Date:   Fri, 12 Feb 2021 15:46:24 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Harshad Shirwadkar <harshadshirwadkar@...il.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        Alex Zhuravlev <bzzz@...mcloud.com>,
        artem.blagodarenko@...il.com, Shuichi Ihara <sihara@....com>
Subject: Re: [PATCH v2 4/5] ext4: improve cr 0 / cr 1 group scanning

On Feb 9, 2021, at 1:28 PM, 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".

Harshad, if you are going to refresh this patch, I would recommend to
include all or most of what you write in the commit message into the
below comment at the start of mballoc.c, possibly with some editing in
the expectation that "mb_optimized_scan" will become the default so
that the description of the list/rbtree for cr0/cr1 is mentioned first,
and the sequential group scanning is mentioned afterward.

Otherwise, the existing comment only mentions "groups are traversed in
an optimal order" which doesn't really explain much useful to the reader.

Cheers, Andreas

> 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.
> + */
> +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
> 


Cheers, Andreas






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