| Message ID | 20241212184214.2018411-2-paulmck@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | No-CB changes for v6.14 | expand |
Le Thu, Dec 12, 2024 at 10:42:14AM -0800, Paul E. McKenney a écrit : > From: Frederic Weisbecker <frederic@kernel.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@intel.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@kernel.org> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> You can drop this patch, it has been replaced with another version upstream. Thanks!
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@kernel.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@intel.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@kernel.org> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > > You can drop this patch, it has been replaced with another version upstream. > Dropped. Thank you! -- Uladzislau Rezki
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index a9a811d9d7a37..7ed060edd12b1 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -290,6 +290,7 @@ struct rcu_data { #define RCU_NOCB_WAKE_LAZY 2 #define RCU_NOCB_WAKE 3 #define RCU_NOCB_WAKE_FORCE 4 +#define RCU_NOCB_WAKE_OFFLINE 5 #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500)) /* For jiffies_till_first_fqs and */ diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 0923d60c5a338..78841346e1c13 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -295,6 +295,8 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype, case RCU_NOCB_WAKE_FORCE: if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE) mod_timer(&rdp_gp->nocb_timer, jiffies + 1); + fallthrough; + case RCU_NOCB_WAKE_OFFLINE: if (rdp_gp->nocb_defer_wakeup < waketype) WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype); break; @@ -562,8 +564,16 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, lazy_len = READ_ONCE(rdp->lazy_len); if (was_alldone) { rdp->qlen_last_fqs_check = len; - // Only lazy CBs in bypass list - if (lazy_len && bypass_len == lazy_len) { + if (cpu_is_offline(rdp->cpu)) { + /* + * Offline CPUs can't call swake_up_one_online() from IRQs. Rely + * on the final deferred wake-up rcutree_report_cpu_dead() + */ + rcu_nocb_unlock(rdp); + wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_OFFLINE, + TPS("WakeEmptyIsDeferredOffline")); + } else if (lazy_len && bypass_len == lazy_len) { + // Only lazy CBs in bypass list rcu_nocb_unlock(rdp); wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY, TPS("WakeLazy"));