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

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Aug 05, 2021 at 12:51:48PM +0100, Valentin Schneider wrote:
> On 04/08/21 15:53, Paul E. McKenney wrote:
> > On Wed, Aug 04, 2021 at 11:18:11AM +0100, Valentin Schneider wrote:
> >>
> >> 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?
> 
> I actually run rcutorture as a builtin, but from what I can tell the above
> patch came in v5.14-rc1, and ofc I'm running my tests on v5.13-rt1... I
> should have paid closer attention to what was in the latest mainline,
> apologies for the noise. 

Not a problem, and thank you for giving rcutorture a try!

> FWIW tweaking ksoftirqd priority only when the torture module is builtin
> makes sense to me.

Very good, I will stick with the status quo, then.

							Thanx, Paul