Add a background scanning timer to restore the watermarks to the pages_lots
level and only call on it if kswapd has not been called upon for the last 5
seconds. This allows us to balance all zones to the more generous pages_lots
watermark at a time unrelated to page allocation thus leading to lighter
levels of vm load when called upon under page allocation.

Signed-off-by: Con Kolivas <kernel@kolivas.org>

The -ck patches modify mm/vmscan.c and add a timer to wake up kswapd every 5 
seconds. This timer is initialized after the creation of the kswapd thread.

The kswapd() thread function calls mod_timer at the front of its infinite 
service loop (to reset the timer to 5 seconds in the future). mod_timer() 
includes a BUG_ON() to assert that the timer's callback function is set.

Since the wakeup timer is initialized after the kswapd thread is created, if 
kswapd gets scheduled before kswapd_run() has prepared the timer, the 
BUG_ON() check will throw a stack trace and immediately terminate the kswapd 
thread.

This patch modifies the kswapd_run() function in mm/vmscan.c to initialize the 
watermark timer before starting the kswapd thread.

Signed-off-by: Chase Venters <chase.venters at clientec.com>

 include/linux/mmzone.h |    2 ++
 mm/vmscan.c            |   42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

Index: linux-2.6.21-ck2/include/linux/mmzone.h
===================================================================
--- linux-2.6.21-ck2.orig/include/linux/mmzone.h	2007-05-14 19:49:55.000000000 +1000
+++ linux-2.6.21-ck2/include/linux/mmzone.h	2007-05-14 19:49:56.000000000 +1000
@@ -13,6 +13,7 @@
 #include <linux/init.h>
 #include <linux/seqlock.h>
 #include <linux/nodemask.h>
+#include <linux/timer.h>
 #include <asm/atomic.h>
 #include <asm/page.h>
 
@@ -449,6 +450,7 @@ typedef struct pglist_data {
 	wait_queue_head_t kswapd_wait;
 	struct task_struct *kswapd;
 	int kswapd_max_order;
+	struct timer_list watermark_timer;
 } pg_data_t;
 
 #define node_present_pages(nid)	(NODE_DATA(nid)->node_present_pages)
Index: linux-2.6.21-ck2/mm/vmscan.c
===================================================================
--- linux-2.6.21-ck2.orig/mm/vmscan.c	2007-05-14 19:49:56.000000000 +1000
+++ linux-2.6.21-ck2/mm/vmscan.c	2007-05-14 19:49:56.000000000 +1000
@@ -37,6 +37,7 @@
 #include <linux/rwsem.h>
 #include <linux/delay.h>
 #include <linux/kthread.h>
+#include <linux/timer.h>
 #include <linux/freezer.h>
 
 #include <asm/tlbflush.h>
@@ -1335,6 +1336,8 @@ out:
 	return nr_reclaimed;
 }
 
+#define WT_EXPIRY	(HZ * 5)	/* Time to wakeup watermark_timer */
+
 /*
  * The background pageout daemon, started as a kernel thread
  * from the init process. 
@@ -1384,6 +1387,8 @@ static int kswapd(void *p)
 
 		try_to_freeze();
 
+		/* kswapd has been busy so delay watermark_timer */
+		mod_timer(&pgdat->watermark_timer, jiffies + WT_EXPIRY);
 		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 		new_order = pgdat->kswapd_max_order;
 		pgdat->kswapd_max_order = 0;
@@ -1604,20 +1609,57 @@ static int __devinit cpu_callback(struct
 }
 
 /*
+ * We wake up kswapd every WT_EXPIRY till free ram is above pages_lots
+ */
+static void watermark_wakeup(unsigned long data)
+{
+	pg_data_t *pgdat = (pg_data_t *)data;
+	struct timer_list *wt = &pgdat->watermark_timer;
+	int i;
+
+	if (!waitqueue_active(&pgdat->kswapd_wait) || above_background_load())
+		goto out;
+	for (i = pgdat->nr_zones - 1; i >= 0; i--) {
+		struct zone *z = pgdat->node_zones + i;
+
+		if (!populated_zone(z) || is_highmem(z)) {
+			/* We are better off leaving highmem full */
+			continue;
+		}
+		if (!zone_watermark_ok(z, 0, z->pages_lots, 0, 0)) {
+			wake_up_interruptible(&pgdat->kswapd_wait);
+			goto out;
+		}
+	}
+out:
+	mod_timer(wt, jiffies + WT_EXPIRY);
+	return;
+}
+
+/*
  * This kswapd start function will be called by init and node-hot-add.
  * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
  */
 int kswapd_run(int nid)
 {
 	pg_data_t *pgdat = NODE_DATA(nid);
+	struct timer_list *wt;
 	int ret = 0;
 
 	if (pgdat->kswapd)
 		return 0;
 
+	wt = &pgdat->watermark_timer;
+	init_timer(wt);
+	wt->data = (unsigned long)pgdat;
+	wt->function = watermark_wakeup;
+	wt->expires = jiffies + WT_EXPIRY;
+	add_timer(wt);
+
 	pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
 	if (IS_ERR(pgdat->kswapd)) {
 		/* failure at boot is fatal */
+		del_timer(wt);
 		BUG_ON(system_state == SYSTEM_BOOTING);
 		printk("Failed to start kswapd on node %d\n",nid);
 		ret = -1;