writeback: make the super_block pinning more efficient

Currently we pin the inode->i_sb for every single inode. This
increases cache traffic on sb->s_umount sem. Lets instead
cache the inode sb pin state and keep the super_block pinned
for as long as keep writing out inodes from the same
super_block.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 225c7316344e676fdaf700f3ea3638ecbc2126a7..c6bf775e641a45422b45525b7ea3a5c3c3d2a4c8 100644 (file)
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -540,6 +540,17 @@ writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
        return ret;
 }
 
+static void unpin_sb_for_writeback(struct super_block **psb)
+{
+       struct super_block *sb = *psb;
+
+       if (sb) {
+               up_read(&sb->s_umount);
+               put_super(sb);
+               *psb = NULL;
+       }
+}
+
 /*
  * For WB_SYNC_NONE writeback, the caller does not have the sb pinned
  * before calling writeback. So make sure that we do pin it, so it doesn't
@@ -549,10 +560,19 @@ writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
  * 1 if we failed.
  */
 static int pin_sb_for_writeback(struct writeback_control *wbc,
-                                  struct inode *inode)
+                               struct inode *inode, struct super_block **psb)
 {
        struct super_block *sb = inode->i_sb;
 
+       /*
+        * If this sb is already pinned, nothing more to do. If not and
+        * *psb is non-NULL, unpin the old one first
+        */
+       if (sb == *psb)
+               return 0;
+       else if (*psb)
+               unpin_sb_for_writeback(psb);
+
        /*
         * Caller must already hold the ref for this
         */
@@ -566,7 +586,7 @@ static int pin_sb_for_writeback(struct writeback_control *wbc,
        if (down_read_trylock(&sb->s_umount)) {
                if (sb->s_root) {
                        spin_unlock(&sb_lock);
-                       return 0;
+                       goto pinned;
                }
                /*
                 * umounted, drop rwsem again and fall through to failure
@@ -577,24 +597,15 @@ static int pin_sb_for_writeback(struct writeback_control *wbc,
        sb->s_count--;
        spin_unlock(&sb_lock);
        return 1;
-}
-
-static void unpin_sb_for_writeback(struct writeback_control *wbc,
-                                  struct inode *inode)
-{
-       struct super_block *sb = inode->i_sb;
-
-       if (wbc->sync_mode == WB_SYNC_ALL)
-               return;
-
-       up_read(&sb->s_umount);
-       put_super(sb);
+pinned:
+       *psb = sb;
+       return 0;
 }
 
 static void writeback_inodes_wb(struct bdi_writeback *wb,
                                struct writeback_control *wbc)
 {
-       struct super_block *sb = wbc->sb;
+       struct super_block *sb = wbc->sb, *pin_sb = NULL;
        const int is_blkdev_sb = sb_is_blkdev_sb(sb);
        const unsigned long start = jiffies;    /* livelock avoidance */
 
@@ -653,7 +664,7 @@ static void writeback_inodes_wb(struct bdi_writeback *wb,
                if (inode_dirtied_after(inode, start))
                        break;
 
-               if (pin_sb_for_writeback(wbc, inode)) {
+               if (pin_sb_for_writeback(wbc, inode, &pin_sb)) {
                        requeue_io(inode);
                        continue;
                }
@@ -662,7 +673,6 @@ static void writeback_inodes_wb(struct bdi_writeback *wb,
                __iget(inode);
                pages_skipped = wbc->pages_skipped;
                writeback_single_inode(inode, wbc);
-               unpin_sb_for_writeback(wbc, inode);
                if (wbc->pages_skipped != pages_skipped) {
                        /*
                         * writeback is not making progress due to locked
@@ -682,6 +692,8 @@ static void writeback_inodes_wb(struct bdi_writeback *wb,
                        wbc->more_io = 1;
        }
 
+       unpin_sb_for_writeback(&pin_sb);
+
        spin_unlock(&inode_lock);
        /* Leave any unwritten inodes on b_io */
 }