linux-kernel - Re: [PATCH rcu 2/2] rcu/nocb: Fix rcuog wake-up from offline softirq

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Message-ID: <Z12vW37q35hvP5n6@pc636>
Date: Sat, 14 Dec 2024 17:16:27 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH rcu 2/2] rcu/nocb: Fix rcuog wake-up from offline softirq

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@...nel.org>
> > 
> > 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@...el.com>
> > Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@intel.com
> > Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU")
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> You can drop this patch, it has been replaced with another version upstream.
> 
Dropped. Thank you!

--
Uladzislau Rezki