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
