ext4: improve fast_commit performance and scalability

Currently ext4_fc_commit_dentry_updates() is of quadratic time
complexity, which is causing performance bottlenecks with high
threads/file/dir count with fs_mark.

This patch makes commit dentry updates (and hence ext4_fc_commit()) path
to linear time complexity. Hence improves the performance of workloads
which does fsync on multiple threads/open files one-by-one.

Absolute numbers in avg file creates per sec (from fs_mark in 1K order)
=======================================================================
no.     Order   without-patch(K)   with-patch(K)   Diff(%)
1       1        16.90              17.51           +3.60
2       2,2      32.08              31.80           -0.87
3       3,3      53.97              55.01           +1.92
4       4,4      78.94              76.90           -2.58
5       5,5      95.82              95.37           -0.46
6       6,6      87.92              103.38          +17.58
7       6,10      0.73              126.13          +17178.08
8       6,14      2.33              143.19          +6045.49

workload type
==============
For e.g. 7th row order of 6,10 (2^6 == 64 && 2^10 == 1024)
echo /run/riteshh/mnt/{1..64} |sed -E 's/[[:space:]]+/ -d /g' \
  | xargs -I {} bash -c "sudo fs_mark -L 100 -D 1024 -n 1024 -s0 -S5 -d {}"

Perf profile
(w/o patches)
=============================
87.15%  [kernel]  [k] ext4_fc_commit           --> Heavy contention/bottleneck
 1.98%  [kernel]  [k] perf_event_interrupt
 0.96%  [kernel]  [k] power_pmu_enable
 0.91%  [kernel]  [k] update_sd_lb_stats.constprop.0
 0.67%  [kernel]  [k] ktime_get

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/930f35d4fd5f83e2673c868781d9ebf15e91bf4e.1645426817.git.riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -1046,6 +1046,8 @@ struct ext4_inode_info {
 
 	/* Fast commit related info */
 
+	/* For tracking dentry create updates */
+	struct list_head i_fc_dilist;
 	struct list_head i_fc_list;	/*
 					 * inodes that need fast commit
 					 * protected by sbi->s_fc_lock.

fs/ext4/fast_commit.c

@@ -199,6 +199,7 @@ void ext4_fc_init_inode(struct inode *inode)
 	ext4_fc_reset_inode(inode);
 	ext4_clear_inode_state(inode, EXT4_STATE_FC_COMMITTING);
 	INIT_LIST_HEAD(&ei->i_fc_list);
+	INIT_LIST_HEAD(&ei->i_fc_dilist);
 	init_waitqueue_head(&ei->i_fc_wait);
 	atomic_set(&ei->i_fc_updates, 0);
 }
@@ -279,14 +280,16 @@ void ext4_fc_stop_update(struct inode *inode)
 void ext4_fc_del(struct inode *inode)
 {
 	struct ext4_inode_info *ei = EXT4_I(inode);
+	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
+	struct ext4_fc_dentry_update *fc_dentry;
 
 	if (!test_opt2(inode->i_sb, JOURNAL_FAST_COMMIT) ||
 	    (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY))
 		return;
 
 restart:
 	spin_lock(&EXT4_SB(inode->i_sb)->s_fc_lock);
-	if (list_empty(&ei->i_fc_list)) {
+	if (list_empty(&ei->i_fc_list) && list_empty(&ei->i_fc_dilist)) {
 		spin_unlock(&EXT4_SB(inode->i_sb)->s_fc_lock);
 		return;
 	}
@@ -295,8 +298,33 @@ void ext4_fc_del(struct inode *inode)
 		ext4_fc_wait_committing_inode(inode);
 		goto restart;
 	}
-	list_del_init(&ei->i_fc_list);
-	spin_unlock(&EXT4_SB(inode->i_sb)->s_fc_lock);
+
+	if (!list_empty(&ei->i_fc_list))
+		list_del_init(&ei->i_fc_list);
+
+	/*
+	 * Since this inode is getting removed, let's also remove all FC
+	 * dentry create references, since it is not needed to log it anyways.
+	 */
+	if (list_empty(&ei->i_fc_dilist)) {
+		spin_unlock(&sbi->s_fc_lock);
+		return;
+	}
+
+	fc_dentry = list_first_entry(&ei->i_fc_dilist, struct ext4_fc_dentry_update, fcd_dilist);
+	WARN_ON(fc_dentry->fcd_op != EXT4_FC_TAG_CREAT);
+	list_del_init(&fc_dentry->fcd_list);
+	list_del_init(&fc_dentry->fcd_dilist);
+
+	WARN_ON(!list_empty(&ei->i_fc_dilist));
+	spin_unlock(&sbi->s_fc_lock);
+
+	if (fc_dentry->fcd_name.name &&
+		fc_dentry->fcd_name.len > DNAME_INLINE_LEN)
+		kfree(fc_dentry->fcd_name.name);
+	kmem_cache_free(ext4_fc_dentry_cachep, fc_dentry);
+
+	return;
 }
 
 /*
@@ -427,14 +455,28 @@ static int __track_dentry_update(struct inode *inode, void *arg, bool update)
 		node->fcd_name.name = node->fcd_iname;
 	}
 	node->fcd_name.len = dentry->d_name.len;
-
+	INIT_LIST_HEAD(&node->fcd_dilist);
 	spin_lock(&sbi->s_fc_lock);
 	if (sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
 		sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING)
 		list_add_tail(&node->fcd_list,
 				&sbi->s_fc_dentry_q[FC_Q_STAGING]);
 	else
 		list_add_tail(&node->fcd_list, &sbi->s_fc_dentry_q[FC_Q_MAIN]);
+
+	/*
+	 * This helps us keep a track of all fc_dentry updates which is part of
+	 * this ext4 inode. So in case the inode is getting unlinked, before
+	 * even we get a chance to fsync, we could remove all fc_dentry
+	 * references while evicting the inode in ext4_fc_del().
+	 * Also with this, we don't need to loop over all the inodes in
+	 * sbi->s_fc_q to get the corresponding inode in
+	 * ext4_fc_commit_dentry_updates().
+	 */
+	if (dentry_update->op == EXT4_FC_TAG_CREAT) {
+		WARN_ON(!list_empty(&ei->i_fc_dilist));
+		list_add_tail(&node->fcd_dilist, &ei->i_fc_dilist);
+	}
 	spin_unlock(&sbi->s_fc_lock);
 	mutex_lock(&ei->i_fc_lock);
 
@@ -954,7 +996,7 @@ __releases(&sbi->s_fc_lock)
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_fc_dentry_update *fc_dentry, *fc_dentry_n;
 	struct inode *inode;
-	struct ext4_inode_info *ei, *ei_n;
+	struct ext4_inode_info *ei;
 	int ret;
 
 	if (list_empty(&sbi->s_fc_dentry_q[FC_Q_MAIN]))
@@ -970,21 +1012,16 @@ __releases(&sbi->s_fc_lock)
 			spin_lock(&sbi->s_fc_lock);
 			continue;
 		}
-
-		inode = NULL;
-		list_for_each_entry_safe(ei, ei_n, &sbi->s_fc_q[FC_Q_MAIN],
-					 i_fc_list) {
-			if (ei->vfs_inode.i_ino == fc_dentry->fcd_ino) {
-				inode = &ei->vfs_inode;
-				break;
-			}
-		}
 		/*
-		 * If we don't find inode in our list, then it was deleted,
-		 * in which case, we don't need to record it's create tag.
+		 * With fcd_dilist we need not loop in sbi->s_fc_q to get the
+		 * corresponding inode pointer
 		 */
-		if (!inode)
-			continue;
+		WARN_ON(list_empty(&fc_dentry->fcd_dilist));
+		ei = list_first_entry(&fc_dentry->fcd_dilist,
+				struct ext4_inode_info, i_fc_dilist);
+		inode = &ei->vfs_inode;
+		WARN_ON(inode->i_ino != fc_dentry->fcd_ino);
+
 		spin_unlock(&sbi->s_fc_lock);
 
 		/*
@@ -1228,6 +1265,7 @@ static void ext4_fc_cleanup(journal_t *journal, int full, tid_t tid)
 					     struct ext4_fc_dentry_update,
 					     fcd_list);
 		list_del_init(&fc_dentry->fcd_list);
+		list_del_init(&fc_dentry->fcd_dilist);
 		spin_unlock(&sbi->s_fc_lock);
 
 		if (fc_dentry->fcd_name.name &&

fs/ext4/fast_commit.h

@@ -109,6 +109,7 @@ struct ext4_fc_dentry_update {
 	struct qstr fcd_name;	/* Dirent name */
 	unsigned char fcd_iname[DNAME_INLINE_LEN];	/* Dirent name string */
 	struct list_head fcd_list;
+	struct list_head fcd_dilist;
 };
 
 struct ext4_fc_stats {