Re: [PATCH] EXT2: Remove superblock lock contention in ext2_statfs

Dave Kleikamp
2006-09-18T23:38:19Z

On Mon, 2006-09-18 at 15:36 -0500, Dave Kleikamp wrote:
> EXT2: Remove superblock lock contention in ext2_statfs
>
> Fix a performance degradation introduced in 2.6.17. (30% degradation running
> dbench with 16 threads)
>
> Patch 21730eed11de42f22afcbd43f450a1872a0b5ea1, which claims to make
> EXT2_DEBUG work again, moves the taking of the kernel lock out of debug-only
> code in ext2_count_free_inodes and ext2_count_free_blocks and into
> ext2_statfs. This patch reverses that part of the patch.
>
> Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>

Eric Sandeen pointed out to me that taking the superblock lock in
ext2_count_free_* will cause a deadlock when EXT2FS_DEBUG is enabled,
since the superblock is locked in write_super().

We found that the same problem was fixed in ext3 with this patch
(forgive the long link):
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5b11687924e40790deb0d5f959247ade82196665;hp=2384f55f8aa520172c995965bd2f8a9740d53095

The patch below just removes the use of the superblock lock in the debug
code.

> diff --git a/fs/ext2/balloc.c b/fs/ext2/balloc.c
> index d487043..fddefff 100644
> --- a/fs/ext2/balloc.c
> +++ b/fs/ext2/balloc.c
> @@ -539,7 +539,6 @@ unsigned long ext2_count_free (struct bu
>
> #endif /* EXT2FS_DEBUG */
>
> -/* Superblock must be locked */
> unsigned long ext2_count_free_blocks (struct super_block * sb)
> {
> struct ext2_group_desc * desc;
> @@ -549,6 +548,7 @@ #ifdef EXT2FS_DEBUG
> unsigned long bitmap_count, x;
> struct ext2_super_block *es;
>
> + lock_super(sb);
> es = EXT2_SB(sb)->s_es;
> desc_count = 0;
> bitmap_count = 0;
> @@ -572,6 +572,7 @@ #ifdef EXT2FS_DEBUG
> printk("ext2_count_free_blocks: stored = %lu, computed = %lu, %lu\n",
> (long)le32_to_cpu(es->s_free_blocks_count),
> desc_count, bitmap_count);
> + unlock_super(sb);
> return bitmap_count;
> #else
> for (i = 0; i < EXT2_SB(sb)->s_groups_count; i++) {
> diff --git a/fs/ext2/ialloc.c b/fs/ext2/ialloc.c
> index de85c61..5d1d1c9 100644
> --- a/fs/ext2/ialloc.c
> +++ b/fs/ext2/ialloc.c
> @@ -637,7 +637,6 @@ fail:
> return ERR_PTR(err);
> }
>
> -/* Superblock must be locked */
> unsigned long ext2_count_free_inodes (struct super_block * sb)
> {
> struct ext2_group_desc *desc;
> @@ -649,6 +648,7 @@ #ifdef EXT2FS_DEBUG
> unsigned long bitmap_count = 0;
> struct buffer_head *bitmap_bh = NULL;
>
> + lock_super(sb);
> es = EXT2_SB(sb)->s_es;
> for (i = 0; i < EXT2_SB(sb)->s_groups_count; i++) {
> unsigned x;
> @@ -671,6 +671,7 @@ #ifdef EXT2FS_DEBUG
> printk("ext2_count_free_inodes: stored = %lu, computed = %lu, %lu\n",
> percpu_counter_read(&EXT2_SB(sb)->s_freeinodes_counter),
> desc_count, bitmap_count);
> + unlock_super(sb);
> return desc_count;
> #else
> for (i = 0; i < EXT2_SB(sb)->s_groups_count; i++) {
> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> index ca5bfb6..4286ff6 100644
> --- a/fs/ext2/super.c
> +++ b/fs/ext2/super.c
> @@ -1083,7 +1083,6 @@ static int ext2_statfs (struct dentry *
> unsigned long overhead;
> int i;
>
> - lock_super(sb);
> if (test_opt (sb, MINIX_DF))
> overhead = 0;
> else {
> @@ -1124,7 +1123,6 @@ static int ext2_statfs (struct dentry *
> buf->f_files = le32_to_cpu(sbi->s_es->s_inodes_count);
> buf->f_ffree = ext2_count_free_inodes (sb);
> buf->f_namelen = EXT2_NAME_LEN;
> - unlock_super(sb);
> return 0;
> }
>
EXT2: Remove superblock lock contention in ext2_statfs

Fix a performance degradation introduced in 2.6.17. (30% degradation
running
dbench with 16 threads)

Patch 21730eed11de42f22afcbd43f450a1872a0b5ea1, which claims to make
EXT2_DEBUG work again, moves the taking of the kernel lock out of
debug-only
code in ext2_count_free_inodes and ext2_count_free_blocks and into
ext2_statfs.

The same problem was fixed in ext3 by removing the lock completely
(patch 5b11687924e40790deb0d5f959247ade82196665)

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>

diff --git a/fs/ext2/balloc.c b/fs/ext2/balloc.c
index d487043..b1981d0 100644
--- a/fs/ext2/balloc.c
+++ b/fs/ext2/balloc.c
@@ -539,7 +539,6 @@ unsigned long ext2_count_free (struct bu

#endif /* EXT2FS_DEBUG */

-/* Superblock must be locked */
unsigned long ext2_count_free_blocks (struct super_block * sb)
{
struct ext2_group_desc * desc;
diff --git a/fs/ext2/ialloc.c b/fs/ext2/ialloc.c
index de85c61..695f69c 100644
--- a/fs/ext2/ialloc.c
+++ b/fs/ext2/ialloc.c
@@ -637,7 +637,6 @@ fail:
return ERR_PTR(err);
}

-/* Superblock must be locked */
unsigned long ext2_count_free_inodes (struct super_block * sb)
{
struct ext2_group_desc *desc;
diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index ca5bfb6..4286ff6 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -1083,7 +1083,6 @@ static int ext2_statfs (struct dentry *
unsigned long overhead;
int i;

- lock_super(sb);
if (test_opt (sb, MINIX_DF))
overhead = 0;
else {
@@ -1124,7 +1123,6 @@ static int ext2_statfs (struct dentry *
buf->f_files = le32_to_cpu(sbi->s_es->s_inodes_count);
buf->f_ffree = ext2_count_free_inodes (sb);
buf->f_namelen = EXT2_NAME_LEN;
- unlock_super(sb);
return 0;
}

--
David Kleikamp
IBM Linux Technology Center