refperf: Add holdoff parameter to allow CPUs to come online

This commit adds an rcuperf module parameter named "holdoff" that
defaults to 10 seconds if refperf is built in and to zero otherwise.
The assumption is that all the CPUs are online by the time that the
modprobe and insmod commands are going to do anything, and that normal
systems will have all the CPUs online within ten seconds.

Larger systems may take many tens of seconds or even minutes to get
to this point, hence this being a module parameter instead of being a
hard-coded constant.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/refperf.c

@@ -57,7 +57,10 @@ MODULE_PARM_DESC(perf_type, "Type of test (rcu, srcu, refcnt, rwsem, rwlock.");
 
 torture_param(int, verbose, 0, "Enable verbose debugging printk()s");
 
-// Number of loops per experiment, all readers execute an operation concurrently
+// Wait until there are multiple CPUs before starting test.
+torture_param(int, holdoff, IS_BUILTIN(CONFIG_RCU_REF_PERF_TEST) ? 10 : 0,
+	      "Holdoff time before test start (s)");
+// Number of loops per experiment, all readers execute operations concurrently.
 torture_param(long, loops, 10000000, "Number of loops per experiment.");
 
 #ifdef MODULE
@@ -248,6 +251,8 @@ ref_perf_reader(void *arg)
 	set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
 	set_user_nice(current, MAX_NICE);
 	atomic_inc(&n_init);
+	if (holdoff)
+		schedule_timeout_interruptible(holdoff * HZ);
 repeat:
 	VERBOSE_PERFOUT("ref_perf_reader %ld: waiting to start next experiment on cpu %d", me, smp_processor_id());
 
@@ -357,6 +362,8 @@ static int main_func(void *arg)
 
 	// Wait for all threads to start.
 	wait_event(main_wq, atomic_read(&n_init) == (nreaders + 1));
+	if (holdoff)
+		schedule_timeout_interruptible(holdoff * HZ);
 
 	// Start exp readers up per experiment
 	for (exp = 0; exp < nreaders && !torture_must_stop(); exp++) {
@@ -420,8 +427,8 @@ static void
 ref_perf_print_module_parms(struct ref_perf_ops *cur_ops, const char *tag)
 {
 	pr_alert("%s" PERF_FLAG
-		 "--- %s:  verbose=%d shutdown=%d loops=%ld\n", perf_type, tag,
-		 verbose, shutdown, loops);
+		 "--- %s:  verbose=%d shutdown=%d holdoff=%d loops=%ld\n", perf_type, tag,
+		 verbose, shutdown, holdoff, loops);
 }
 
 static void