[PATCH 3/6] tracing, vmscan: Add trace event when a page is written

[Mel Gorman <mel@csn.ul.ie>]

This patch adds a trace event for when page reclaim queues a page for IO and
records whether it is synchronous or asynchronous. Excessive synchronous
IO for a process can result in noticeable stalls during direct reclaim.
Excessive IO from page reclaim may indicate that the system is seriously
under provisioned for the amount of dirty pages that exist.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
---
 include/trace/events/vmscan.h |   23 +++++++++++++++++++++++
 mm/vmscan.c                   |    2 ++
 2 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
index a331454..b26daa9 100644
--- a/include/trace/events/vmscan.h
+++ b/include/trace/events/vmscan.h
@@ -154,6 +154,29 @@ TRACE_EVENT(mm_vmscan_lru_isolate,
 		__entry->nr_lumpy_dirty,
 		__entry->nr_lumpy_failed)
 );
+
+TRACE_EVENT(mm_vmscan_writepage,
+
+	TP_PROTO(struct page *page,
+		int sync_io),
+
+	TP_ARGS(page, sync_io),
+
+	TP_STRUCT__entry(
+		__field(struct page *, page)
+		__field(int, sync_io)
+	),
+
+	TP_fast_assign(
+		__entry->page = page;
+		__entry->sync_io = sync_io;
+	),
+
+	TP_printk("page=%p pfn=%lu sync_io=%d",
+		__entry->page,
+		page_to_pfn(__entry->page),
+		__entry->sync_io)
+);
 		
 #endif /* _TRACE_VMSCAN_H */
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 25bf05a..58527c4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -399,6 +399,8 @@ static pageout_t pageout(struct page *page, struct address_space *mapping,
 			/* synchronous write or broken a_ops? */
 			ClearPageReclaim(page);
 		}
+		trace_mm_vmscan_writepage(page,
+			sync_writeback == PAGEOUT_IO_SYNC);
 		inc_zone_page_state(page, NR_VMSCAN_WRITE);
 		return PAGE_SUCCESS;
 	}
-- 
1.7.1

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