staging: android: lowmemkiller: Substantially reduce overhead during reclaim

This patch optimizes lowmemkiller to not do any work when it has an outstanding
kill-request. This greatly reduces the pressure on the task_list lock
(improving interactivity), as well as improving the vmscan performance
when under heavy memory pressure (by up to 20x in tests).

Note: For this enhancement to work, you need CONFIG_PROFILING

Signed-off-by: San Mehat <san@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/android/lowmemorykiller.c

@@ -34,6 +34,8 @@
 #include <linux/mm.h>
 #include <linux/oom.h>
 #include <linux/sched.h>
+#include <linux/profile.h>
+#include <linux/notifier.h>
 
 static uint32_t lowmem_debug_level = 2;
 static int lowmem_adj[6] = {
@@ -51,12 +53,32 @@ static size_t lowmem_minfree[6] = {
 };
 static int lowmem_minfree_size = 4;
 
+static struct task_struct *lowmem_deathpending;
+
 #define lowmem_print(level, x...)			\
 	do {						\
 		if (lowmem_debug_level >= (level))	\
 			printk(x);			\
 	} while (0)
 
+static int
+task_notify_func(struct notifier_block *self, unsigned long val, void *data);
+
+static struct notifier_block task_nb = {
+	.notifier_call	= task_notify_func,
+};
+
+static int
+task_notify_func(struct notifier_block *self, unsigned long val, void *data)
+{
+	struct task_struct *task = data;
+	if (task == lowmem_deathpending) {
+		lowmem_deathpending = NULL;
+		task_handoff_unregister(&task_nb);
+	}
+	return NOTIFY_OK;
+}
+
 static int lowmem_shrink(int nr_to_scan, gfp_t gfp_mask)
 {
 	struct task_struct *p;
@@ -71,6 +93,18 @@ static int lowmem_shrink(int nr_to_scan, gfp_t gfp_mask)
 	int other_free = global_page_state(NR_FREE_PAGES);
 	int other_file = global_page_state(NR_FILE_PAGES);
 
+	/*
+	 * If we already have a death outstanding, then
+	 * bail out right away; indicating to vmscan
+	 * that we have nothing further to offer on
+	 * this pass.
+	 *
+	 * Note: Currently you need CONFIG_PROFILING
+	 * for this to work correctly.
+	 */
+	if (lowmem_deathpending)
+		return 0;
+
 	if (lowmem_adj_size < array_size)
 		array_size = lowmem_adj_size;
 	if (lowmem_minfree_size < array_size)
@@ -142,6 +176,15 @@ static int lowmem_shrink(int nr_to_scan, gfp_t gfp_mask)
 		lowmem_print(1, "send sigkill to %d (%s), adj %d, size %d\n",
 			     selected->pid, selected->comm,
 			     selected_oom_adj, selected_tasksize);
+		/*
+		 * If CONFIG_PROFILING is off, then task_handoff_register()
+		 * is a nop. In that case we don't want to stall the killer
+		 * by setting lowmem_deathpending.
+		 */
+#ifdef CONFIG_PROFILING
+		lowmem_deathpending = selected;
+		task_handoff_register(&task_nb);
+#endif
 		force_sig(SIGKILL, selected);
 		rem -= selected_tasksize;
 	}