From patchwork Thu Dec 12 18:42:14 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. 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McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, Frederic Weisbecker , kernel test robot , "Paul E . McKenney" Subject: [PATCH rcu 2/2] rcu/nocb: Fix rcuog wake-up from offline softirq Date: Thu, 12 Dec 2024 10:42:14 -0800 Message-Id: <20241212184214.2018411-2-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Frederic Weisbecker 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 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 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 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 Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.h | 1 + kernel/rcu/tree_nocb.h | 14 ++++++++++++-- 2 files changed, 13 insertions(+), 2 deletions(-) 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"));