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Date: Mon, 29 Mar 2021 09:26:19 -0700
From: harshad shirwadkar <harshadshirwadkar@...il.com>
To: Ritesh Harjani <riteshh@...ux.ibm.com>
Cc: Ext4 Developers List <linux-ext4@...r.kernel.org>,
kernel test robot <lkp@...el.com>,
Dan Carpenter <dan.carpenter@...cle.com>,
Andreas Dilger <adilger@...ger.ca>
Subject: Re: [PATCH v5 5/6] ext4: improve cr 0 / cr 1 group scanning
On Thu, Mar 25, 2021 at 6:50 AM Ritesh Harjani <riteshh@...ux.ibm.com> wrote:
>
>
>
> 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?
Thanks Ritesh, that's a good catch! So group choosing needs to happen
at the start of the group while increment needs to happen at the end.
I'll fix this in the next version.
- Harshad
>
>
> -ritesh
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