We have a commit_root_sem, which is a read-write semaphore that protects the
commit roots.
But it is also used to protect the list of caching block groups.
As a result, while doing "slow" caching, the following issue is seen:
Some of the caching threads are scanning the extent tree with
commit_root_sem
acquired in shared mode, with stack like:
[<ffffffffc0ad27b2>] read_extent_buffer_pages+0x2d2/0x300 [btrfs]
[<ffffffffc0a9fbe7>] btree_read_extent_buffer_pages.constprop.50+0xb7/0x1e0
[btrfs]
[<ffffffffc0aa1550>] read_tree_block+0x40/0x70 [btrfs]
[<ffffffffc0a7aa7c>] read_block_for_search.isra.33+0x12c/0x370 [btrfs]
[<ffffffffc0a7ce86>] btrfs_search_slot+0x3c6/0xb10 [btrfs]
[<ffffffffc0a975b9>] caching_thread+0x1b9/0x820 [btrfs]
[<ffffffffc0adfa36>] normal_work_helper+0xc6/0x340 [btrfs]
[<ffffffffc0adfd22>] btrfs_cache_helper+0x12/0x20 [btrfs]
IO requests that want to allocate space are waiting in cache_block_group()
to acquire the commit_root_sem in exclusive mode. But they only want to add
the caching control structure to the list of caching block-groups:
[<ffffffff817136c9>] schedule+0x29/0x70
[<ffffffff81716085>] rwsem_down_write_failed+0x145/0x320
[<ffffffff813a1ae3>] call_rwsem_down_write_failed+0x13/0x20
[<ffffffffc0a86d0b>] cache_block_group+0x25b/0x450 [btrfs]
[<ffffffffc0a94d36>] find_free_extent+0xd16/0xdb0 [btrfs]
[<ffffffffc0a94e7f>] btrfs_reserve_extent+0xaf/0x160 [btrfs]
Other caching threads want to continue their scanning, and for that they
are waiting to acquire commit_root_sem in shared mode. But since there are
IO threads that want the exclusive lock, the caching threads are unable
to continue the scanning, because (I presume) rw_semaphore guarantees some
fairness:
[<ffffffff817136c9>] schedule+0x29/0x70
[<ffffffff81715ee5>] rwsem_down_read_failed+0xc5/0x120
[<ffffffff813a1ab4>] call_rwsem_down_read_failed+0x14/0x30
[<ffffffffc0a975a1>] caching_thread+0x1a1/0x820 [btrfs]
[<ffffffffc0adfa36>] normal_work_helper+0xc6/0x340 [btrfs]
[<ffffffffc0adfd22>] btrfs_cache_helper+0x12/0x20 [btrfs]
[<ffffffff8108bd56>] process_one_work+0x146/0x410
This causes slowness of the IO, especially when there are many block groups
that need to be scanned for free space. In some cases it takes minutes
until a single IO thread is able to allocate free space.
I don't see a deadlock here, because the caching threads that were able to
acquire
the commit_root_sem will call rwsem_is_contended() and should give up the
semaphore,
so that IO threads are able to acquire it in exclusive mode.
However, introducing a separate mutex that protects only the list of caching
block groups makes things move forward much faster.
This patch is based on kernel 3.18.
Unfortunately, I am not able to submit a patch based on one of the latest
kernels, because
here btrfs is part of the larger system, and upgrading the kernel is a
significant effort.
Hence marking the patch as RFC.
Hopefully, this patch still has some value to the community.
Signed-off-by: Alex Lyakas <a...@zadarastorage.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 42d11e7..74feacb 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1490,6 +1490,8 @@ struct btrfs_fs_info {
struct list_head trans_list;
struct list_head dead_roots;
struct list_head caching_block_groups;
+ /* protects the above list */
+ struct mutex caching_block_groups_mutex;
spinlock_t delayed_iput_lock;
struct list_head delayed_iputs;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 5177954..130ec58 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2229,6 +2229,7 @@ int open_ctree(struct super_block *sb,
INIT_LIST_HEAD(&fs_info->delayed_iputs);
INIT_LIST_HEAD(&fs_info->delalloc_roots);
INIT_LIST_HEAD(&fs_info->caching_block_groups);
+ mutex_init(&fs_info->caching_block_groups_mutex);
spin_lock_init(&fs_info->delalloc_root_lock);
spin_lock_init(&fs_info->trans_lock);
spin_lock_init(&fs_info->fs_roots_radix_lock);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index a067065..906fb08 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -637,10 +637,10 @@ static int cache_block_group(struct
btrfs_block_group_cache *cache,
return 0;
}
- down_write(&fs_info->commit_root_sem);
+ mutex_lock(&fs_info->caching_block_groups_mutex);
atomic_inc(&caching_ctl->count);
list_add_tail(&caching_ctl->list, &fs_info->caching_block_groups);
- up_write(&fs_info->commit_root_sem);
+ mutex_unlock(&fs_info->caching_block_groups_mutex);
btrfs_get_block_group(cache);
@@ -5693,6 +5693,7 @@ void btrfs_prepare_extent_commit(struct
btrfs_trans_handle *trans,
down_write(&fs_info->commit_root_sem);
+ mutex_lock(&fs_info->caching_block_groups_mutex);
list_for_each_entry_safe(caching_ctl, next,
&fs_info->caching_block_groups, list) {
cache = caching_ctl->block_group;
@@ -5704,6 +5705,7 @@ void btrfs_prepare_extent_commit(struct
btrfs_trans_handle *trans,
cache->last_byte_to_unpin = caching_ctl->progress;
}
}
+ mutex_unlock(&fs_info->caching_block_groups_mutex);
if (fs_info->pinned_extents == &fs_info->freed_extents[0])
fs_info->pinned_extents = &fs_info->freed_extents[1];
@@ -8849,14 +8851,14 @@ int btrfs_free_block_groups(struct btrfs_fs_info
*info)
struct btrfs_caching_control *caching_ctl;
struct rb_node *n;
- down_write(&info->commit_root_sem);
+ mutex_lock(&info->caching_block_groups_mutex);
while (!list_empty(&info->caching_block_groups)) {
caching_ctl = list_entry(info->caching_block_groups.next,
struct btrfs_caching_control, list);
list_del(&caching_ctl->list);
put_caching_control(caching_ctl);
}
- up_write(&info->commit_root_sem);
+ mutex_unlock(&info->caching_block_groups_mutex);
spin_lock(&info->unused_bgs_lock);
while (!list_empty(&info->unused_bgs)) {
--
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