[tip: core/rcu] torture: Periodically pause in stutter_wait()

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     19012b786ecccb29a9fa20c4ec0a67e2cdfbc010
Gitweb:        https://git.kernel.org/tip/19012b786ecccb29a9fa20c4ec0a67e2cdfbc010
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Tue, 01 Sep 2020 16:58:41 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Fri, 06 Nov 2020 17:13:48 -08:00

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.  This commit also uses hrtimers to provide
very short sleeps to avoid degrading the sudden-on testing that stutter
is supposed to provide.

Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/torture.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/torture.c b/kernel/torture.c
index 1061492..be09377 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -602,8 +602,11 @@ static int stutter_gap;
  */
 bool stutter_wait(const char *title)
 {
-	int spt;
+	ktime_t delay;
+	unsigned int i = 0;
+	int oldnice;
 	bool ret = false;
+	int spt;
 
 	cond_resched_tasks_rcu_qs();
 	spt = READ_ONCE(stutter_pause_test);
@@ -612,8 +615,17 @@ bool stutter_wait(const char *title)
 		if (spt == 1) {
 			schedule_timeout_interruptible(1);
 		} else if (spt == 2) {
-			while (READ_ONCE(stutter_pause_test))
+			oldnice = task_nice(current);
+			set_user_nice(current, MAX_NICE);
+			while (READ_ONCE(stutter_pause_test)) {
+				if (!(i++ & 0xffff)) {
+					set_current_state(TASK_INTERRUPTIBLE);
+					delay = 10 * NSEC_PER_USEC;
+					schedule_hrtimeout(&delay, HRTIMER_MODE_REL);
+				}
 				cond_resched();
+			}
+			set_user_nice(current, oldnice);
 		} else {
 			schedule_timeout_interruptible(round_jiffies_relative(HZ));
 		}