ext4: address scalability issue by removing extent cache statistics

commit 9cd70b34 upstream.

Andi Kleen and Tim Chen have reported that under certain circumstances
the extent cache statistics are causing scalability problems due to
cache line bounces.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/ext4/ext4.h

@@ -1185,9 +1185,6 @@ struct ext4_sb_info {
 	unsigned long s_ext_blocks;
 	unsigned long s_ext_extents;
 #endif
-	/* ext4 extent cache stats */
-	unsigned long extent_cache_hits;
-	unsigned long extent_cache_misses;
 
 	/* for buddy allocator */
 	struct ext4_group_info ***s_group_info;

fs/ext4/extents.c

@@ -2052,10 +2052,6 @@ static int ext4_ext_check_cache(struct inode *inode, ext4_lblk_t block,
 		ret = 1;
 	}
 errout:
-	if (!ret)
-		sbi->extent_cache_misses++;
-	else
-		sbi->extent_cache_hits++;
 	trace_ext4_ext_in_cache(inode, block, ret);
 	spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
 	return ret;

fs/ext4/super.c

@@ -2504,18 +2504,6 @@ static ssize_t lifetime_write_kbytes_show(struct ext4_attr *a,
 			  EXT4_SB(sb)->s_sectors_written_start) >> 1)));
 }
 
-static ssize_t extent_cache_hits_show(struct ext4_attr *a,
-				      struct ext4_sb_info *sbi, char *buf)
-{
-	return snprintf(buf, PAGE_SIZE, "%lu\n", sbi->extent_cache_hits);
-}
-
-static ssize_t extent_cache_misses_show(struct ext4_attr *a,
-					struct ext4_sb_info *sbi, char *buf)
-{
-	return snprintf(buf, PAGE_SIZE, "%lu\n", sbi->extent_cache_misses);
-}
-
 static ssize_t inode_readahead_blks_store(struct ext4_attr *a,
 					  struct ext4_sb_info *sbi,
 					  const char *buf, size_t count)
@@ -2573,8 +2561,6 @@ static struct ext4_attr ext4_attr_##name = __ATTR(name, mode, show, store)
 EXT4_RO_ATTR(delayed_allocation_blocks);
 EXT4_RO_ATTR(session_write_kbytes);
 EXT4_RO_ATTR(lifetime_write_kbytes);
-EXT4_RO_ATTR(extent_cache_hits);
-EXT4_RO_ATTR(extent_cache_misses);
 EXT4_ATTR_OFFSET(inode_readahead_blks, 0644, sbi_ui_show,
 		 inode_readahead_blks_store, s_inode_readahead_blks);
 EXT4_RW_ATTR_SBI_UI(inode_goal, s_inode_goal);
@@ -2590,8 +2576,6 @@ static struct attribute *ext4_attrs[] = {
 	ATTR_LIST(delayed_allocation_blocks),
 	ATTR_LIST(session_write_kbytes),
 	ATTR_LIST(lifetime_write_kbytes),
-	ATTR_LIST(extent_cache_hits),
-	ATTR_LIST(extent_cache_misses),
 	ATTR_LIST(inode_readahead_blks),
 	ATTR_LIST(inode_goal),
 	ATTR_LIST(mb_stats),