writeback: account for time spent congestion_waited

There is strong evidence to indicate a lot of time is being spent in
congestion_wait(), some of it unnecessarily.  This patch adds a tracepoint
for congestion_wait to record when congestion_wait() was called, how long
the timeout was for and how long it actually slept.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 33 insertions, 0 deletions

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 0bb01ab2e984..d2b2654606ec 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -151,6 +151,34 @@ DEFINE_WBC_EVENT(wbc_balance_dirty_written);
 DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
 DEFINE_WBC_EVENT(wbc_writepage);
 
+DECLARE_EVENT_CLASS(writeback_congest_waited_template,
+
+        TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),
+
+        TP_ARGS(usec_timeout, usec_delayed),
+
+        TP_STRUCT__entry(
+                __field(        unsigned int,   usec_timeout    )
+                __field(        unsigned int,   usec_delayed    )
+        ),
+
+        TP_fast_assign(
+                __entry->usec_timeout   = usec_timeout;
+                __entry->usec_delayed   = usec_delayed;
+        ),
+
+        TP_printk("usec_timeout=%u usec_delayed=%u",
+                        __entry->usec_timeout,
+                        __entry->usec_delayed)
+);
+
+DEFINE_EVENT(writeback_congest_waited_template, writeback_congestion_wait,
+
+        TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),
+
+        TP_ARGS(usec_timeout, usec_delayed)
+);
+
 #endif /* _TRACE_WRITEBACK_H */
 
 /* This part must be outside protection */
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 65d420499a61..55627306abe0 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -764,12 +764,17 @@ EXPORT_SYMBOL(set_bdi_congested);
 long congestion_wait(int sync, long timeout)
 {
         long ret;
+        unsigned long start = jiffies;
         DEFINE_WAIT(wait);
         wait_queue_head_t *wqh = &congestion_wqh[sync];
 
         prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
         ret = io_schedule_timeout(timeout);
         finish_wait(wqh, &wait);
+
+        trace_writeback_congestion_wait(jiffies_to_usecs(timeout),
+                                        jiffies_to_usecs(jiffies - start));
+
         return ret;
 }
 EXPORT_SYMBOL(congestion_wait);