Re: [PATCH 2/2] rcutorture: Nudge ksoftirqd priority for RCU boost testing

On 03/08/21 16:42, Paul E. McKenney wrote:

On Tue, Aug 03, 2021 at 11:54:37PM +0100, Valentin Schneider wrote:

quoted

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 680f66b65f14..3dd5fa75f469 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c

@@ -948,12 +948,26 @@ static int rcu_torture_boost(void *arg)
     unsigned long endtime;
     unsigned long oldstarttime;
     struct rcu_boost_inflight rbi = { .inflight = 0 };
+	struct task_struct *ksoftirqd = this_cpu_ksoftirqd();

     VERBOSE_TOROUT_STRING("rcu_torture_boost started");

     /* Set real-time priority. */
     sched_set_fifo_low(current);

+	/*
+	 * Boost testing requires TIMER_SOFTIRQ to run at a higher priority
+	 * than the CPU-hogging torture kthreads, otherwise said threads
+	 * will never let timer expiry for the RCU GP kthread happen, which will
+	 * prevent any boosting.
+	 */
+	if (current->normal_prio < ksoftirqd->normal_prio) {

Would it make sense to add IS_ENABLED(CONFIG_PREEMPT_RT) to the above
condition?

Hm so v5.13-rt1 has this commit:

  5e59fba573e6 ("rcutorture: Fix testing of RCU priority boosting")

which gates RCU boost torture testing under CONFIG_PREEMPT_RT. Now, AFAICT
the TIMER_SOFTIRQ priority problem is there regardless of
CONFIG_PREEMPT_RT, so this patch would (should?) make sense even on
!CONFIG_PREEMPT_RT.

                                                      Thanx, Paul

quoted

+		struct sched_param sp = { .sched_priority = 2 };
+
+		pr_alert("%s(): Adjusting %s priority\n", __func__, ksoftirqd->comm);
+		sched_setscheduler_nocheck(ksoftirqd, SCHED_FIFO, &sp);
+	}
+
     init_rcu_head_on_stack(&rbi.rcu);
     /* Each pass through the following loop does one boost-test cycle. */
     do {
--
2.25.1