linux-kernel - Re: rcutorture: Can not Disable RT throttling

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Date:   Wed, 23 Aug 2023 09:50:37 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Z qiang <qiang.zhang1211@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: rcutorture: Can not Disable RT throttling

On Tue, Aug 22, 2023 at 3:37 AM Z qiang <qiang.zhang1211@...il.com> 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.

Thoughts?

Thanks.