Message-ID: <f34cd26f-60b6-b622-e51b-7f84112e4219@linux.ibm.com>
Date:   Thu, 25 Mar 2021 19:20:37 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     Harshad Shirwadkar <harshadshirwadkar@...il.com>,
        linux-ext4@...r.kernel.org
Cc:     kernel test robot <lkp@...el.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Andreas Dilger <adilger@...ger.ca>, riteshh@...ux.ibm.com
Subject: Re: [PATCH v5 5/6] ext4: improve cr 0 / cr 1 group scanning



On 3/25/21 4:49 AM, Harshad Shirwadkar 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".
> 
> 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@...il.com>
> Reported-by: kernel test robot <lkp@...el.com>
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> Reviewed-by: Andreas Dilger <adilger@...ger.ca>
> ---
>   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;

hmm, Please help me correct if I am missing something here.
So, if *new_cr >= 2, then we will directly come here
increment the group and return to calling function.
But then we ended up incrementing the group while not
even searching in the given group to see if we could
find some blocks.



> +	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;

Note we start from a goal group or optimal 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;
> +			}


So ext4_mb_choose_next_group() will do group++ _even_ for cr >=2.
This would mean that we will never start our search from ac_g_ex.fe_group.
Why is that? Did I miss anything?


-ritesh

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