[PATCH] prune_icache_sb

[Wendy Cheng <wcheng@redhat.com>]

There seems to have a need to prune inode cache entries for specific 
mount points (per vfs superblock) due to performance issues found after 
some io intensive commands ("rsyn" for example).  The problem is 
particularly serious for one of our kernel modules where it caches its 
(cluster) locks based on vfs inode implementation. These locks are 
created by inode creation call and get purged when s_op->clear_inode() 
is invoked. With larger servers that equipped with plenty of memory, the 
page dirty ratio may not pass the threshold to trigger VM reclaim logic 
but the accumulated inode counts (and its associated cluster locks) 
could causes unacceptable performance degradation for latency sensitive 
applications.

After adding the uploaded inode trimming patch, together with 
shrink_dcache_sb(), we are able to keep the latency for one real world 
application within a satisfactory bound (consistently stayed within 5 
seconds, compared to the original fluctuation between 5 to 16 seconds). 
The calls are placed in one of our kernel daemons that wakes up in a 
tunable interval to do the trimming work as shown in the following code 
segment. Would appreciate if this patch can get accepted into mainline 
kernel.

                 i_percent = sdp->sd_tune.gt_inoded_purge;
                 if (i_percent) {
                         if (i_percent > 100) i_percent = 100;
                         a_count = atomic_read(&sdp->sd_inode_count);
                         i_count = a_count * i_percent / 100;
                         (void) shrink_dcache_sb(sdp->sd_vfs);
                         (void) prune_icache_sb(i_count, sdp->sd_vfs);
                  }

-- Wendy




Signed-off-by: S. Wendy Cheng <wcheng@redhat.com>

 fs/inode.c         |   14 +++++++++++---
 include/linux/fs.h |    3 ++-
 2 files changed, 13 insertions(+), 4 deletions(-)

--- linux-2.6.18/include/linux/fs.h	2006-09-19 23:42:06.000000000 -0400
+++ ups-kernel/include/linux/fs.h	2006-11-22 13:55:55.000000000 -0500
@@ -1625,7 +1625,8 @@ extern void remove_inode_hash(struct ino
 static inline void insert_inode_hash(struct inode *inode) {
 	__insert_inode_hash(inode, inode->i_ino);
 }
-
+extern void prune_icache_sb(int nr_to_scan, struct super_block *sb);
+ 
 extern struct file * get_empty_filp(void);
 extern void file_move(struct file *f, struct list_head *list);
 extern void file_kill(struct file *f);
--- linux-2.6.18/fs/inode.c	2006-09-19 23:42:06.000000000 -0400
+++ ups-kernel/fs/inode.c	2006-11-22 14:12:28.000000000 -0500
@@ -411,7 +411,7 @@ static int can_unuse(struct inode *inode
  * If the inode has metadata buffers attached to mapping->private_list then
  * try to remove them.
  */
-static void prune_icache(int nr_to_scan)
+static void __prune_icache(int nr_to_scan, struct super_block *sb)
 {
 	LIST_HEAD(freeable);
 	int nr_pruned = 0;
@@ -428,7 +428,8 @@ static void prune_icache(int nr_to_scan)
 
 		inode = list_entry(inode_unused.prev, struct inode, i_list);
 
-		if (inode->i_state || atomic_read(&inode->i_count)) {
+		if (inode->i_state || atomic_read(&inode->i_count) 
+			|| (sb && (inode->i_sb != sb))) {
 			list_move(&inode->i_list, &inode_unused);
 			continue;
 		}
@@ -461,6 +462,13 @@ static void prune_icache(int nr_to_scan)
 	mutex_unlock(&iprune_mutex);
 }
 
+void prune_icache_sb(int nr_to_scan, struct super_block *sb)
+{
+	__prune_icache(nr_to_scan, sb);
+}
+
+EXPORT_SYMBOL(prune_icache_sb);
+
 /*
  * shrink_icache_memory() will attempt to reclaim some unused inodes.  Here,
  * "unused" means that no dentries are referring to the inodes: the files are
@@ -480,7 +488,7 @@ static int shrink_icache_memory(int nr, 
 	 	 */
 		if (!(gfp_mask & __GFP_FS))
 			return -1;
-		prune_icache(nr);
+		__prune_icache(nr, NULL);
 	}
 	return (inodes_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
 }