writeback: Add tracepoint to track pending inode switches

Add trace_inode_switch_wbs_queue tracepoint to allow insight into how
many inodes are queued to switch their bdi_writeback structure.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: jankara

fs/fs-writeback.c

@@ -667,6 +667,7 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 
 	isw->inodes[0] = inode;
 
+	trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1);
 	wb_queue_isw(new_wb, isw);
 	return;
 
@@ -752,6 +753,7 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 		return restart;
 	}
 
+	trace_inode_switch_wbs_queue(wb, new_wb, nr);
 	wb_queue_isw(new_wb, isw);
 
 	return restart;

include/trace/events/writeback.h

@@ -213,6 +213,35 @@ TRACE_EVENT(inode_foreign_history,
 	)
 );
 
+TRACE_EVENT(inode_switch_wbs_queue,
+
+	TP_PROTO(struct bdi_writeback *old_wb, struct bdi_writeback *new_wb,
+		 unsigned int count),
+
+	TP_ARGS(old_wb, new_wb, count),
+
+	TP_STRUCT__entry(
+		__array(char,		name, 32)
+		__field(ino_t,		old_cgroup_ino)
+		__field(ino_t,		new_cgroup_ino)
+		__field(unsigned int,	count)
+	),
+
+	TP_fast_assign(
+		strscpy_pad(__entry->name, bdi_dev_name(old_wb->bdi), 32);
+		__entry->old_cgroup_ino	= __trace_wb_assign_cgroup(old_wb);
+		__entry->new_cgroup_ino	= __trace_wb_assign_cgroup(new_wb);
+		__entry->count		= count;
+	),
+
+	TP_printk("bdi %s: old_cgroup_ino=%lu new_cgroup_ino=%lu count=%u",
+		__entry->name,
+		(unsigned long)__entry->old_cgroup_ino,
+		(unsigned long)__entry->new_cgroup_ino,
+		__entry->count
+	)
+);
+
 TRACE_EVENT(inode_switch_wbs,
 
 	TP_PROTO(struct inode *inode, struct bdi_writeback *old_wb,