[PATCH] Instrument congestion_wait

[Mel Gorman]

This patch instruments how long congestion_wait() really waited for a
given caller.

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 3d3accb..fc945e0 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -10,6 +10,7 @@
 #include <linux/module.h>
 #include <linux/writeback.h>
 #include <linux/device.h>
+#include <linux/kallsyms.h>
 
 void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
 {
@@ -729,6 +730,11 @@ EXPORT_SYMBOL(set_bdi_congested);
  */
 long congestion_wait(int sync, long timeout)
 {
+	unsigned long jiffies_start = jiffies;
+	char *module;
+	char buf[128];
+	const char *symbol;
+	unsigned long offset, symbolsize;
 	long ret;
 	DEFINE_WAIT(wait);
 	wait_queue_head_t *wqh = &congestion_wqh[sync];
@@ -736,6 +742,13 @@ long congestion_wait(int sync, long timeout)
 	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
 	ret = io_schedule_timeout(timeout);
 	finish_wait(wqh, &wait);
+
+	symbol = kallsyms_lookup(_RET_IP_, &symbolsize, &offset, &module, buf),
+	printk(KERN_INFO "%-20s congestion_wait sync=%d delay %lu timeout %ld\n",
+		symbol,
+		sync,
+		jiffies - jiffies_start,
+		timeout);
 	return ret;
 }
 EXPORT_SYMBOL(congestion_wait);
--
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