On Wed, Aug 04, 2021 at 11:18:11AM +0100, Valentin Schneider wrote:
> On 03/08/21 16:42, Paul E. McKenney wrote:
> > On Tue, Aug 03, 2021 at 11:54:37PM +0100, Valentin Schneider wrote:
> >> 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.
What rcutorture scenario TREE03 does is to boot with tree.use_softirq=0
and threadirqs. I see your point about timers and softirq, but this
does run reliably for me.
Ah, I see why. Commit ea6d962e80b6 ("rcutorture: Judge RCU priority
boosting on grace periods, not callbacks") includes boosting the priority
of the ksoftirqd kthreads. But only when running rcutorture builtin,
not as a module. Here is the code in rcu_torture_init():
// Testing RCU priority boosting requires rcutorture do
// some serious abuse. Counter this by running ksoftirqd
// at higher priority.
if (IS_BUILTIN(CONFIG_RCU_TORTURE_TEST)) {
for_each_online_cpu(cpu) {
struct sched_param sp;
struct task_struct *t;
t = per_cpu(ksoftirqd, cpu);
WARN_ON_ONCE(!t);
sp.sched_priority = 2;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
}
}
I take it that you were running rcutorture as a module?
This describes how to run it built-in, if that works for you:
https://paulmck.livejournal.com/61432.html
More specifically: https://paulmck.livejournal.com/57769.html
Alternatively, the "IS_BUILTIN(CONFIG_RCU_TORTURE_TEST)" check could be
removed in the above code, and the ksoftirqd kthreads could have their
original priority restored in rcu_torture_cleanup().
Thoughts?
Thanx, Paul
> >> + 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
> >>
