writeback: trace global_dirty_state

[It seems beneficial to queue this simple trace event for
next/upstream after the review?]

Add trace event balance_dirty_state for showing the global dirty page counts and thresholds at each global_dirty_limits() invocation. This will cover the callers throttle_vm_writeout(), over_bground_thresh() and each balance_dirty_pages() loop.

Signed-off-by Wu Fengguang <[email protected]>

---
 include/trace/events/writeback.h |   36 +++++++++++++++++++++++++++++
 mm/page-writeback.c              |    1 
 2 files changed, 37 insertions(+)

mm/page-writeback.c +1 -0

@@ -430,6 +430,7 @@ void global_dirty_limits(unsigned long *
 	}
 	*pbackground = background;
 	*pdirty = dirty;
+	trace_global_dirty_state(background, dirty);
 }
 
 /**

include/trace/events/writeback.h +36 -0

@@ -187,6 +187,42 @@ TRACE_EVENT(writeback_queue_io,
 		__entry->moved)
 );
 
+TRACE_EVENT(global_dirty_state,
+
+	TP_PROTO(unsigned long background_thresh,
+		 unsigned long dirty_thresh
+	),
+
+	TP_ARGS(background_thresh,
+		dirty_thresh
+	),
+
+	TP_STRUCT__entry(
+		__field(unsigned long,	nr_dirty)
+		__field(unsigned long,	nr_writeback)
+		__field(unsigned long,	nr_unstable)
+		__field(unsigned long,	background_thresh)
+		__field(unsigned long,	dirty_thresh)
+	),
+
+	TP_fast_assign(
+		__entry->nr_dirty	= global_page_state(NR_FILE_DIRTY);
+		__entry->nr_writeback	= global_page_state(NR_WRITEBACK);
+		__entry->nr_unstable	= global_page_state(NR_UNSTABLE_NFS);
+		__entry->background_thresh = background_thresh;
+		__entry->dirty_thresh	= dirty_thresh;
+	),
+
+	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
+		  "bg_thresh=%lu thresh=%lu",
+		  __entry->nr_dirty,
+		  __entry->nr_writeback,
+		  __entry->nr_unstable,
+		  __entry->background_thresh,
+		  __entry->dirty_thresh
+	)
+);
+
 DECLARE_EVENT_CLASS(writeback_congest_waited_template,
 
 	TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),

Quoted 7 lines · Wu Fengguang

[It seems beneficial to queue this simple trace event for
next/upstream after the review?]

Add trace event balance_dirty_state for showing the global dirty page counts and thresholds at each global_dirty_limits() invocation. This will cover the callers throttle_vm_writeout(), over_bground_thresh() and each balance_dirty_pages() loop.

OK, this might be useful. But shouldn't we also add similar trace point
for bdi limits? Otherwise the information is of limited use...