writeback: trace event bdi_dirty_ratelimit

It helps understand how various throttle bandwidths are updated.
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

include/trace/events/writeback.h

@@ -226,6 +226,51 @@ TRACE_EVENT(global_dirty_state,
 	)
 );
 
+#define KBps(x)			((x) << (PAGE_SHIFT - 10))
+
+TRACE_EVENT(bdi_dirty_ratelimit,
+
+	TP_PROTO(struct backing_dev_info *bdi,
+		 unsigned long dirty_rate,
+		 unsigned long task_ratelimit),
+
+	TP_ARGS(bdi, dirty_rate, task_ratelimit),
+
+	TP_STRUCT__entry(
+		__array(char,		bdi, 32)
+		__field(unsigned long,	write_bw)
+		__field(unsigned long,	avg_write_bw)
+		__field(unsigned long,	dirty_rate)
+		__field(unsigned long,	dirty_ratelimit)
+		__field(unsigned long,	task_ratelimit)
+		__field(unsigned long,	balanced_dirty_ratelimit)
+	),
+
+	TP_fast_assign(
+		strlcpy(__entry->bdi, dev_name(bdi->dev), 32);
+		__entry->write_bw	= KBps(bdi->write_bandwidth);
+		__entry->avg_write_bw	= KBps(bdi->avg_write_bandwidth);
+		__entry->dirty_rate	= KBps(dirty_rate);
+		__entry->dirty_ratelimit = KBps(bdi->dirty_ratelimit);
+		__entry->task_ratelimit	= KBps(task_ratelimit);
+		__entry->balanced_dirty_ratelimit =
+					  KBps(bdi->balanced_dirty_ratelimit);
+	),
+
+	TP_printk("bdi %s: "
+		  "write_bw=%lu awrite_bw=%lu dirty_rate=%lu "
+		  "dirty_ratelimit=%lu task_ratelimit=%lu "
+		  "balanced_dirty_ratelimit=%lu",
+		  __entry->bdi,
+		  __entry->write_bw,		/* write bandwidth */
+		  __entry->avg_write_bw,	/* avg write bandwidth */
+		  __entry->dirty_rate,		/* bdi dirty rate */
+		  __entry->dirty_ratelimit,	/* base ratelimit */
+		  __entry->task_ratelimit, /* ratelimit with position control */
+		  __entry->balanced_dirty_ratelimit /* the balanced ratelimit */
+	)
+);
+
 DECLARE_EVENT_CLASS(writeback_congest_waited_template,
 
 	TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),

mm/page-writeback.c

@@ -875,6 +875,8 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
 
 	bdi->dirty_ratelimit = max(dirty_ratelimit, 1UL);
 	bdi->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
+
+	trace_bdi_dirty_ratelimit(bdi, dirty_rate, task_ratelimit);
 }
 
 void __bdi_update_bandwidth(struct backing_dev_info *bdi,