[PATCH tip/core/rcu 04/28] torture: Periodically pause in stutter_wait()

paulmck
2020-11-05T23:49:01Z

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@suse.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/torture.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/torture.c b/kernel/torture.c
index 1061492..5488ad2 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 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));
}
--
2.9.5