[PATCH] Add tracepoints to track pagecache transition

[Atsushi Tsuji]

Hi,

The below patch adds instrumentation for pagecache.

I thought it would be useful to trace pagecache behavior for problem
analysis (performance bottlenecks, behavior differences between stable
time and trouble time).

By using those tracepoints, we can describe and visualize pagecache
transition (file-by-file basis) in kernel and  pagecache
consumes most of the memory in running system and pagecache hit rate
and writeback behavior will influence system load and performance.

I attached an example which is visualization of pagecache status using
SystemTap. That graph describes pagecache transition of File A and File B
on a file-by-file basis with the situation where regular I/O to File A
is delayed because of other I/O to File B. We visually understand
pagecache for File A is narrowed down due to I/O pressure from File B.


Signed-off-by: Atsushi Tsuji <a-tsuji@xxxxxxxxxxxxx>
---
diff --git a/include/trace/filemap.h b/include/trace/filemap.h
new file mode 100644
index 0000000..196955e
--- /dev/null
+++ b/include/trace/filemap.h
@@ -0,0 +1,13 @@
+#ifndef _TRACE_FILEMAP_H
+#define _TRACE_FILEMAP_H
+
+#include <linux/tracepoint.h>
+
+DECLARE_TRACE(filemap_add_to_page_cache,
+	TPPROTO(struct address_space *mapping, pgoff_t offset),
+	TPARGS(mapping, offset));
+DECLARE_TRACE(filemap_remove_from_page_cache,
+	TPPROTO(struct address_space *mapping),
+	TPARGS(mapping));
+
+#endif
diff --git a/mm/filemap.c b/mm/filemap.c
index 23acefe..76a6887 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -34,6 +34,7 @@
 #include <linux/hardirq.h> /* for BUG_ON(!in_atomic()) only */
 #include <linux/memcontrol.h>
 #include <linux/mm_inline.h> /* for page_is_file_cache() */
+#include <trace/filemap.h>
 #include "internal.h"
 
 /*
@@ -43,6 +44,8 @@
 
 #include <asm/mman.h>
 
+DEFINE_TRACE(filemap_add_to_page_cache);
+DEFINE_TRACE(filemap_remove_from_page_cache);
 
 /*
  * Shared mappings implemented 30.11.1994. It's not fully working yet,
@@ -120,6 +123,7 @@ void __remove_from_page_cache(struct page *page)
 	page->mapping = NULL;
 	mapping->nrpages--;
 	__dec_zone_page_state(page, NR_FILE_PAGES);
+	trace_filemap_remove_from_page_cache(mapping);
 	BUG_ON(page_mapped(page));
 	mem_cgroup_uncharge_cache_page(page);
 
@@ -475,6 +479,7 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
 		if (likely(!error)) {
 			mapping->nrpages++;
 			__inc_zone_page_state(page, NR_FILE_PAGES);
+			trace_filemap_add_to_page_cache(mapping, offset);
 		} else {
 			page->mapping = NULL;
 			mem_cgroup_uncharge_cache_page(page);