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Subject[PATCH 1/3] 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() occurred
and how long was spent.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
---
include/trace/events/writeback.h | 17 +++++++++++++++++
mm/backing-dev.c | 4 ++++
2 files changed, 21 insertions(+), 0 deletions(-)
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index f345f66..e3bee61 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -153,6 +153,23 @@ DEFINE_WBC_EVENT(wbc_balance_dirty_written);
DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
DEFINE_WBC_EVENT(wbc_writepage);

+TRACE_EVENT(writeback_congest_waited,
+
+ TP_PROTO(unsigned int usec_delayed),
+
+ TP_ARGS(usec_delayed),
+
+ TP_STRUCT__entry(
+ __field( unsigned int, usec_delayed )
+ ),
+
+ TP_fast_assign(
+ __entry->usec_delayed = usec_delayed;
+ ),
+
+ TP_printk("usec_delayed=%u", __entry->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..7ae33e2 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -759,12 +759,16 @@ 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_congest_waited(jiffies_to_usecs(jiffies - start));
+
return ret;
}
EXPORT_SYMBOL(congestion_wait);
--
1.7.1


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