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 <[email protected]>
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;
}
--
David Kleikamp
IBM Linux Technology Center
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 <[email protected]>
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 <[email protected]>
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
On Mon, Sep 18, 2006 at 06:38:05PM -0500, Dave Kleikamp wrote:
> 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 <[email protected]>
>
> 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.
(Sorry for the delay; been on vacation.)
Heh, I ran into the same lock nesting issues as you when I first tried
to fix this; the lock debugging code found it for me. I asked for
feedback on the locking issue when I submitted the patch, but no one
had any opinions then, so I chose consistency over possible
contention. Al Viro snorted at the idea of consistency in the results
of statfs (I paraphrase his IRC remarks), and thinking about it
further, I realized the debug code should not be doing these checks in
statfs anyway; only on mount and unmount. This is because it appears
that the block group accounting and the overall fs accounting are done
non-atomically - see group_reserve_blocks() for example - and part of
what the code does is reconcile these two numbers. It is legal for a
valid fs to have the block group summaries and the fs-wide summaries
out of sync, so the debug code could erroneously report an error,
leading some poor soul on a wild goose chase. Removing this code from
statfs also happens to fix the locking issues nicely.
Rewriting this has been on my todo list for about 6 months now -
anyone interested in grabbing it? I'm on #linuxfs on irc.oftc.net if
anyone wants to chat about it.
-VAL