Re: rcutorture: Can not Disable RT throttling

[Paul E. McKenney]

On Wed, Aug 23, 2023 at 09:50:37AM -0400, Joel Fernandes wrote:
> On Tue, Aug 22, 2023 at 3:37 AM Z qiang <qiang.zhang1211@xxxxxxxxx> wrote:
> >
> > When running build-in rcutorture tests in 6.5.0-rc4-rt, and found that,
> > although the value of /proc/sys/kernel/sched_rt_runtime_us is -1,
> >
> > cat /sys/kernel/debug/sched/debug
> > ....
> > rt_rq[6]:
> >   .rt_nr_running                 : 4
> >   .rt_nr_migratory               : 0
> >   .rt_throttled                  : 0
> >   .rt_time                       : 0.000000
> >   .rt_runtime                    : 950.000000
> >
> > but the rt_runtime still is 950.000000.
> > set sysctl_sched_rt_runtime in rcu_torture_disable_rt_throttle()
> > does not disable rt-throttling.
> 
> I think you have hit a bug. I think the problem is
> rcu_torture_disable_rt_throttle() modifies the sysctl knobs, but does
> not change def_rt_bandwidth and reinitialize the rt_rq. I think we
> need to call sched_rt_do_global() like the sysfs handler does, or
> change the sysctl knobs to be earlier in the boot process before the
> rt_rq are initialized.
> 
> Or better yet (not sure if it is possible) trigger the sysctl change
> via the sysctl layer and let it do the same logic.

That would be difficult in built-in rcutorture testing due to the fact
that there is not much in the way of userspace.  I suppose we could
invoke the sysfs handler so as to mock up a userspace access, but that
might not be the most robust approach.

Another way is to disable preemption in the real-time kthreads.  Which
might need careful implementation to avoid "scheduling while atomic"
and friends.

							Thanx, Paul