ext4: Don't allow lg prealloc list to be grow large.

Currently, the locality group prealloc list is freed only when there
is a block allocation failure. This can result in large number of
entries in the preallocation list making ext4_mb_use_preallocated()
expensive.

To fix this, we convert the locality group prealloc list to a hash
list. The hash index is the order of number of blocks in the prealloc
space with a max order of 9. When adding prealloc space to the list we
make sure total entries for each order does not exceed 8. If it is
more than 8 we discard few entries and make sure the we have only <= 5
entries.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

2 files changed, 193 insertions, 30 deletions

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 49bec8404c5f..865e9ddb44d4 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2480,7 +2480,7 @@ err_freesgi:
 int ext4_mb_init(struct super_block *sb, int needs_recovery)
 {
         struct ext4_sb_info *sbi = EXT4_SB(sb);
-        unsigned i;
+        unsigned i, j;
         unsigned offset;
         unsigned max;
         int ret;
@@ -2552,7 +2552,8 @@ int ext4_mb_init(struct super_block *sb, int needs_recovery)
                 struct ext4_locality_group *lg;
                 lg = &sbi->s_locality_groups[i];
                 mutex_init(&lg->lg_mutex);
-                INIT_LIST_HEAD(&lg->lg_prealloc_list);
+                for (j = 0; j < PREALLOC_TB_SIZE; j++)
+                        INIT_LIST_HEAD(&lg->lg_prealloc_list[j]);
                 spin_lock_init(&lg->lg_prealloc_lock);
         }
 
@@ -3263,6 +3264,7 @@ static void ext4_mb_use_group_pa(struct ext4_allocation_context *ac,
                                 struct ext4_prealloc_space *pa)
 {
         unsigned int len = ac->ac_o_ex.fe_len;
+
         ext4_get_group_no_and_offset(ac->ac_sb, pa->pa_pstart,
                                         &ac->ac_b_ex.fe_group,
                                         &ac->ac_b_ex.fe_start);
@@ -3285,6 +3287,7 @@ static void ext4_mb_use_group_pa(struct ext4_allocation_context *ac,
 static noinline_for_stack int
 ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
 {
+        int order, i;
         struct ext4_inode_info *ei = EXT4_I(ac->ac_inode);
         struct ext4_locality_group *lg;
         struct ext4_prealloc_space *pa;
@@ -3325,22 +3328,29 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
         lg = ac->ac_lg;
         if (lg == NULL)
                 return 0;
-
-        rcu_read_lock();
-        list_for_each_entry_rcu(pa, &lg->lg_prealloc_list, pa_inode_list) {
-                spin_lock(&pa->pa_lock);
-                if (pa->pa_deleted == 0 && pa->pa_free >= ac->ac_o_ex.fe_len) {
-                        atomic_inc(&pa->pa_count);
-                        ext4_mb_use_group_pa(ac, pa);
+        order  = fls(ac->ac_o_ex.fe_len) - 1;
+        if (order > PREALLOC_TB_SIZE - 1)
+                /* The max size of hash table is PREALLOC_TB_SIZE */
+                order = PREALLOC_TB_SIZE - 1;
+
+        for (i = order; i < PREALLOC_TB_SIZE; i++) {
+                rcu_read_lock();
+                list_for_each_entry_rcu(pa, &lg->lg_prealloc_list[i],
+                                        pa_inode_list) {
+                        spin_lock(&pa->pa_lock);
+                        if (pa->pa_deleted == 0 &&
+                                        pa->pa_free >= ac->ac_o_ex.fe_len) {
+                                atomic_inc(&pa->pa_count);
+                                ext4_mb_use_group_pa(ac, pa);
+                                spin_unlock(&pa->pa_lock);
+                                ac->ac_criteria = 20;
+                                rcu_read_unlock();
+                                return 1;
+                        }
                         spin_unlock(&pa->pa_lock);
-                        ac->ac_criteria = 20;
-                        rcu_read_unlock();
-                        return 1;
                 }
-                spin_unlock(&pa->pa_lock);
+                rcu_read_unlock();
         }
-        rcu_read_unlock();
-
         return 0;
 }
 
@@ -3563,6 +3573,7 @@ ext4_mb_new_group_pa(struct ext4_allocation_context *ac)
         pa->pa_free = pa->pa_len;
         atomic_set(&pa->pa_count, 1);
         spin_lock_init(&pa->pa_lock);
+        INIT_LIST_HEAD(&pa->pa_inode_list);
         pa->pa_deleted = 0;
         pa->pa_linear = 1;
 
@@ -3583,10 +3594,10 @@ ext4_mb_new_group_pa(struct ext4_allocation_context *ac)
         list_add(&pa->pa_group_list, &grp->bb_prealloc_list);
         ext4_unlock_group(sb, ac->ac_b_ex.fe_group);
 
-        spin_lock(pa->pa_obj_lock);
-        list_add_tail_rcu(&pa->pa_inode_list, &lg->lg_prealloc_list);
-        spin_unlock(pa->pa_obj_lock);
-
+        /*
+         * We will later add the new pa to the right bucket
+         * after updating the pa_free in ext4_mb_release_context
+         */
         return 0;
 }
 
@@ -4123,22 +4134,168 @@ ext4_mb_initialize_context(struct ext4_allocation_context *ac,
 
 }
 
+static noinline_for_stack void
+ext4_mb_discard_lg_preallocations(struct super_block *sb,
+                                        struct ext4_locality_group *lg,
+                                        int order, int total_entries)
+{
+        ext4_group_t group = 0;
+        struct ext4_buddy e4b;
+        struct list_head discard_list;
+        struct ext4_prealloc_space *pa, *tmp;
+        struct ext4_allocation_context *ac;
+
+        mb_debug("discard locality group preallocation\n");
+
+        INIT_LIST_HEAD(&discard_list);
+        ac = kmem_cache_alloc(ext4_ac_cachep, GFP_NOFS);
+
+        spin_lock(&lg->lg_prealloc_lock);
+        list_for_each_entry_rcu(pa, &lg->lg_prealloc_list[order],
+                                                pa_inode_list) {
+                spin_lock(&pa->pa_lock);
+                if (atomic_read(&pa->pa_count)) {
+                        /*
+                         * This is the pa that we just used
+                         * for block allocation. So don't
+                         * free that
+                         */
+                        spin_unlock(&pa->pa_lock);
+                        continue;
+                }
+                if (pa->pa_deleted) {
+                        spin_unlock(&pa->pa_lock);
+                        continue;
+                }
+                /* only lg prealloc space */
+                BUG_ON(!pa->pa_linear);
+
+                /* seems this one can be freed ... */
+                pa->pa_deleted = 1;
+                spin_unlock(&pa->pa_lock);
+
+                list_del_rcu(&pa->pa_inode_list);
+                list_add(&pa->u.pa_tmp_list, &discard_list);
+
+                total_entries--;
+                if (total_entries <= 5) {
+                        /*
+                         * we want to keep only 5 entries
+                         * allowing it to grow to 8. This
+                         * mak sure we don't call discard
+                         * soon for this list.
+                         */
+                        break;
+                }
+        }
+        spin_unlock(&lg->lg_prealloc_lock);
+
+        list_for_each_entry_safe(pa, tmp, &discard_list, u.pa_tmp_list) {
+
+                ext4_get_group_no_and_offset(sb, pa->pa_pstart, &group, NULL);
+                if (ext4_mb_load_buddy(sb, group, &e4b)) {
+                        ext4_error(sb, __func__, "Error in loading buddy "
+                                        "information for %lu\n", group);
+                        continue;
+                }
+                ext4_lock_group(sb, group);
+                list_del(&pa->pa_group_list);
+                ext4_mb_release_group_pa(&e4b, pa, ac);
+                ext4_unlock_group(sb, group);
+
+                ext4_mb_release_desc(&e4b);
+                list_del(&pa->u.pa_tmp_list);
+                call_rcu(&(pa)->u.pa_rcu, ext4_mb_pa_callback);
+        }
+        if (ac)
+                kmem_cache_free(ext4_ac_cachep, ac);
+}
+
+/*
+ * We have incremented pa_count. So it cannot be freed at this
+ * point. Also we hold lg_mutex. So no parallel allocation is
+ * possible from this lg. That means pa_free cannot be updated.
+ *
+ * A parallel ext4_mb_discard_group_preallocations is possible.
+ * which can cause the lg_prealloc_list to be updated.
+ */
+
+static void ext4_mb_add_n_trim(struct ext4_allocation_context *ac)
+{
+        int order, added = 0, lg_prealloc_count = 1;
+        struct super_block *sb = ac->ac_sb;
+        struct ext4_locality_group *lg = ac->ac_lg;
+        struct ext4_prealloc_space *tmp_pa, *pa = ac->ac_pa;
+
+        order = fls(pa->pa_free) - 1;
+        if (order > PREALLOC_TB_SIZE - 1)
+                /* The max size of hash table is PREALLOC_TB_SIZE */
+                order = PREALLOC_TB_SIZE - 1;
+        /* Add the prealloc space to lg */
+        rcu_read_lock();
+        list_for_each_entry_rcu(tmp_pa, &lg->lg_prealloc_list[order],
+                                                pa_inode_list) {
+                spin_lock(&tmp_pa->pa_lock);
+                if (tmp_pa->pa_deleted) {
+                        spin_unlock(&pa->pa_lock);
+                        continue;
+                }
+                if (!added && pa->pa_free < tmp_pa->pa_free) {
+                        /* Add to the tail of the previous entry */
+                        list_add_tail_rcu(&pa->pa_inode_list,
+                                                &tmp_pa->pa_inode_list);
+                        added = 1;
+                        /*
+                         * we want to count the total
+                         * number of entries in the list
+                         */
+                }
+                spin_unlock(&tmp_pa->pa_lock);
+                lg_prealloc_count++;
+        }
+        if (!added)
+                list_add_tail_rcu(&pa->pa_inode_list,
+                                        &lg->lg_prealloc_list[order]);
+        rcu_read_unlock();
+
+        /* Now trim the list to be not more than 8 elements */
+        if (lg_prealloc_count > 8) {
+                ext4_mb_discard_lg_preallocations(sb, lg,
+                                                order, lg_prealloc_count);
+                return;
+        }
+        return ;
+}
+
 /*
  * release all resource we used in allocation
  */
 static int ext4_mb_release_context(struct ext4_allocation_context *ac)
 {
-        if (ac->ac_pa) {
-                if (ac->ac_pa->pa_linear) {
+        struct ext4_prealloc_space *pa = ac->ac_pa;
+        if (pa) {
+                if (pa->pa_linear) {
                         /* see comment in ext4_mb_use_group_pa() */
-                        spin_lock(&ac->ac_pa->pa_lock);
-                        ac->ac_pa->pa_pstart += ac->ac_b_ex.fe_len;
-                        ac->ac_pa->pa_lstart += ac->ac_b_ex.fe_len;
-                        ac->ac_pa->pa_free -= ac->ac_b_ex.fe_len;
-                        ac->ac_pa->pa_len -= ac->ac_b_ex.fe_len;
-                        spin_unlock(&ac->ac_pa->pa_lock);
+                        spin_lock(&pa->pa_lock);
+                        pa->pa_pstart += ac->ac_b_ex.fe_len;
+                        pa->pa_lstart += ac->ac_b_ex.fe_len;
+                        pa->pa_free -= ac->ac_b_ex.fe_len;
+                        pa->pa_len -= ac->ac_b_ex.fe_len;
+                        spin_unlock(&pa->pa_lock);
+                        /*
+                         * We want to add the pa to the right bucket.
+                         * Remove it from the list and while adding
+                         * make sure the list to which we are adding
+                         * doesn't grow big.
+                         */
+                        if (likely(pa->pa_free)) {
+                                spin_lock(pa->pa_obj_lock);
+                                list_del_rcu(&pa->pa_inode_list);
+                                spin_unlock(pa->pa_obj_lock);
+                                ext4_mb_add_n_trim(ac);
+                        }
                 }
-                ext4_mb_put_pa(ac, ac->ac_sb, ac->ac_pa);
+                ext4_mb_put_pa(ac, ac->ac_sb, pa);
         }
         if (ac->ac_bitmap_page)
                 page_cache_release(ac->ac_bitmap_page);
diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index bfe6add46bcf..c7c9906c2a75 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h
@@ -164,11 +164,17 @@ struct ext4_free_extent {
  * Locality group:
  *   we try to group all related changes together
  *   so that writeback can flush/allocate them together as well
+ *   Size of lg_prealloc_list hash is determined by MB_DEFAULT_GROUP_PREALLOC
+ *   (512). We store prealloc space into the hash based on the pa_free blocks
+ *   order value.ie, fls(pa_free)-1;
  */
+#define PREALLOC_TB_SIZE 10
 struct ext4_locality_group {
         /* for allocator */
-        struct mutex            lg_mutex;       /* to serialize allocates */
-        struct list_head        lg_prealloc_list;/* list of preallocations */
+        /* to serialize allocates */
+        struct mutex            lg_mutex;
+        /* list of preallocations */
+        struct list_head        lg_prealloc_list[PREALLOC_TB_SIZE];
         spinlock_t              lg_prealloc_lock;
 };