rcu: Check the range of jiffies_till_{first,next}_fqs when setting them

Currently, the range of jiffies_till_{first,next}_fqs are checked and
adjusted on and on in the loop of rcu_gp_kthread on runtime.

However, it's enough to check them only when setting them, not every
time in the loop. So make them handled on a setting time via sysfs.
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -510,8 +510,38 @@ static ulong jiffies_till_first_fqs = ULONG_MAX;
 static ulong jiffies_till_next_fqs = ULONG_MAX;
 static bool rcu_kick_kthreads;
 
-module_param(jiffies_till_first_fqs, ulong, 0644);
-module_param(jiffies_till_next_fqs, ulong, 0644);
+static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp)
+{
+	ulong j;
+	int ret = kstrtoul(val, 0, &j);
+
+	if (!ret)
+		WRITE_ONCE(*(ulong *)kp->arg, (j > HZ) ? HZ : j);
+	return ret;
+}
+
+static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param *kp)
+{
+	ulong j;
+	int ret = kstrtoul(val, 0, &j);
+
+	if (!ret)
+		WRITE_ONCE(*(ulong *)kp->arg, (j > HZ) ? HZ : (j ?: 1));
+	return ret;
+}
+
+static struct kernel_param_ops first_fqs_jiffies_ops = {
+	.set = param_set_first_fqs_jiffies,
+	.get = param_get_ulong,
+};
+
+static struct kernel_param_ops next_fqs_jiffies_ops = {
+	.set = param_set_next_fqs_jiffies,
+	.get = param_get_ulong,
+};
+
+module_param_cb(jiffies_till_first_fqs, &first_fqs_jiffies_ops, &jiffies_till_first_fqs, 0644);
+module_param_cb(jiffies_till_next_fqs, &next_fqs_jiffies_ops, &jiffies_till_next_fqs, 0644);
 module_param(rcu_kick_kthreads, bool, 0644);
 
 /*
@@ -2180,10 +2210,6 @@ static int __noreturn rcu_gp_kthread(void *arg)
 		/* Handle quiescent-state forcing. */
 		first_gp_fqs = true;
 		j = jiffies_till_first_fqs;
-		if (j > HZ) {
-			j = HZ;
-			jiffies_till_first_fqs = HZ;
-		}
 		ret = 0;
 		for (;;) {
 			if (!ret) {
@@ -2218,13 +2244,6 @@ static int __noreturn rcu_gp_kthread(void *arg)
 				WRITE_ONCE(rsp->gp_activity, jiffies);
 				ret = 0; /* Force full wait till next FQS. */
 				j = jiffies_till_next_fqs;
-				if (j > HZ) {
-					j = HZ;
-					jiffies_till_next_fqs = HZ;
-				} else if (j < 1) {
-					j = 1;
-					jiffies_till_next_fqs = 1;
-				}
 			} else {
 				/* Deal with stray signal. */
 				cond_resched_tasks_rcu_qs();