torture: Allow inter-stutter interval to be specified

Currently, the inter-stutter interval is the same as the stutter duration,
that is, whatever number of jiffies is passed into torture_stutter_init().
This has worked well for quite some time, but the addition of
forward-progress testing to rcutorture can delay processes for several
seconds, which can triple the time that they are stuttered.

This commit therefore adds a second argument to torture_stutter_init()
that specifies the inter-stutter interval.  While locktorture preserves
the current behavior, rcutorture uses the RCU CPU stall warning interval
to provide a wider inter-stutter interval.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

include/linux/torture.h

@@ -66,7 +66,7 @@ int torture_shutdown_init(int ssecs, void (*cleanup)(void));
 
 /* Task stuttering, which forces load/no-load transitions. */
 bool stutter_wait(const char *title);
-int torture_stutter_init(int s);
+int torture_stutter_init(int s, int sgap);
 
 /* Initialization and cleanup. */
 bool torture_init_begin(char *ttype, int v);

kernel/locking/locktorture.c

@@ -975,7 +975,7 @@ static int __init lock_torture_init(void)
 			goto unwind;
 	}
 	if (stutter > 0) {
-		firsterr = torture_stutter_init(stutter);
+		firsterr = torture_stutter_init(stutter, stutter);
 		if (firsterr)
 			goto unwind;
 	}

kernel/rcu/rcutorture.c

@@ -2373,7 +2373,10 @@ rcu_torture_init(void)
 	if (stutter < 0)
 		stutter = 0;
 	if (stutter) {
-		firsterr = torture_stutter_init(stutter * HZ);
+		int t;
+
+		t = cur_ops->stall_dur ? cur_ops->stall_dur() : stutter * HZ;
+		firsterr = torture_stutter_init(stutter * HZ, t);
 		if (firsterr)
 			goto unwind;
 	}

kernel/torture.c

@@ -570,6 +570,7 @@ static void torture_shutdown_cleanup(void)
 static struct task_struct *stutter_task;
 static int stutter_pause_test;
 static int stutter;
+static int stutter_gap;
 
 /*
  * Block until the stutter interval ends.  This must be called periodically
@@ -621,7 +622,7 @@ static int torture_stutter(void *arg)
 		}
 		WRITE_ONCE(stutter_pause_test, 0);
 		if (!torture_must_stop())
-			schedule_timeout_interruptible(stutter);
+			schedule_timeout_interruptible(stutter_gap);
 		torture_shutdown_absorb("torture_stutter");
 	} while (!torture_must_stop());
 	torture_kthread_stopping("torture_stutter");
@@ -631,9 +632,10 @@ static int torture_stutter(void *arg)
 /*
  * Initialize and kick off the torture_stutter kthread.
  */
-int torture_stutter_init(const int s)
+int torture_stutter_init(const int s, const int sgap)
 {
 	stutter = s;
+	stutter_gap = sgap;
 	return torture_create_kthread(torture_stutter, NULL, stutter_task);
 }
 EXPORT_SYMBOL_GPL(torture_stutter_init);