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Subject[PATCH 02/10] writeback: Account for time spent congestion_waited
Date
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>
---
include/trace/events/writeback.h | 28 ++++++++++++++++++++++++++++
mm/backing-dev.c | 5 +++++
2 files changed, 33 insertions(+), 0 deletions(-)

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index f345f66..275d477 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -153,6 +153,34 @@ 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 eaa4a5b..298975a 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -759,12 +759,17 @@ 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);
--
1.7.1


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