gfs2: Glock dump performance regression fix

Restore an optimization removed in commit 7f19449553 "Fix debugfs glocks
dump": keep the glock hash table iterator active while the glock dump
file is held open.  This avoids having to rescan the hash table from the
start for each read, with quadratically rising runtime.

In addition, use rhastable_walk_peek for resuming a glock dump at the
current position: when a glock doesn't fit in the provided buffer
anymore, the next read must revisit the same glock.

Finally, also restart the dump from the first entry when we notice that
the hash table has been resized in gfs2_glock_seq_start.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>

fs/gfs2/glock.c

@@ -1921,19 +1921,29 @@ void gfs2_glock_exit(void)
 	destroy_workqueue(gfs2_delete_workqueue);
 }
 
-static void gfs2_glock_iter_next(struct gfs2_glock_iter *gi)
+static void gfs2_glock_iter_next(struct gfs2_glock_iter *gi, loff_t n)
 {
-	while ((gi->gl = rhashtable_walk_next(&gi->hti))) {
-		if (IS_ERR(gi->gl)) {
-			if (PTR_ERR(gi->gl) == -EAGAIN)
-				continue;
-			gi->gl = NULL;
-			return;
+	if (n == 0)
+		gi->gl = rhashtable_walk_peek(&gi->hti);
+	else {
+		gi->gl = rhashtable_walk_next(&gi->hti);
+		n--;
+	}
+	for (;;) {
+		if (IS_ERR_OR_NULL(gi->gl)) {
+			if (!gi->gl)
+				return;
+			if (PTR_ERR(gi->gl) != -EAGAIN) {
+				gi->gl = NULL;
+				return;
+			}
+			n = 0;
+		} else if (gi->sdp == gi->gl->gl_name.ln_sbd &&
+			   !__lockref_is_dead(&gi->gl->gl_lockref)) {
+			if (!n--)
+				break;
 		}
-		/* Skip entries for other sb and dead entries */
-		if (gi->sdp == gi->gl->gl_name.ln_sbd &&
-		    !__lockref_is_dead(&gi->gl->gl_lockref))
-			return;
+		gi->gl = rhashtable_walk_next(&gi->hti);
 	}
 }
 
@@ -1941,18 +1951,24 @@ static void *gfs2_glock_seq_start(struct seq_file *seq, loff_t *pos)
 	__acquires(RCU)
 {
 	struct gfs2_glock_iter *gi = seq->private;
-	loff_t n = *pos;
+	loff_t n;
 
-	rhashtable_walk_enter(&gl_hash_table, &gi->hti);
-	if (rhashtable_walk_start_check(&gi->hti) != 0)
-		return NULL;
+	/*
+	 * We can either stay where we are, skip to the next hash table
+	 * entry, or start from the beginning.
+	 */
+	if (*pos < gi->last_pos) {
+		rhashtable_walk_exit(&gi->hti);
+		rhashtable_walk_enter(&gl_hash_table, &gi->hti);
+		n = *pos + 1;
+	} else {
+		n = *pos - gi->last_pos;
+	}
 
-	do {
-		gfs2_glock_iter_next(gi);
-	} while (gi->gl && n--);
+	rhashtable_walk_start(&gi->hti);
 
+	gfs2_glock_iter_next(gi, n);
 	gi->last_pos = *pos;
-
 	return gi->gl;
 }
 
@@ -1963,8 +1979,7 @@ static void *gfs2_glock_seq_next(struct seq_file *seq, void *iter_ptr,
 
 	(*pos)++;
 	gi->last_pos = *pos;
-	gfs2_glock_iter_next(gi);
-
+	gfs2_glock_iter_next(gi, 1);
 	return gi->gl;
 }
 
@@ -1975,7 +1990,6 @@ static void gfs2_glock_seq_stop(struct seq_file *seq, void *iter_ptr)
 
 	gi->gl = NULL;
 	rhashtable_walk_stop(&gi->hti);
-	rhashtable_walk_exit(&gi->hti);
 }
 
 static int gfs2_glock_seq_show(struct seq_file *seq, void *iter_ptr)
@@ -2041,7 +2055,13 @@ static int __gfs2_glocks_open(struct inode *inode, struct file *file,
 		seq->buf = kmalloc(GFS2_SEQ_GOODSIZE, GFP_KERNEL | __GFP_NOWARN);
 		if (seq->buf)
 			seq->size = GFS2_SEQ_GOODSIZE;
+		/*
+		 * Initially, we are "before" the first hash table entry; the
+		 * first call to rhashtable_walk_next gets us the first entry.
+		 */
+		gi->last_pos = -1;
 		gi->gl = NULL;
+		rhashtable_walk_enter(&gl_hash_table, &gi->hti);
 	}
 	return ret;
 }
@@ -2057,6 +2077,7 @@ static int gfs2_glocks_release(struct inode *inode, struct file *file)
 	struct gfs2_glock_iter *gi = seq->private;
 
 	gi->gl = NULL;
+	rhashtable_walk_exit(&gi->hti);
 	return seq_release_private(inode, file);
 }