refperf: Convert nreaders to a module parameter

This commit converts nreaders to a module parameter, with the default
of -1 specifying the old behavior of using 75% of the readers.

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

kernel/rcu/refperf.c

@@ -62,6 +62,12 @@ 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.");
+// Number of readers, with -1 defaulting to about 75% of the CPUs.
+torture_param(int, nreaders, -1, "Number of readers, -1 for 75% of CPUs.");
+// Number of runs.
+torture_param(int, nruns, 30, "Number of experiments to run.");
+// Reader delay in nanoseconds, 0 for no delay.
+torture_param(int, readdelay, 0, "Read-side delay in nanoseconds.");
 
 #ifdef MODULE
 # define REFPERF_SHUTDOWN 0
@@ -93,7 +99,6 @@ static wait_queue_head_t main_wq;
 static int shutdown_start;
 
 static struct reader_task *reader_tasks;
-static int nreaders;
 
 // Number of readers that are part of the current experiment.
 static atomic_t nreaders_exp;
@@ -411,8 +416,8 @@ 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 holdoff=%d loops=%ld\n", perf_type, tag,
-		 verbose, shutdown, holdoff, loops);
+		 "--- %s:  verbose=%d shutdown=%d holdoff=%d loops=%ld nreaders=%d\n", perf_type, tag,
+		 verbose, shutdown, holdoff, loops, nreaders);
 }
 
 static void
@@ -501,7 +506,8 @@ ref_perf_init(void)
 		schedule_timeout_uninterruptible(1);
 	}
 
-	// Reader tasks (~75% of online CPUs).
+	// Reader tasks (default to ~75% of online CPUs).
+	if (nreaders < 0)
 		nreaders = (num_online_cpus() >> 1) + (num_online_cpus() >> 2);
 	reader_tasks = kcalloc(nreaders, sizeof(reader_tasks[0]),
 			       GFP_KERNEL);