netdev - [PATCH] vfs: avoid taking locks if inode not in lists

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Date:	Wed, 27 Jul 2011 17:21:05 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	David Miller <davem@...emloft.net>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	Anton Blanchard <anton@...ba.org>, npiggin@...nel.dk,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: [PATCH] vfs: avoid taking locks if inode not in lists

Le mardi 26 juillet 2011 à 11:36 +0200, Eric Dumazet a écrit :
> Le mardi 26 juillet 2011 à 05:03 -0400, Christoph Hellwig a écrit :
> > On Tue, Jul 26, 2011 at 10:21:06AM +0200, Eric Dumazet wrote:
> > > Well, not 'last' contention point, as we still hit remove_inode_hash(),
> > 
> > There should be no ned to put pipe or anon inodes on the inode hash.
> > Probably sockets don't need it either, but I'd need to look at it in
> > detail.
> > 
> > > inode_wb_list_del()
> > 
> > The should never be on the wb list either, doing an unlocked check for
> > actually beeing on the list before taking the lock should help you.
> 
> Yes, it might even help regular inodes ;)
> 
> > 
> > > inode_lru_list_del(),
> > 
> > No real need to keep inodes in the LRU if we only allocate them using
> > new_inode but never look them up either.  You might want to try setting
> > .drop_inode to generic_delete_inode for these.
> 
> Yes, I'll take a look, thanks.

If I am not mistaken, we can add unlocked checks on the three hot spots.

After following patch, a close(socket(PF_INET, SOCK_DGRAM, 0)) pair on
my dev machine takes ~3us instead of ~9us.

Maybe its better to split it in three patches, just let me know.

22us -> 3us, thats a nice patch series ;)

Thanks

[PATCH] vfs: avoid taking locks if inode not in lists

sockets and pipes inodes destruction hits three possibly contended
locks :

system-wide inode_hash_lock in remove_inode_hash()
superblock s_inode_lru_lock in inode_lru_list_del()
bdi wb.list_lock in inode_wb_list_del()

Before even taking locks, we can perform an unlocked test to check if
inode can possibly be in the lists.

On a 2x4x2 machine, a close(socket()) pair can be 200% faster with these
changes.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 fs/fs-writeback.c |   10 ++++++----
 fs/inode.c        |    6 ++++++
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 1599aa9..8b90bdb 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -182,11 +182,13 @@ void bdi_start_background_writeback(struct backing_dev_info *bdi)
  */
 void inode_wb_list_del(struct inode *inode)
 {
-	struct backing_dev_info *bdi = inode_to_bdi(inode);
+	if (!list_empty(&inode->i_wb_list)) {
+		struct backing_dev_info *bdi = inode_to_bdi(inode);
 
-	spin_lock(&bdi->wb.list_lock);
-	list_del_init(&inode->i_wb_list);
-	spin_unlock(&bdi->wb.list_lock);
+		spin_lock(&bdi->wb.list_lock);
+		list_del_init(&inode->i_wb_list);
+		spin_unlock(&bdi->wb.list_lock);
+	}
 }
 
 /*
diff --git a/fs/inode.c b/fs/inode.c
index d0c72ff..796a420 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -338,6 +338,9 @@ static void inode_lru_list_add(struct inode *inode)
 
 static void inode_lru_list_del(struct inode *inode)
 {
+	if (list_empty(&inode->i_lru))
+		return;
+
 	spin_lock(&inode->i_sb->s_inode_lru_lock);
 	if (!list_empty(&inode->i_lru)) {
 		list_del_init(&inode->i_lru);
@@ -406,6 +409,9 @@ EXPORT_SYMBOL(__insert_inode_hash);
  */
 void remove_inode_hash(struct inode *inode)
 {
+	if (inode_unhashed(inode))
+		return;
+
 	spin_lock(&inode_hash_lock);
 	spin_lock(&inode->i_lock);
 	hlist_del_init(&inode->i_hash);


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