On Fri 07-10-22 02:16:18, Ojaswin Mujoo wrote:
> Currently, the kernel uses i_prealloc_list to hold all the inode
> preallocations. This is known to cause degradation in performance in
> workloads which perform large number of sparse writes on a single file.
> This is mainly because functions like ext4_mb_normalize_request() and
> ext4_mb_use_preallocated() iterate over this complete list, resulting in
> slowdowns when large number of PAs are present.
>
> Patch 27bc446e2 partially fixed this by enforcing a limit of 512 for
> the inode preallocation list and adding logic to continually trim the
> list if it grows above the threshold, however our testing revealed that
> a hardcoded value is not suitable for all kinds of workloads.
>
> To optimize this, add an rbtree to the inode and hold the inode
> preallocations in this rbtree. This will make iterating over inode PAs
> faster and scale much better than a linked list. Additionally, we also
> had to remove the LRU logic that was added during trimming of the list
> (in ext4_mb_release_context()) as it will add extra overhead in rbtree.
> The discards now happen in the lowest-logical-offset-first order.
>
> ** Locking notes **
>
> With the introduction of rbtree to maintain inode PAs, we can't use RCU
> to walk the tree for searching since it can result in partial traversals
> which might miss some nodes(or entire subtrees) while discards happen
> in parallel (which happens under a lock). Hence this patch converts the
> ei->i_prealloc_lock spin_lock to rw_lock.
>
> Almost all the codepaths that read/modify the PA rbtrees are protected
> by the higher level inode->i_data_sem (except
> ext4_mb_discard_group_preallocations() and ext4_clear_inode()) IIUC, the
> only place we need lock protection is when one thread is reading
> "searching" the PA rbtree (earlier protected under rcu_read_lock()) and
> another is "deleting" the PAs in ext4_mb_discard_group_preallocations()
> function (which iterates all the PAs using the grp->bb_prealloc_list and
> deletes PAs from the tree without taking any inode lock (i_data_sem)).
>
> So, this patch converts all rcu_read_lock/unlock() paths for inode list
> PA to use read_lock() and all places where we were using
> ei->i_prealloc_lock spinlock will now be using write_lock().
>
> Note that this makes the fast path (searching of the right PA e.g.
> ext4_mb_use_preallocated() or ext4_mb_normalize_request()), now use
> read_lock() instead of rcu_read_lock/unlock(). Ths also will now block
> due to slow discard path (ext4_mb_discard_group_preallocations()) which
> uses write_lock().
>
> But this is not as bad as it looks. This is because -
>
> 1. The slow path only occurs when the normal allocation failed and we
> can say that we are low on disk space. One can argue this scenario
> won't be much frequent.
>
> 2. ext4_mb_discard_group_preallocations(), locks and unlocks the rwlock
> for deleting every individual PA. This gives enough opportunity for
> the fast path to acquire the read_lock for searching the PA inode
> list.
>
> Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
Looks mostly good to me now. Just three nits below. With those fixes feel
free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
> @@ -4031,19 +4054,27 @@ ext4_mb_pa_adjust_overlap(struct ext4_allocation_context *ac,
> new_end = *end;
>
> /* check we don't cross already preallocated blocks */
> - rcu_read_lock();
> - list_for_each_entry_rcu(tmp_pa, &ei->i_prealloc_list, pa_node.inode_list) {
> - if (tmp_pa->pa_deleted)
> + read_lock(&ei->i_prealloc_lock);
> + for (iter = ei->i_prealloc_node.rb_node; iter;
> + iter = ext4_mb_pa_rb_next_iter(new_start, tmp_pa_start, iter)) {
> + tmp_pa = rb_entry(iter, struct ext4_prealloc_space,
> + pa_node.inode_node);
> + tmp_pa_start = tmp_pa->pa_lstart;
> + tmp_pa_end = tmp_pa->pa_lstart + EXT4_C2B(sbi, tmp_pa->pa_len);
> +
> + /*
> + * If pa is deleted, ignore overlaps and just iterate in rbtree
> + * based on tmp_pa_start
> + */
> + if (tmp_pa->pa_deleted) {
> continue;
> + }
Curly braces here are pointless.
> @@ -4408,17 +4439,21 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
> return false;
>
> /* first, try per-file preallocation */
> - rcu_read_lock();
> - list_for_each_entry_rcu(tmp_pa, &ei->i_prealloc_list, pa_node.inode_list) {
> + read_lock(&ei->i_prealloc_lock);
> + for (iter = ei->i_prealloc_node.rb_node; iter;
> + iter = ext4_mb_pa_rb_next_iter(ac->ac_o_ex.fe_logical, tmp_pa_start, iter)) {
> + tmp_pa = rb_entry(iter, struct ext4_prealloc_space, pa_node.inode_node);
>
> /* all fields in this condition don't change,
> * so we can skip locking for them */
> tmp_pa_start = tmp_pa->pa_lstart;
> tmp_pa_end = tmp_pa->pa_lstart + EXT4_C2B(sbi, tmp_pa->pa_len);
>
> + /* original request start doesn't lie in this PA */
> if (ac->ac_o_ex.fe_logical < tmp_pa_start ||
> - ac->ac_o_ex.fe_logical >= tmp_pa_end)
> + ac->ac_o_ex.fe_logical >= tmp_pa_end) {
> continue;
> + }
Again, curly braces here are pointless.
> +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);
> +}
I think I wrote it already last time: ext4_mb_rb_insert() is always called
with ext4_mb_pa_cmp() as the comparison function. Furthemore
ext4_mb_pa_cmp() is used nowhere else. So I'd just opencode
ext4_mb_pa_cmp() in ext4_mb_rb_insert() and get rid of the indirect call.
Better for speed as well as readability.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
