Re: [RFC v4 Patch 0/4] fs/inode.c: optimization for inode lock usage

From: Dave Chinner
Date: Mon Sep 24 2012 - 04:26:58 EST

On Mon, Sep 24, 2012 at 03:08:52PM +0800, Guo Chao wrote:
> On Mon, Sep 24, 2012 at 04:28:12PM +1000, Dave Chinner wrote:
> > > Ah, this is intended to be a code clean patchset actually. I thought these
> > > locks are redundant in an obvious and trivial manner. If, on the contrary,
> > > they are such tricky, then never mind :) Thanks for your patient.
> >
> > The RCU conversion is actually trivial - everything is already set
> > up for it to be done, and is simpler than this patch set. It pretty
> > much is simply replacing all the read side inode_hash_lock pairs
> > with rcu_read_lock()/rcu_read_unlock() pairs. Like I said, if you
> > want to clean up this code, then RCU traversals are the conversion
> > to make.
> >
>
> Thanks for your suggestion. Though I doubt it's such trivial, I will try this
> after a little investigation.

Probably best to start with the patch below - it's run under heavy
concurrent load on ext4 with a working set of inodes about 20x
larger than can fit in memory for the past hour....

Cheers,

Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx

fs: Use RCU lookups for inode cache

From: Dave Chinner <dchinner@xxxxxxxxxx>

Convert inode cache lookups to be protected by RCU locking rather
than the global inode_hash_lock. This will improve scalability of
inode lookup intensive workloads.

Smoke tested w/ ext4 on concurrent fsmark/lookup/unlink workloads
over 50 million or so inodes...

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
fs/inode.c | 74 ++++++++++++++++++++++++++++++++++--------------------------
1 file changed, 42 insertions(+), 32 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index ac8d904..2e92674 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -464,7 +464,7 @@ void __insert_inode_hash(struct inode *inode, unsigned long hashval)

spin_lock(&inode_hash_lock);
spin_lock(&inode->i_lock);
- hlist_add_head(&inode->i_hash, b);
+ hlist_add_head_rcu(&inode->i_hash, b);
spin_unlock(&inode->i_lock);
spin_unlock(&inode_hash_lock);
}
@@ -480,7 +480,7 @@ void __remove_inode_hash(struct inode *inode)
{
spin_lock(&inode_hash_lock);
spin_lock(&inode->i_lock);
- hlist_del_init(&inode->i_hash);
+ hlist_del_init_rcu(&inode->i_hash);
spin_unlock(&inode->i_lock);
spin_unlock(&inode_hash_lock);
}
@@ -783,14 +783,19 @@ static void __wait_on_freeing_inode(struct inode *inode);
static struct inode *find_inode(struct super_block *sb,
struct hlist_head *head,
int (*test)(struct inode *, void *),
- void *data)
+ void *data, bool locked)
{
struct hlist_node *node;
struct inode *inode = NULL;

repeat:
- hlist_for_each_entry(inode, node, head, i_hash) {
+ rcu_read_lock();
+ hlist_for_each_entry_rcu(inode, node, head, i_hash) {
spin_lock(&inode->i_lock);
+ if (inode_unhashed(inode)) {
+ spin_unlock(&inode->i_lock);
+ continue;
+ }
if (inode->i_sb != sb) {
spin_unlock(&inode->i_lock);
continue;
@@ -800,13 +805,20 @@ repeat:
continue;
}
if (inode->i_state & (I_FREEING|I_WILL_FREE)) {
+ rcu_read_unlock();
+ if (locked)
+ spin_unlock(&inode_hash_lock);
__wait_on_freeing_inode(inode);
+ if (locked)
+ spin_lock(&inode_hash_lock);
goto repeat;
}
__iget(inode);
spin_unlock(&inode->i_lock);
+ rcu_read_unlock();
return inode;
}
+ rcu_read_unlock();
return NULL;
}

@@ -815,14 +827,20 @@ repeat:
* iget_locked for details.
*/
static struct inode *find_inode_fast(struct super_block *sb,
- struct hlist_head *head, unsigned long ino)
+ struct hlist_head *head,
+ unsigned long ino, bool locked)
{
struct hlist_node *node;
struct inode *inode = NULL;

repeat:
- hlist_for_each_entry(inode, node, head, i_hash) {
+ rcu_read_lock();
+ hlist_for_each_entry_rcu(inode, node, head, i_hash) {
spin_lock(&inode->i_lock);
+ if (inode_unhashed(inode)) {
+ spin_unlock(&inode->i_lock);
+ continue;
+ }
if (inode->i_ino != ino) {
spin_unlock(&inode->i_lock);
continue;
@@ -832,13 +850,20 @@ repeat:
continue;
}
if (inode->i_state & (I_FREEING|I_WILL_FREE)) {
+ rcu_read_unlock();
+ if (locked)
+ spin_unlock(&inode_hash_lock);
__wait_on_freeing_inode(inode);
+ if (locked)
+ spin_lock(&inode_hash_lock);
goto repeat;
}
__iget(inode);
spin_unlock(&inode->i_lock);
+ rcu_read_unlock();
return inode;
}
+ rcu_read_unlock();
return NULL;
}

@@ -995,9 +1020,7 @@ struct inode *iget5_locked(struct super_block *sb, unsigned long hashval,
struct hlist_head *head = inode_hashtable + hash(sb, hashval);
struct inode *inode;

- spin_lock(&inode_hash_lock);
- inode = find_inode(sb, head, test, data);
- spin_unlock(&inode_hash_lock);
+ inode = find_inode(sb, head, test, data, false);

if (inode) {
wait_on_inode(inode);
@@ -1009,8 +1032,7 @@ struct inode *iget5_locked(struct super_block *sb, unsigned long hashval,
struct inode *old;

spin_lock(&inode_hash_lock);
- /* We released the lock, so.. */
- old = find_inode(sb, head, test, data);
+ old = find_inode(sb, head, test, data, true);
if (!old) {
if (set(inode, data))
goto set_failed;
@@ -1065,9 +1087,7 @@ struct inode *iget_locked(struct super_block *sb, unsigned long ino)
struct hlist_head *head = inode_hashtable + hash(sb, ino);
struct inode *inode;

- spin_lock(&inode_hash_lock);
- inode = find_inode_fast(sb, head, ino);
- spin_unlock(&inode_hash_lock);
+ inode = find_inode_fast(sb, head, ino, false);
if (inode) {
wait_on_inode(inode);
return inode;
@@ -1078,8 +1098,7 @@ struct inode *iget_locked(struct super_block *sb, unsigned long ino)
struct inode *old;

spin_lock(&inode_hash_lock);
- /* We released the lock, so.. */
- old = find_inode_fast(sb, head, ino);
+ old = find_inode_fast(sb, head, ino, true);
if (!old) {
inode->i_ino = ino;
spin_lock(&inode->i_lock);
@@ -1122,14 +1141,15 @@ static int test_inode_iunique(struct super_block *sb, unsigned long ino)
struct hlist_node *node;
struct inode *inode;

- spin_lock(&inode_hash_lock);
+ rcu_read_lock();
hlist_for_each_entry(inode, node, b, i_hash) {
- if (inode->i_ino == ino && inode->i_sb == sb) {
- spin_unlock(&inode_hash_lock);
+ if (inode->i_ino == ino && inode->i_sb == sb &&
+ !inode_unhashed(inode)) {
+ rcu_read_unlock();
return 0;
}
}
- spin_unlock(&inode_hash_lock);
+ rcu_read_unlock();

return 1;
}
@@ -1210,13 +1230,8 @@ struct inode *ilookup5_nowait(struct super_block *sb, unsigned long hashval,
int (*test)(struct inode *, void *), void *data)
{
struct hlist_head *head = inode_hashtable + hash(sb, hashval);
- struct inode *inode;
-
- spin_lock(&inode_hash_lock);
- inode = find_inode(sb, head, test, data);
- spin_unlock(&inode_hash_lock);

- return inode;
+ return find_inode(sb, head, test, data, false);
}
EXPORT_SYMBOL(ilookup5_nowait);

@@ -1261,10 +1276,7 @@ struct inode *ilookup(struct super_block *sb, unsigned long ino)
struct hlist_head *head = inode_hashtable + hash(sb, ino);
struct inode *inode;

- spin_lock(&inode_hash_lock);
- inode = find_inode_fast(sb, head, ino);
- spin_unlock(&inode_hash_lock);
-
+ inode = find_inode_fast(sb, head, ino, false);
if (inode)
wait_on_inode(inode);
return inode;
@@ -1711,10 +1723,8 @@ static void __wait_on_freeing_inode(struct inode *inode)
wq = bit_waitqueue(&inode->i_state, __I_NEW);
prepare_to_wait(wq, &wait.wait, TASK_UNINTERRUPTIBLE);
spin_unlock(&inode->i_lock);
- spin_unlock(&inode_hash_lock);
schedule();
finish_wait(wq, &wait.wait);
- spin_lock(&inode_hash_lock);
}

static __initdata unsigned long ihash_entries;
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