rcuscale: Add minruntime module parameter

By default, rcuscale collects only 100 points of data per writer, but
arranging for all kthreads to be actively collecting (if not recording)
data during the time that any kthread might be recording.  This works
well, but does not allow much time to bring external performance tools
to bear.  This commit therefore adds a minruntime module parameter
that specifies a minimum data-collection interval in seconds.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4953,6 +4953,12 @@
 			Number of loops doing rcuscale.kfree_alloc_num number
 			of allocations and frees.
 
+	rcuscale.minruntime= [KNL]
+			Set the minimum test run time in seconds.  This
+			does not affect the data-collection interval,
+			but instead allows better measurement of things
+			like CPU consumption.
+
 	rcuscale.nreaders= [KNL]
 			Set number of RCU readers.  The value -1 selects
 			N, where N is the number of CPUs.  A value

kernel/rcu/rcuscale.c

@@ -87,6 +87,7 @@ torture_param(bool, gp_async, false, "Use asynchronous GP wait primitives");
 torture_param(int, gp_async_max, 1000, "Max # outstanding waits per writer");
 torture_param(bool, gp_exp, false, "Use expedited GP wait primitives");
 torture_param(int, holdoff, 10, "Holdoff time before test start (s)");
+torture_param(int, minruntime, 0, "Minimum run time (s)");
 torture_param(int, nreaders, -1, "Number of RCU reader threads");
 torture_param(int, nwriters, -1, "Number of RCU updater threads");
 torture_param(bool, shutdown, RCUSCALE_SHUTDOWN,
@@ -411,6 +412,7 @@ rcu_scale_writer(void *arg)
 {
 	int i = 0;
 	int i_max;
+	unsigned long jdone;
 	long me = (long)arg;
 	struct rcu_head *rhp = NULL;
 	bool started = false, done = false, alldone = false;
@@ -447,6 +449,7 @@ rcu_scale_writer(void *arg)
 		}
 	}
 
+	jdone = jiffies + minruntime * HZ;
 	do {
 		if (writer_holdoff)
 			udelay(writer_holdoff);
@@ -479,7 +482,7 @@ rcu_scale_writer(void *arg)
 		if (!started &&
 		    atomic_read(&n_rcu_scale_writer_started) >= nrealwriters)
 			started = true;
-		if (!done && i >= MIN_MEAS) {
+		if (!done && i >= MIN_MEAS && time_after(jiffies, jdone)) {
 			done = true;
 			sched_set_normal(current, 0);
 			pr_alert("%s%s rcu_scale_writer %ld has %d measurements\n",