rcu-tasks: Add rcupdate.rcu_task_enqueue_lim to set initial queueing

This commit adds a rcupdate.rcu_task_enqueue_lim module parameter that
sets the initial number of callback queues to use for the RCU Tasks
family of RCU implementations.  This parameter allows testing of various
fanout values.
Reported-by: Martin Lau <kafai@fb.com>
Cc: Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4805,6 +4805,13 @@
 			period to instead use normal non-expedited
 			grace-period processing.
 
+	rcupdate.rcu_task_enqueue_lim= [KNL]
+			Set the number of callback queues to use for the
+			RCU Tasks family of RCU flavors.  The default
+			of -1 allows this to be automatically (and
+			dynamically) adjusted.	This parameter is intended
+			for use in testing.
+
 	rcupdate.rcu_task_ipi_delay= [KNL]
 			Set time in jiffies during which RCU tasks will
 			avoid sending IPIs, starting with the beginning

kernel/rcu/tasks.h

@@ -130,6 +130,9 @@ module_param(rcu_task_ipi_delay, int, 0644);
 static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
 module_param(rcu_task_stall_timeout, int, 0644);
 
+static int rcu_task_enqueue_lim __read_mostly = -1;
+module_param(rcu_task_enqueue_lim, int, 0444);
+
 /* RCU tasks grace-period state for debugging. */
 #define RTGS_INIT		 0
 #define RTGS_WAIT_WAIT_CBS	 1
@@ -192,10 +195,19 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
 {
 	int cpu;
 	unsigned long flags;
+	int lim;
 
 	raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
-	rtp->percpu_enqueue_shift = ilog2(nr_cpu_ids);
-	rtp->percpu_enqueue_lim = 1;
+	if (rcu_task_enqueue_lim < 0)
+		rcu_task_enqueue_lim = nr_cpu_ids;
+	else if (rcu_task_enqueue_lim == 0)
+		rcu_task_enqueue_lim = 1;
+	lim = rcu_task_enqueue_lim;
+
+	if (lim > nr_cpu_ids)
+		lim = nr_cpu_ids;
+	WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids / lim));
+	smp_store_release(&rtp->percpu_enqueue_lim, lim);
 	for_each_possible_cpu(cpu) {
 		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 
@@ -211,7 +223,7 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
 		raw_spin_unlock_rcu_node(rtpcp); // irqs remain disabled.
 	}
 	raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
-
+	pr_info("%s: Setting shift to %d and lim to %d.\n", __func__, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim));
 }
 
 // Enqueue a callback for the specified flavor of Tasks RCU.
@@ -307,7 +319,7 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
 	unsigned long flags;
 	int needgpcb = 0;
 
-	for (cpu = 0; cpu < rtp->percpu_enqueue_lim; cpu++) {
+	for (cpu = 0; cpu < smp_load_acquire(&rtp->percpu_enqueue_lim); cpu++) {
 		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 
 		/* Advance and accelerate any new callbacks. */
@@ -338,11 +350,11 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
 
 	cpu = rtpcp->cpu;
 	cpunext = cpu * 2 + 1;
-	if (cpunext < rtp->percpu_enqueue_lim) {
+	if (cpunext < smp_load_acquire(&rtp->percpu_enqueue_lim)) {
 		rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
 		queue_work_on(cpunext, system_wq, &rtpcp_next->rtp_work);
 		cpunext++;
-		if (cpunext < rtp->percpu_enqueue_lim) {
+		if (cpunext < smp_load_acquire(&rtp->percpu_enqueue_lim)) {
 			rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
 			queue_work_on(cpunext, system_wq, &rtpcp_next->rtp_work);
 		}