torture: Periodically pause in stutter_wait()

Running locktorture scenario LOCK05 results in hangs:

tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --torture lock --duration 3 --configs LOCK05

The lock_torture_writer() kthreads set themselves to MAX_NICE while
running SCHED_OTHER.  Other locktorture kthreads run at default niceness,
also SCHED_OTHER.  This results in these other locktorture kthreads
indefinitely preempting the lock_torture_writer() kthreads.  Note that
the cond_resched() in the stutter_wait() function's loop is ineffective
because this scenario is built with CONFIG_PREEMPT=y.

It is not clear that such indefinite preemption is supposed to happen, but
in the meantime this commit prevents kthreads running in stutter_wait()
from being completely CPU-bound, thus allowing the other threads to get
some CPU in a timely fashion.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/torture.c

diff --git a/kernel/torture.c b/kernel/torture.c
index 1061492..9310c1d 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -602,6 +602,7 @@ static int stutter_gap;
  */
 bool stutter_wait(const char *title)
 {
+	unsigned i = 0;
 	int spt;
 	bool ret = false;
 
@@ -612,8 +613,13 @@ bool stutter_wait(const char *title)
 		if (spt == 1) {
 			schedule_timeout_interruptible(1);
 		} else if (spt == 2) {
-			while (READ_ONCE(stutter_pause_test))
+			while (READ_ONCE(stutter_pause_test)) {
+				if (!(++i & 0xffff))
+					schedule_timeout_interruptible(1);
+				else if (!(i & 0xfff))
+					udelay(1);
 				cond_resched();
+			}
 		} else {
 			schedule_timeout_interruptible(round_jiffies_relative(HZ));
 		}