ext4: fix fio regression

We (Linux Kernel Performance project) found a regression introduced
by commit:

  f7fec032aa ext4: track all extent status in extent status tree

The commit causes about 20% performance decrease in fio random write
test. Profiler shows that rb_next() uses a lot of CPU time. The call
stack is:

  rb_next
  ext4_es_find_delayed_extent
  ext4_map_blocks
  _ext4_get_block
  ext4_get_block_write
  __blockdev_direct_IO
  ext4_direct_IO
  generic_file_direct_write
  __generic_file_aio_write
  ext4_file_write
  aio_rw_vect_retry
  aio_run_iocb
  do_io_submit
  sys_io_submit
  system_call_fastpath
  io_submit
  td_io_getevents
  io_u_queued_complete
  thread_main
  main
  __libc_start_main

The cause is that ext4_es_find_delayed_extent() doesn't have an
upper bound, it keeps searching until a delayed extent is found.
When there are a lots of non-delayed entries in the extent state
tree, ext4_es_find_delayed_extent() may uses a lot of CPU time.

Reported-by: LKP project <lkp@linux.intel.com>
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 12 insertions, 5 deletions

diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
index fe3337a85ede..e6941e622d31 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c
@@ -232,14 +232,16 @@ static struct extent_status *__es_tree_search(struct rb_root *root,
 }
 
 /*
- * ext4_es_find_delayed_extent: find the 1st delayed extent covering @es->lblk
- * if it exists, otherwise, the next extent after @es->lblk.
+ * ext4_es_find_delayed_extent_range: find the 1st delayed extent covering
+ * @es->lblk if it exists, otherwise, the next extent after @es->lblk.
  *
  * @inode: the inode which owns delayed extents
  * @lblk: the offset where we start to search
+ * @end: the offset where we stop to search
  * @es: delayed extent that we found
  */
-void ext4_es_find_delayed_extent(struct inode *inode, ext4_lblk_t lblk,
+void ext4_es_find_delayed_extent_range(struct inode *inode,
+                                 ext4_lblk_t lblk, ext4_lblk_t end,
                                  struct extent_status *es)
 {
         struct ext4_es_tree *tree = NULL;
@@ -247,7 +249,8 @@ void ext4_es_find_delayed_extent(struct inode *inode, ext4_lblk_t lblk,
         struct rb_node *node;
 
         BUG_ON(es == NULL);
-        trace_ext4_es_find_delayed_extent_enter(inode, lblk);
+        BUG_ON(end < lblk);
+        trace_ext4_es_find_delayed_extent_range_enter(inode, lblk);
 
         read_lock(&EXT4_I(inode)->i_es_lock);
         tree = &EXT4_I(inode)->i_es_tree;
@@ -270,6 +273,10 @@ out:
         if (es1 && !ext4_es_is_delayed(es1)) {
                 while ((node = rb_next(&es1->rb_node)) != NULL) {
                         es1 = rb_entry(node, struct extent_status, rb_node);
+                        if (es1->es_lblk > end) {
+                                es1 = NULL;
+                                break;
+                        }
                         if (ext4_es_is_delayed(es1))
                                 break;
                 }
@@ -285,7 +292,7 @@ out:
         read_unlock(&EXT4_I(inode)->i_es_lock);
 
         ext4_es_lru_add(inode);
-        trace_ext4_es_find_delayed_extent_exit(inode, es);
+        trace_ext4_es_find_delayed_extent_range_exit(inode, es);
 }
 
 static struct extent_status *