Message-Id: <BF6E7BBD-794A-4C02-9219-A302956F7BFA@gmail.com>
Date:   Tue, 16 Feb 2021 22:39:22 +0300
From:   Благодаренко Артём 
        <artem.blagodarenko@...il.com>
To:     Harshad Shirwadkar <harshadshirwadkar@...il.com>
Cc:     linux-ext4@...r.kernel.org, "Theodore Y. Ts'o" <tytso@....edu>,
        bzzz@...mcloud.com, sihara@....com,
        Andreas Dilger <adilger@...ger.ca>
Subject: Re: [PATCH v2 4/5] ext4: improve cr 0 / cr 1 group scanning

Hello Harshad,

Thanks for this useful optimisation.

Some comments bellow.

> On 9 Feb 2021, at 23:28, Harshad Shirwadkar <harshadshirwadkar@...il.com> 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.
> 
> 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".
> 
> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>
> ---
> fs/ext4/ext4.h    |  13 +-
> fs/ext4/mballoc.c | 316 ++++++++++++++++++++++++++++++++++++++++++++--
> fs/ext4/mballoc.h |   1 +
> fs/ext4/super.c   |   6 +-
> 4 files changed, 322 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 317b43420ecf..0601c997c87f 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -162,6 +162,8 @@ 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
> +/* Avg fragment size rb tree lookup succeeded at least once for cr = 1 */
> +#define EXT4_MB_CR1_OPTIMIZED		0x8000
> 
> struct ext4_allocation_request {
> 	/* target inode for block we're allocating */
> @@ -1247,7 +1249,9 @@ 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
> @@ -1527,6 +1531,10 @@ 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;
> @@ -3308,11 +3316,14 @@ 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 b7f25120547d..63562f5f42f1 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -147,7 +147,12 @@
>  * the group specified as the goal value in allocation context via
>  * ac_g_ex. Each group is first checked based on the criteria whether it
>  * can be used for allocation. ext4_mb_good_group explains how the groups are
> - * checked.
> + * checked. If "mb_optimize_scan" mount option is set, instead of traversing
> + * groups linearly starting at the goal, the groups are traversed in an optimal
> + * order according to each cr level, so as to minimize considering groups which
> + * would anyway be rejected by ext4_mb_good_group. This has a side effect
> + * though - subsequent allocations may not be close to each other. And so,
> + * the underlying device may get filled up in a non-linear fashion.
>  *
>  * 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
> @@ -299,6 +304,8 @@
>  *  - 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 +335,9 @@
>  *    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 +361,9 @@ 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 +757,243 @@ 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))
> +			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_1 < num_frags_2);
> +}
> +
> +/*
> + * Reinsert grpinfo into the avg_fragment_size tree with new average
> + * fragment size.
> + */

Walk along the ngroups linked elements in worst case for every mb_free_blocks and mb_mark_used which are quite frequently executed actions.
If double-linked list is used for avg_fragments this function will make this change without iterating through the list:
1. Check with previous element. If smaller, then commute
2. Check with next element. If greater, then commute.

> +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))
> +		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;
> +
> +	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) {
> +			/*
> +			 * Perform this check without a lock, once we lock
> +			 * the group, we'll perform this check again.
> +			 */
> +			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;
> +	}
> +	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 (ac->ac_flags & EXT4_MB_CR1_OPTIMIZED) {
> +		/* 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);
> +			/*
> +			 * Perform this check without locking, we'll lock later
> +			 * to confirm.
> +			 */
> +			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);
> +		/*
> +		 * 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 (*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 +1001,32 @@ 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) {
> +		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 +1082,7 @@ 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 +1782,7 @@ 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 +1919,7 @@ 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);
> 
> @@ -2346,17 +2613,20 @@ 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;
> 		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
> @@ -2696,7 +2966,10 @@ 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;
> @@ -2886,6 +3159,22 @@ 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)
> +		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)
> +		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;
> @@ -2949,6 +3238,8 @@ 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);
> @@ -3005,6 +3296,7 @@ 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_offsets);
> 	kfree(sbi->s_mb_maxs);
> 	iput(sbi->s_buddy_cache);
> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
> index 02861406932f..1e86a8a0460d 100644
> --- a/fs/ext4/mballoc.h
> +++ b/fs/ext4/mballoc.h
> @@ -166,6 +166,7 @@ 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;
> 	__u16 ac_groups_scanned;
> 	__u16 ac_found;
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 0f0db49031dc..a14363654cfd 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -154,6 +154,7 @@ static inline void __ext4_read_bh(struct buffer_head *bh, int op_flags,
> 	clear_buffer_verified(bh);
> 
> 	bh->b_end_io = end_io ? end_io : end_buffer_read_sync;
> +
> 	get_bh(bh);
> 	submit_bh(REQ_OP_READ, op_flags, bh);
> }
> @@ -1687,7 +1688,7 @@ 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 +1789,7 @@ 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"},
> 	{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 */
> @@ -2008,6 +2010,8 @@ 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_SET | MOPT_2 | MOPT_EXT4_ONLY},
> #ifdef CONFIG_EXT4_DEBUG
> 	{Opt_fc_debug_force, EXT4_MOUNT2_JOURNAL_FAST_COMMIT,
> 	 MOPT_SET | MOPT_2 | MOPT_EXT4_ONLY},
> -- 
> 2.30.0.478.g8a0d178c01-goog
> 

With your parch we have actual information about all groups in memory but ext4_mb_seq_groups_show() process all groups to show output. Should we improve this function somehow? Or probably add new statistics there? Or news seq, that outputs sorted lists. This is optional. Just for discussion.

Best regards,
Artem Blagodarenko.

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