rcuscale: Add test for using call_rcu_lazy() to emulate kfree_rcu()

Reuse the kfree_rcu() test in order to be able to compare the memory reclaiming
properties of call_rcu_lazy() with kfree_rcu().

With this test, we find similar memory footprint and time call_rcu_lazy()
free'ing takes compared to kfree_rcu(). Also we confirm that call_rcu_lazy()
can survive OOM during extremely frequent calls.

If we really push it, i.e. boot system with low memory and compare
kfree_rcu() with call_rcu_lazy(), I find that call_rcu_lazy() is more
resilient and is much harder to produce OOM as compared to kfree_rcu().

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

kernel/rcu/rcu.h

@@ -473,6 +473,12 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 			       unsigned long c);
 void rcu_gp_set_torture_wait(int duration);
 void rcu_force_call_rcu_to_lazy(bool force);
+
+#if IS_ENABLED(CONFIG_RCU_SCALE_TEST)
+unsigned long rcu_scale_get_jiffies_till_flush(void);
+void rcu_scale_set_jiffies_till_flush(unsigned long j);
+#endif
+
 #else
 static inline void rcutorture_get_gp_data(enum rcutorture_type test_type,
 					  int *flags, unsigned long *gp_seq)

kernel/rcu/rcuscale.c

@@ -95,6 +95,7 @@ torture_param(int, verbose, 1, "Enable verbose debugging printk()s");
 torture_param(int, writer_holdoff, 0, "Holdoff (us) between GPs, zero to disable");
 torture_param(int, kfree_rcu_test, 0, "Do we run a kfree_rcu() scale test?");
 torture_param(int, kfree_mult, 1, "Multiple of kfree_obj size to allocate.");
+torture_param(int, kfree_rcu_by_lazy, 0, "Use call_rcu_lazy() to emulate kfree_rcu()?");
 
 static char *scale_type = "rcu";
 module_param(scale_type, charp, 0444);
@@ -658,6 +659,13 @@ struct kfree_obj {
 	struct rcu_head rh;
 };
 
+/* Used if doing RCU-kfree'ing via call_rcu_lazy(). */
+void kfree_rcu_lazy(struct rcu_head *rh)
+{
+	struct kfree_obj *obj = container_of(rh, struct kfree_obj, rh);
+	kfree(obj);
+}
+
 static int
 kfree_scale_thread(void *arg)
 {
@@ -695,6 +703,11 @@ kfree_scale_thread(void *arg)
 			if (!alloc_ptr)
 				return -ENOMEM;
 
+			if (kfree_rcu_by_lazy) {
+				call_rcu_lazy(&(alloc_ptr->rh), kfree_rcu_lazy);
+				continue;
+			}
+
 			// By default kfree_rcu_test_single and kfree_rcu_test_double are
 			// initialized to false. If both have the same value (false or true)
 			// both are randomly tested, otherwise only the one with value true
@@ -737,6 +750,9 @@ kfree_scale_cleanup(void)
 {
 	int i;
 
+	if (kfree_rcu_by_lazy)
+		rcu_force_call_rcu_to_lazy(false);
+
 	if (torture_cleanup_begin())
 		return;
 
@@ -766,11 +782,55 @@ kfree_scale_shutdown(void *arg)
 	return -EINVAL;
 }
 
+// Used if doing RCU-kfree'ing via call_rcu_lazy().
+unsigned long jiffies_at_lazy_cb;
+struct rcu_head lazy_test1_rh;
+int rcu_lazy_test1_cb_called;
+void call_rcu_lazy_test1(struct rcu_head *rh)
+{
+	jiffies_at_lazy_cb = jiffies;
+	WRITE_ONCE(rcu_lazy_test1_cb_called, 1);
+}
+
 static int __init
 kfree_scale_init(void)
 {
 	long i;
 	int firsterr = 0;
+	unsigned long orig_jif, jif_start;
+
+	// Force all call_rcu() to call_rcu_lazy() so that non-lazy CBs
+	// do not remove laziness of the lazy ones (since the test tries
+	// to stress call_rcu_lazy() for OOM).
+	//
+	// Also, do a quick self-test to ensure laziness is as much as
+	// expected.
+	if (kfree_rcu_by_lazy) {
+		/* do a test to check the timeout. */
+		orig_jif = rcu_scale_get_jiffies_till_flush();
+
+		rcu_force_call_rcu_to_lazy(true);
+		rcu_scale_set_jiffies_till_flush(2 * HZ);
+		rcu_barrier();
+
+		jif_start = jiffies;
+		jiffies_at_lazy_cb = 0;
+		call_rcu_lazy(&lazy_test1_rh, call_rcu_lazy_test1);
+
+		smp_cond_load_relaxed(&rcu_lazy_test1_cb_called, VAL == 1);
+
+		rcu_scale_set_jiffies_till_flush(orig_jif);
+
+		if (WARN_ON_ONCE(jiffies_at_lazy_cb - jif_start < 2 * HZ)) {
+			pr_alert("Lazy CBs are not being lazy as expected!\n");
+			return -1;
+		}
+
+		if (WARN_ON_ONCE(jiffies_at_lazy_cb - jif_start > 3 * HZ)) {
+			pr_alert("Lazy CBs are being too lazy!\n");
+			return -1;
+		}
+	}
 
 	kfree_nrealthreads = compute_real(kfree_nthreads);
 	/* Start up the kthreads. */
@@ -783,7 +843,9 @@ kfree_scale_init(void)
 		schedule_timeout_uninterruptible(1);
 	}
 
-	pr_alert("kfree object size=%zu\n", kfree_mult * sizeof(struct kfree_obj));
+	pr_alert("kfree object size=%zu, kfree_rcu_by_lazy=%d\n",
+			kfree_mult * sizeof(struct kfree_obj),
+			kfree_rcu_by_lazy);
 
 	kfree_reader_tasks = kcalloc(kfree_nrealthreads, sizeof(kfree_reader_tasks[0]),
 			       GFP_KERNEL);

kernel/rcu/tree_nocb.h

@@ -257,6 +257,21 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
 }
 
 #define LAZY_FLUSH_JIFFIES (10 * HZ)
+unsigned long jiffies_till_flush = LAZY_FLUSH_JIFFIES;
+
+#ifdef CONFIG_RCU_SCALE_TEST
+void rcu_scale_set_jiffies_till_flush(unsigned long jif)
+{
+	jiffies_till_flush = jif;
+}
+EXPORT_SYMBOL(rcu_scale_set_jiffies_till_flush);
+
+unsigned long rcu_scale_get_jiffies_till_flush(void)
+{
+	return jiffies_till_flush;
+}
+EXPORT_SYMBOL(rcu_scale_get_jiffies_till_flush);
+#endif
 
 /*
  * Arrange to wake the GP kthread for this NOCB group at some future
@@ -275,7 +290,7 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 	 * of callback storm, no need to wake up too early.
 	 */
 	if (waketype == RCU_NOCB_WAKE_LAZY) {
-		mod_timer(&rdp_gp->nocb_timer, jiffies + LAZY_FLUSH_JIFFIES);
+		mod_timer(&rdp_gp->nocb_timer, jiffies + jiffies_till_flush);
 		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
 	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
 		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);