On 29.06.2018 11:27, Ethan Lien wrote:
> We use percpu_counter to reduce lock contention of frequently updated
> counter. But the default 32 batch size is suitable only for inc/dec
> update, not for sector size update. The end result is we still acquire
> spin lock for every percpu_counter_add(). So use customized batch size
> for total_bytes_pinned as we already done for dirty_metadata_bytes
> and delalloc_bytes. Also we fix dirty_metadata_bytes to use
> __percpu_counter_compare of customized batch so we get precise value of
> dirty_metadata_bytes.
Do you have any numbers to prove this is problematic? Perhaps a fio
workload + lockdep contention numbers before/after the patch ?
>
> Signed-off-by: Ethan Lien <ethanl...@synology.com>
> ---
> fs/btrfs/ctree.h | 1 +
> fs/btrfs/disk-io.c | 10 ++++++----
> fs/btrfs/extent-tree.c | 43 +++++++++++++++++++++++++++---------------
> 3 files changed, 35 insertions(+), 19 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 118346aceea9..df682a521635 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -422,6 +422,7 @@ struct btrfs_space_info {
> * time the transaction commits.
> */
> struct percpu_counter total_bytes_pinned;
> + s32 total_bytes_pinned_batch;
>
> struct list_head list;
> /* Protected by the spinlock 'lock'. */
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 205092dc9390..dfed08e70ec1 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -961,8 +961,9 @@ static int btree_writepages(struct address_space *mapping,
>
> fs_info = BTRFS_I(mapping->host)->root->fs_info;
> /* this is a bit racy, but that's ok */
> - ret = percpu_counter_compare(&fs_info->dirty_metadata_bytes,
> - BTRFS_DIRTY_METADATA_THRESH);
> + ret = __percpu_counter_compare(&fs_info->dirty_metadata_bytes,
> + BTRFS_DIRTY_METADATA_THRESH,
> + fs_info->dirty_metadata_batch);
> if (ret < 0)
> return 0;
> }
> @@ -4150,8 +4151,9 @@ static void __btrfs_btree_balance_dirty(struct
> btrfs_fs_info *fs_info,
> if (flush_delayed)
> btrfs_balance_delayed_items(fs_info);
>
> - ret = percpu_counter_compare(&fs_info->dirty_metadata_bytes,
> - BTRFS_DIRTY_METADATA_THRESH);
> + ret = __percpu_counter_compare(&fs_info->dirty_metadata_bytes,
> + BTRFS_DIRTY_METADATA_THRESH,
> + fs_info->dirty_metadata_batch);
> if (ret > 0) {
>
> balance_dirty_pages_ratelimited(fs_info->btree_inode->i_mapping);
> }
This hunk needs to be in a separate patch. And the last sentence which
relates to it needs to be expanded further to explain the problem.
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 3d9fe58c0080..a067c047904c 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -758,7 +758,8 @@ static void add_pinned_bytes(struct btrfs_fs_info
> *fs_info, s64 num_bytes,
>
> space_info = __find_space_info(fs_info, flags);
> ASSERT(space_info);
> - percpu_counter_add(&space_info->total_bytes_pinned, num_bytes);
> + percpu_counter_add_batch(&space_info->total_bytes_pinned, num_bytes,
> + space_info->total_bytes_pinned_batch);
> }
>
> /*
> @@ -2598,8 +2599,9 @@ static int cleanup_ref_head(struct btrfs_trans_handle
> *trans,
> flags = BTRFS_BLOCK_GROUP_METADATA;
> space_info = __find_space_info(fs_info, flags);
> ASSERT(space_info);
> - percpu_counter_add(&space_info->total_bytes_pinned,
> - -head->num_bytes);
> + percpu_counter_add_batch(&space_info->total_bytes_pinned,
> + -head->num_bytes,
> + space_info->total_bytes_pinned_batch);
>
> if (head->is_data) {
> spin_lock(&delayed_refs->lock);
> @@ -4035,6 +4037,10 @@ static int create_space_info(struct btrfs_fs_info
> *info, u64 flags)
> INIT_LIST_HEAD(&space_info->ro_bgs);
> INIT_LIST_HEAD(&space_info->tickets);
> INIT_LIST_HEAD(&space_info->priority_tickets);
> + if (flags & BTRFS_BLOCK_GROUP_DATA)
> + space_info->total_bytes_pinned_batch = info->delalloc_batch;
> + else
> + space_info->total_bytes_pinned_batch =
> info->dirty_metadata_batch;
>
> ret = kobject_init_and_add(&space_info->kobj, &space_info_ktype,
> info->space_info_kobj, "%s",
> @@ -4309,9 +4315,10 @@ int btrfs_alloc_data_chunk_ondemand(struct btrfs_inode
> *inode, u64 bytes)
> * allocation, and no removed chunk in current transaction,
> * don't bother committing the transaction.
> */
> - have_pinned_space = percpu_counter_compare(
> + have_pinned_space = __percpu_counter_compare(
> &data_sinfo->total_bytes_pinned,
> - used + bytes - data_sinfo->total_bytes);
> + used + bytes - data_sinfo->total_bytes,
> + data_sinfo->total_bytes_pinned_batch);
> spin_unlock(&data_sinfo->lock);
>
> /* commit the current transaction and try again */
> @@ -4912,8 +4919,9 @@ static int may_commit_transaction(struct btrfs_fs_info
> *fs_info,
> return 0;
>
> /* See if there is enough pinned space to make this reservation */
> - if (percpu_counter_compare(&space_info->total_bytes_pinned,
> - bytes) >= 0)
> + if (__percpu_counter_compare(&space_info->total_bytes_pinned,
> + bytes,
> + space_info->total_bytes_pinned_batch) >= 0)
> goto commit;
>
> /*
> @@ -4930,8 +4938,9 @@ static int may_commit_transaction(struct btrfs_fs_info
> *fs_info,
> bytes -= delayed_rsv->size;
> spin_unlock(&delayed_rsv->lock);
>
> - if (percpu_counter_compare(&space_info->total_bytes_pinned,
> - bytes) < 0) {
> + if (__percpu_counter_compare(&space_info->total_bytes_pinned,
> + bytes,
> + space_info->total_bytes_pinned_batch) < 0) {
> return -ENOSPC;
> }
>
> @@ -6268,8 +6277,9 @@ static int update_block_group(struct btrfs_trans_handle
> *trans,
> trace_btrfs_space_reservation(info, "pinned",
> cache->space_info->flags,
> num_bytes, 1);
> -
> percpu_counter_add(&cache->space_info->total_bytes_pinned,
> - num_bytes);
> +
> percpu_counter_add_batch(&cache->space_info->total_bytes_pinned,
> + num_bytes,
> +
> cache->space_info->total_bytes_pinned_batch);
> set_extent_dirty(info->pinned_extents,
> bytenr, bytenr + num_bytes - 1,
> GFP_NOFS | __GFP_NOFAIL);
> @@ -6347,7 +6357,8 @@ static int pin_down_extent(struct btrfs_fs_info
> *fs_info,
>
> trace_btrfs_space_reservation(fs_info, "pinned",
> cache->space_info->flags, num_bytes, 1);
> - percpu_counter_add(&cache->space_info->total_bytes_pinned, num_bytes);
> + percpu_counter_add_batch(&cache->space_info->total_bytes_pinned,
> + num_bytes, cache->space_info->total_bytes_pinned_batch);
> set_extent_dirty(fs_info->pinned_extents, bytenr,
> bytenr + num_bytes - 1, GFP_NOFS | __GFP_NOFAIL);
> return 0;
> @@ -6711,7 +6722,8 @@ static int unpin_extent_range(struct btrfs_fs_info
> *fs_info,
> trace_btrfs_space_reservation(fs_info, "pinned",
> space_info->flags, len, 0);
> space_info->max_extent_size = 0;
> - percpu_counter_add(&space_info->total_bytes_pinned, -len);
> + percpu_counter_add_batch(&space_info->total_bytes_pinned,
> + -len, space_info->total_bytes_pinned_batch);
> if (cache->ro) {
> space_info->bytes_readonly += len;
> readonly = true;
> @@ -10764,8 +10776,9 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info
> *fs_info)
>
> space_info->bytes_pinned -= block_group->pinned;
> space_info->bytes_readonly += block_group->pinned;
> - percpu_counter_add(&space_info->total_bytes_pinned,
> - -block_group->pinned);
> + percpu_counter_add_batch(&space_info->total_bytes_pinned,
> + -block_group->pinned,
> + space_info->total_bytes_pinned_batch);
> block_group->pinned = 0;
>
> spin_unlock(&block_group->lock);
>
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