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authorQu Wenruo <quwenruo@cn.fujitsu.com>2016-07-20 15:04:18 +0800
committerChris Mason <clm@fb.com>2016-08-25 03:58:16 -0700
commitd8422ba334f9df16e071bc77707e55fd7f8446ae (patch)
treecfcef8f2e9b45840f48eec4e0a8fc57c952c5dbc
parent1c1ea4f781db9f754842b9c31d1eff400d17cddc (diff)
downloadlinux-stericsson-d8422ba334f9df16e071bc77707e55fd7f8446ae.tar.gz
btrfs: backref: Fix soft lockup in __merge_refs function
When over 1000 file extents refers to one extent, find_parent_nodes() will be obviously slow, due to the O(n^2)~O(n^3) loops inside __merge_refs(). The following ftrace shows the cubic growth of execution time: 256 refs 5) + 91.768 us | __add_keyed_refs.isra.12 [btrfs](); 5) 1.447 us | __add_missing_keys.isra.13 [btrfs](); 5) ! 114.544 us | __merge_refs [btrfs](); 5) ! 136.399 us | __merge_refs [btrfs](); 512 refs 6) ! 279.859 us | __add_keyed_refs.isra.12 [btrfs](); 6) 3.164 us | __add_missing_keys.isra.13 [btrfs](); 6) ! 442.498 us | __merge_refs [btrfs](); 6) # 2091.073 us | __merge_refs [btrfs](); and 1024 refs 7) ! 368.683 us | __add_keyed_refs.isra.12 [btrfs](); 7) 4.810 us | __add_missing_keys.isra.13 [btrfs](); 7) # 2043.428 us | __merge_refs [btrfs](); 7) * 18964.23 us | __merge_refs [btrfs](); And sort them into the following char: (Unit: us) ------------------------------------------------------------------------ Trace function | 256 ref | 512 refs | 1024 refs | ------------------------------------------------------------------------ __add_keyed_refs | 91 | 249 | 368 | __add_missing_keys | 1 | 3 | 4 | __merge_refs 1st call | 114 | 442 | 2043 | __merge_refs 2nd call | 136 | 2091 | 18964 | ------------------------------------------------------------------------ We can see the that __add_keyed_refs() grows almost in linear behavior. And __add_missing_keys() in this case doesn't change much or takes much time. While for the 1st __merge_refs() it's square growth for the 2nd __merge_refs() call it's cubic growth. It's no doubt that merge_refs() will take a long long time to execute if the number of refs continues its grows. So add a cond_resced() into the loop of __merge_refs(). Although this will solve the problem of soft lockup, we need to use the new rb_tree based structure introduced by Lu Fengqi to really solve the long execution time. Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com> Signed-off-by: Chris Mason <clm@fb.com>
-rw-r--r--fs/btrfs/backref.c1
1 files changed, 1 insertions, 0 deletions
diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index 2b88439c2ee8..455a6b2fd539 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -589,6 +589,7 @@ static void __merge_refs(struct list_head *head, int mode)
list_del(&ref2->list);
kmem_cache_free(btrfs_prelim_ref_cache, ref2);
+ cond_resched();
}
}