[PATCH] rcu: Make jiffies_till_sched_qs writable

[Byungchul Park]

jiffies_till_sched_qs is useless if it's readonly as it is used to set
jiffies_to_sched_qs with its value regardless of first/next fqs jiffies.
And it should be applied immediately on change through sysfs.

The function for setting jiffies_to_sched_qs,
adjust_jiffies_till_sched_qs() will be called only if
the value from sysfs != ULONG_MAX. And the value won't be adjusted
unlike first/next fqs jiffies.

While at it, changed the positions of two module_param()s downward.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/rcu/tree.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a2f8ba2..a28e2fe 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -422,9 +422,7 @@ static int rcu_is_cpu_rrupt_from_idle(void)
  * quiescent-state help from rcu_note_context_switch().
  */
 static ulong jiffies_till_sched_qs = ULONG_MAX;
-module_param(jiffies_till_sched_qs, ulong, 0444);
 static ulong jiffies_to_sched_qs; /* See adjust_jiffies_till_sched_qs(). */
-module_param(jiffies_to_sched_qs, ulong, 0444); /* Display only! */
 
 /*
  * Make sure that we give the grace-period kthread time to detect any
@@ -450,6 +448,18 @@ static void adjust_jiffies_till_sched_qs(void)
 	WRITE_ONCE(jiffies_to_sched_qs, j);
 }
 
+static int param_set_sched_qs_jiffies(const char *val, const struct kernel_param *kp)
+{
+	ulong j;
+	int ret = kstrtoul(val, 0, &j);
+
+	if (!ret && j != ULONG_MAX) {
+		WRITE_ONCE(*(ulong *)kp->arg, j);
+		adjust_jiffies_till_sched_qs();
+	}
+	return ret;
+}
+
 static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp)
 {
 	ulong j;
@@ -474,6 +484,11 @@ static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param
 	return ret;
 }
 
+static struct kernel_param_ops sched_qs_jiffies_ops = {
+	.set = param_set_sched_qs_jiffies,
+	.get = param_get_ulong,
+};
+
 static struct kernel_param_ops first_fqs_jiffies_ops = {
 	.set = param_set_first_fqs_jiffies,
 	.get = param_get_ulong,
@@ -484,8 +499,11 @@ static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param
 	.get = param_get_ulong,
 };
 
+module_param_cb(jiffies_till_sched_qs, &sched_qs_jiffies_ops, &jiffies_till_sched_qs, 0644);
 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(jiffies_to_sched_qs, ulong, 0444); /* Display only! */
 module_param(rcu_kick_kthreads, bool, 0644);
 
 static void force_qs_rnp(int (*f)(struct rcu_data *rdp));
-- 
1.9.1