbi->s_mb_free_pending = 0;
> >>>> @@ -2961,6 +3312,10 @@ int ext4_mb_init(struct super_block *sb)
> >>>> spin_lock_init(&lg->lg_prealloc_lock);
> >>>> }
> >>>>
> >>>> + if (blk_queue_nonrot(bdev_get_queue(sb->s_bdev)))
> >>>> + sbi->s_mb_linear_limit = 0;
> >>>> + else
> >>>> + sbi->s_mb_linear_limit = MB_DEFAULT_LINEAR_LIMIT;
> >>>> /* init file for buddy data */
> >>>> ret = ext4_mb_init_backend(sb);
> >>>> if (ret != 0)
> >>>> @@ -2972,6 +3327,8 @@ int ext4_mb_init(struct super_block *sb)
> >>>> free_percpu(sbi->s_locality_groups);
> >>>> sbi->s_locality_groups = NULL;
> >>>> out:
> >>>> + kfree(sbi->s_mb_largest_free_orders);
> >>>> + kfree(sbi->s_mb_largest_free_orders_locks);
> >>>> kfree(sbi->s_mb_offsets);
> >>>> sbi->s_mb_offsets = NULL;
> >>>> kfree(sbi->s_mb_maxs);
> >>>> @@ -3028,6 +3385,8 @@ int ext4_mb_release(struct super_block *sb)
> >>>> kvfree(group_info);
> >>>> rcu_read_unlock();
> >>>> }
> >>>> + kfree(sbi->s_mb_largest_free_orders);
> >>>> + kfree(sbi->s_mb_largest_free_orders_locks);
> >>>> kfree(sbi->s_mb_offsets);
> >>>> kfree(sbi->s_mb_maxs);
> >>>> iput(sbi->s_buddy_cache);
> >>>> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
> >>>> index 68111a10cfee..02585e3cbcad 100644
> >>>> --- a/fs/ext4/mballoc.h
> >>>> +++ b/fs/ext4/mballoc.h
> >>>> @@ -78,6 +78,18 @@
> >>>> */
> >>>> #define MB_DEFAULT_MAX_INODE_PREALLOC 512
> >>>>
> >>>> +/*
> >>>> + * Number of groups to search linearly before performing group scanning
> >>>> + * optimization.
> >>>> + */
> >>>> +#define MB_DEFAULT_LINEAR_LIMIT 4
> >>>> +
> >>>> +/*
> >>>> + * Minimum number of groups that should be present in the file system to perform
> >>>> + * group scanning optimizations.
> >>>> + */
> >>>> +#define MB_DEFAULT_LINEAR_SCAN_THRESHOLD 16
> >>>> +
> >>>> /*
> >>>> * Number of valid buddy orders
> >>>> */
> >>>> @@ -166,11 +178,14 @@ struct ext4_allocation_context {
> >>>> /* copy of the best found extent taken before preallocation efforts */
> >>>> struct ext4_free_extent ac_f_ex;
> >>>>
> >>>> + ext4_group_t ac_last_optimal_group;
> >>>> + __u32 ac_groups_considered;
> >>>> + __u32 ac_flags; /* allocation hints */
> >>>> __u16 ac_groups_scanned;
> >>>> + __u16 ac_groups_linear_remaining;
> >>>> __u16 ac_found;
> >>>> __u16 ac_tail;
> >>>> __u16 ac_buddy;
> >>>> - __u16 ac_flags; /* allocation hints */
> >>>> __u8 ac_status;
> >>>> __u8 ac_criteria;
> >>>> __u8 ac_2order; /* if request is to allocate 2^N blocks and
> >>>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> >>>> index 3a2cd9fb7e73..6116640081c0 100644
> >>>> --- a/fs/ext4/super.c
> >>>> +++ b/fs/ext4/super.c
> >>>> @@ -1687,7 +1687,7 @@ enum {
> >>>> Opt_dioread_nolock, Opt_dioread_lock,
> >>>> Opt_discard, Opt_nodiscard, Opt_init_itable, Opt_noinit_itable,
> >>>> Opt_max_dir_size_kb, Opt_nojournal_checksum, Opt_nombcache,
> >>>> - Opt_prefetch_block_bitmaps,
> >>>> + Opt_prefetch_block_bitmaps, Opt_mb_optimize_scan,
> >>>> #ifdef CONFIG_EXT4_DEBUG
> >>>> Opt_fc_debug_max_replay, Opt_fc_debug_force
> >>>> #endif
> >>>> @@ -1788,6 +1788,7 @@ static const match_table_t tokens = {
> >>>> {Opt_nombcache, "nombcache"},
> >>>> {Opt_nombcache, "no_mbcache"}, /* for backward compatibility */
> >>>> {Opt_prefetch_block_bitmaps, "prefetch_block_bitmaps"},
> >>>> + {Opt_mb_optimize_scan, "mb_optimize_scan=%d"},
> >>>> {Opt_removed, "check=none"}, /* mount option from ext2/3 */
> >>>> {Opt_removed, "nocheck"}, /* mount option from ext2/3 */
> >>>> {Opt_removed, "reservation"}, /* mount option from ext2/3 */
> >>>> @@ -1820,6 +1821,8 @@ static ext4_fsblk_t get_sb_block(void **data)
> >>>> }
> >>>>
> >>>> #define DEFAULT_JOURNAL_IOPRIO (IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, 3))
> >>>> +#define DEFAULT_MB_OPTIMIZE_SCAN (-1)
> >>>> +
> >>>> static const char deprecated_msg[] =
> >>>> "Mount option \"%s\" will be removed by %s\n"
> >>>> "Contact linux-ext4@xxxxxxxxxxxxxxx if you think we should keep it.\n";
> >>>> @@ -2008,6 +2011,7 @@ static const struct mount_opts {
> >>>> {Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET},
> >>>> {Opt_prefetch_block_bitmaps, EXT4_MOUNT_PREFETCH_BLOCK_BITMAPS,
> >>>> MOPT_SET},
> >>>> + {Opt_mb_optimize_scan, EXT4_MOUNT2_MB_OPTIMIZE_SCAN, MOPT_GTE0},
> >>>> #ifdef CONFIG_EXT4_DEBUG
> >>>> {Opt_fc_debug_force, EXT4_MOUNT2_JOURNAL_FAST_COMMIT,
> >>>> MOPT_SET | MOPT_2 | MOPT_EXT4_ONLY},
> >>>> @@ -2092,6 +2096,7 @@ static int ext4_set_test_dummy_encryption(struct super_block *sb,
> >>>> struct ext4_parsed_options {
> >>>> unsigned long journal_devnum;
> >>>> unsigned int journal_ioprio;
> >>>> + int mb_optimize_scan;
> >>>> };
> >>>>
> >>>> static int handle_mount_opt(struct super_block *sb, char *opt, int token,
> >>>> @@ -2388,6 +2393,13 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
> >>>> sbi->s_mount_opt |= m->mount_opt;
> >>>> } else if (token == Opt_data_err_ignore) {
> >>>> sbi->s_mount_opt &= ~m->mount_opt;
> >>>> + } else if (token == Opt_mb_optimize_scan) {
> >>>> + if (arg != 0 && arg != 1) {
> >>>> + ext4_msg(sb, KERN_WARNING,
> >>>> + "mb_optimize_scan should be set to 0 or 1.");
> >>>> + return -1;
> >>>> + }
> >>>> + parsed_opts->mb_optimize_scan = arg;
> >>>> } else {
> >>>> if (!args->from)
> >>>> arg = 1;
> >>>> @@ -4034,6 +4046,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> >>>> /* Set defaults for the variables that will be set during parsing */
> >>>> parsed_opts.journal_ioprio = DEFAULT_JOURNAL_IOPRIO;
> >>>> parsed_opts.journal_devnum = 0;
> >>>> + parsed_opts.mb_optimize_scan = DEFAULT_MB_OPTIMIZE_SCAN;
> >>>>
> >>>> if ((data && !orig_data) || !sbi)
> >>>> goto out_free_base;
> >>>> @@ -4984,6 +4997,19 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> >>>> ext4_fc_replay_cleanup(sb);
> >>>>
> >>>> ext4_ext_init(sb);
> >>>> +
> >>>> + /*
> >>>> + * Enable optimize_scan if number of groups is > threshold. This can be
> >>>> + * turned off by passing "mb_optimize_scan=0". This can also be
> >>>> + * turned on forcefully by passing "mb_optimize_scan=1".
> >>>> + */
> >>>> + if (parsed_opts.mb_optimize_scan == 1)
> >>>> + set_opt2(sb, MB_OPTIMIZE_SCAN);
> >>>> + else if (parsed_opts.mb_optimize_scan == 0)
> >>>> + clear_opt2(sb, MB_OPTIMIZE_SCAN);
> >>>> + else if (sbi->s_groups_count >= MB_DEFAULT_LINEAR_SCAN_THRESHOLD)
> >>>> + set_opt2(sb, MB_OPTIMIZE_SCAN);
> >>>> +
> >>>> err = ext4_mb_init(sb);
> >>>> if (err) {
> >>>> ext4_msg(sb, KERN_ERR, "failed to initialize mballoc (%d)",
> >>>> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> >>>> index 59ca9d73b42f..16b8a838f631 100644
> >>>> --- a/fs/ext4/sysfs.c
> >>>> +++ b/fs/ext4/sysfs.c
> >>>> @@ -213,6 +213,7 @@ EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs);
> >>>> EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request);
> >>>> EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc);
> >>>> EXT4_RW_ATTR_SBI_UI(mb_max_inode_prealloc, s_mb_max_inode_prealloc);
> >>>> +EXT4_RW_ATTR_SBI_UI(mb_linear_limit, s_mb_linear_limit);
> >>>> EXT4_RW_ATTR_SBI_UI(extent_max_zeroout_kb, s_extent_max_zeroout_kb);
> >>>> EXT4_ATTR(trigger_fs_error, 0200, trigger_test_error);
> >>>> EXT4_RW_ATTR_SBI_UI(err_ratelimit_interval_ms, s_err_ratelimit_state.interval);
> >>>> @@ -260,6 +261,7 @@ static struct attribute *ext4_attrs[] = {
> >>>> ATTR_LIST(mb_stream_req),
> >>>> ATTR_LIST(mb_group_prealloc),
> >>>> ATTR_LIST(mb_max_inode_prealloc),
> >>>> + ATTR_LIST(mb_linear_limit),
> >>>> ATTR_LIST(max_writeback_mb_bump),
> >>>> ATTR_LIST(extent_max_zeroout_kb),
> >>>> ATTR_LIST(trigger_fs_error),
> >>>> --
> >>>> 2.31.0.291.g576ba9dcdaf-goog
> >>>>
> >>>
> >>>
> >>> Cheers, Andreas
> >>>
> >>>
> >>>
> >>>
> >>>
