Re: [PATCH 2/3] rcu: Defer RCU kthreads wakeup when CPU is dying

Cheng-Jui Wang (王正睿)
2024-08-12T09:11:14Z

On Wed, 2023-12-20 at 16:13 +0100, Frederic Weisbecker wrote:
> Le Wed, Dec 20, 2023 at 04:24:35PM +0800, Z qiang a écrit :
> > >
> >
> > In the rcutree_report_cpu_dead(), the rcuog kthreads may also be
> > wakeup in
> > do_nocb_deferred_wakeup(), if the rcuog kthreads is rt-fifo and
> > wakeup happen,
> > the rt_period_active is set 1 and enqueue hrtimer to offline CPU in
> > do_start_rt_bandwidth(),
> > after that, we invoke swake_up_one_online() send ipi to online CPU,
> > due to the
> > rt_period_active is 1, the rt-bandwith hrtimer will not enqueue to
> > online CPU.
> > any thoughts?
>
> Duh, you're right, that one too. How many more? This hrtimer
> situation is scary...
>
> Thanks.

I encountered the issue pointed out by Z Qiang on kernel-6.6, which
cause RT tasks on the CPU to never be scheduled, leading to a system
reboot. It triggered the WARN_ON_ONCE(!base->cpu_base->online)
introduced in [PATCH 1/3]. Below is the call trace:

Call trace:
enqueue_hrtimer+0x7c/0xf8
hrtimer_start_range_ns+0x2b8/0x300
enqueue_task_rt+0x298/0x3f0
enqueue_task+0x94/0x188
ttwu_do_activate+0xb4/0x27c
try_to_wake_up+0x2d8/0x79c
wake_up_process+0x18/0x28
__wake_nocb_gp+0x80/0x1a0
do_nocb_deferred_wakeup_common+0x3c/0xcc
rcu_report_dead+0x68/0x1ac
cpuhp_report_idle_dead+0x48/0x9c
do_idle+0x288/0x294
cpu_startup_entry+0x34/0x3c
secondary_start_kernel+0x138/0x158