Re: [PATCH rcu 2/2] rcu/nocb: Fix rcuog wake-up from offline softirq
From: Uladzislau Rezki
Date: Sat Dec 14 2024 - 11:16:44 EST
On Sat, Dec 14, 2024 at 12:07:25AM +0100, Frederic Weisbecker wrote:
> Le Thu, Dec 12, 2024 at 10:42:14AM -0800, Paul E. McKenney a écrit :
> > From: Frederic Weisbecker <frederic@xxxxxxxxxx>
> >
> > After a CPU has set itself offline and before it eventually calls
> > rcutree_report_cpu_dead(), there are still opportunities for callbacks
> > to be enqueued, for example from an IRQ. When that happens on NOCB, the
> > rcuog wake-up is deferred through an IPI to an online CPU in order not
> > to call into the scheduler and risk arming the RT-bandwidth after
> > hrtimers have been migrated out and disabled.
> >
> > But performing a synchronized IPI from an IRQ is buggy as reported in
> > the following scenario:
> >
> > WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smp_call_function_single
> > Modules linked in: rcutorture torture
> > CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139f93209d1 #1
> > Stopper: multi_cpu_stop+0x0/0x320 <- __stop_cpus+0xd0/0x120
> > RIP: 0010:smp_call_function_single
> > <IRQ>
> > swake_up_one_online
> > __call_rcu_nocb_wake
> > __call_rcu_common
> > ? rcu_torture_one_read
> > call_timer_fn
> > __run_timers
> > run_timer_softirq
> > handle_softirqs
> > irq_exit_rcu
> > ? tick_handle_periodic
> > sysvec_apic_timer_interrupt
> > </IRQ>
> >
> > The periodic tick must be shutdown when the CPU is offline, just like is
> > done for oneshot tick. This must be fixed but this is not enough:
> > softirqs can happen on any hardirq tail and reproduce the above scenario.
> >
> > Fix this with introducing a special deferred rcuog wake up mode when the
> > CPU is offline. This deferred wake up doesn't arm any timer and simply
> > wait for rcu_report_cpu_dead() to be called in order to flush any
> > pending rcuog wake up.
> >
> > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@xxxxxxxxx
> > Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU")
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> You can drop this patch, it has been replaced with another version upstream.
>
Dropped. Thank you!
--
Uladzislau Rezki