On 11.07.2018 18:59, Ethan Lien wrote:
> In commit b150a4f10d878 ("Btrfs: use a percpu to keep track of possibly
> pinned bytes") we use total_bytes_pinned to track how many bytes we are
> going to free in this transaction. When we are close to ENOSPC, we check it
> and know if we can make the allocation by commit the current transaction.
> For every data/metadata extent we are going to free, we add
> total_bytes_pinned in btrfs_free_extent() and btrfs_free_tree_block(), and
> release it in unpin_extent_range() when we finish the transaction. So this
> is a variable we frequently update but rarely read - just the suitable
> use of percpu_counter. But in previous commit we update total_bytes_pinned
> by default 32 batch size, making every update essentially a spin lock
> protected update. Since every spin lock/unlock operation involves syncing
> a globally used variable and some kind of barrier in a SMP system, this is
> more expensive than using total_bytes_pinned as a simple atomic64_t. So
> fix this by using a customized batch size. Since we only read
> total_bytes_pinned when we are close to ENOSPC and fail to alloc new chunk,
> we can use a really large batch size and have nearly no penalty in most
> cases.
>
>
> [Test]
> We test the patch on a 4-cores x86 machine:
> 1. falloate a 16GiB size test file.
> 2. take snapshot (so all following writes will be cow write).
> 3. run a 180 sec, 4 jobs, 4K random write fio on test file.
>
> We also add a temporary lockdep class on percpu_counter's spin lock used
> by total_bytes_pinned to track lock_stat.
>
>
> [Results]
> unpatched:
> lock_stat version 0.4
> -----------------------------------------------------------------------
> class name con-bounces contentions
> waittime-min waittime-max waittime-total waittime-avg acq-bounces
> acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
>
> total_bytes_pinned_percpu: 82 82
> 0.21 0.61 29.46 0.36 298340
> 635973 0.09 11.01 173476.25 0.27
>
>
> patched:
> lock_stat version 0.4
> -----------------------------------------------------------------------
> class name con-bounces contentions
> waittime-min waittime-max waittime-total waittime-avg acq-bounces
> acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
>
> total_bytes_pinned_percpu: 1 1
> 0.62 0.62 0.62 0.62 13601
> 31542 0.14 9.61 11016.90 0.35
>
According to these numbers though, the best-case (waittime-min and the
average waittime-avg) have actually regressed. So what really saves you
is the fact that the number of time we had to go to the best/average
case is reduced, due to the larger batch. I guess in that regard you are
in the clear. Another pertinent question is did you observe any
significant impact on run times of actual workloads or, say, transaction
commit times or anything like that? I think this case will only be hit
when the filesystem is struggling to satisfy a metadata allocation has
to cycle through all stages .
>
> [Analysis]
> Since the spin lock only protect a single in-memory variable, the
> contentions (number of lock acquisitions that had to wait) in both
> unpatched and patched version are low. But when we see acquisitions and
> acq-bounces, we get much lower counts in patched version. Here the most
> important metric is acq-bounces. It means how many times the lock get
> transferred between different cpus, so the patch can really recude
nit: s/recude/reduce - no need to resend i guess david will fix it on
the way into the tree
> cacheline bouncing of spin lock (also the global counter of percpu_counter)
> in a SMP system.
>
> Fixes: b150a4f10d878 ("Btrfs: use a percpu to keep track of possibly
> pinned bytes")
>
> Signed-off-by: Ethan Lien <ethanl...@synology.com>
Reviewed-by: Nikolay Borisov <nbori...@suse.com>
> ---
>
> V2:
> Rewrite commit comments.
> Add lock_stat test.
> Pull dirty_metadata_bytes out to a separate patch.
>
> fs/btrfs/ctree.h | 1 +
> fs/btrfs/extent-tree.c | 46 ++++++++++++++++++++++++++++--------------
> 2 files changed, 32 insertions(+), 15 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/extent-tree.c b/fs/btrfs/extent-tree.c
> index 3d9fe58c0080..937113534ef4 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);
> @@ -4024,6 +4026,13 @@ static int create_space_info(struct btrfs_fs_info
> *info, u64 flags)
> kfree(space_info);
> return ret;
> }
> + /*
> + * Use large batch size to reduce update overhead.
> + * For reader side, we only read it when we are close to ENOSPC and
> + * the read overhead is mostly related to # of cpus, so it is OK to
> + * use arbitrary large value here.
> + */
> + space_info->total_bytes_pinned_batch = SZ_128M;
>
> for (i = 0; i < BTRFS_NR_RAID_TYPES; i++)
> INIT_LIST_HEAD(&space_info->block_groups[i]);
> @@ -4309,9 +4318,10 @@ 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 +4922,9 @@ 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 +4941,9 @@ 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 +6280,9 @@ 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 +6360,8 @@ 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 +6725,8 @@ 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 +10779,9 @@ 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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