6.6-stable review patch. If anyone has any objections, please let me know.
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From: Frederic Weisbecker frederic@kernel.org
[ Upstream commit 9139f93209d1ffd7f489ab19dee01b7c3a1a43d2 ]
After a CPU is marked offline and until it reaches its final trip to idle, rcuo has several opportunities to be woken up, either because a callback has been queued in the meantime or because rcutree_report_cpu_dead() has issued the final deferred NOCB wake up.
If RCU-boosting is enabled, RCU kthreads are set to SCHED_FIFO policy. And if RT-bandwidth is enabled, the related hrtimer might be armed. However this then happens after hrtimers have been migrated at the CPUHP_AP_HRTIMERS_DYING stage, which is broken as reported by the following warning:
Call trace: enqueue_hrtimer+0x7c/0xf8 hrtimer_start_range_ns+0x2b8/0x300 enqueue_task_rt+0x298/0x3f0 enqueue_task+0x94/0x188 ttwu_do_activate+0xb4/0x27c try_to_wake_up+0x2d8/0x79c wake_up_process+0x18/0x28 __wake_nocb_gp+0x80/0x1a0 do_nocb_deferred_wakeup_common+0x3c/0xcc rcu_report_dead+0x68/0x1ac cpuhp_report_idle_dead+0x48/0x9c do_idle+0x288/0x294 cpu_startup_entry+0x34/0x3c secondary_start_kernel+0x138/0x158
Fix this with waking up rcuo using an IPI if necessary. Since the existing API to deal with this situation only handles swait queue, rcuo is only woken up from offline CPUs if it's not already waiting on a grace period. In the worst case some callbacks will just wait for a grace period to complete before being assigned to a subsequent one.
Reported-by: "Cheng-Jui Wang (王正睿)" Cheng-Jui.Wang@mediatek.com Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier") Signed-off-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Neeraj Upadhyay neeraj.upadhyay@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/rcu/tree_nocb.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 30b34f215ca35..e019f166daa61 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -220,7 +220,10 @@ static bool __wake_nocb_gp(struct rcu_data *rdp_gp, raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags); if (needwake) { trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake")); - wake_up_process(rdp_gp->nocb_gp_kthread); + if (cpu_is_offline(raw_smp_processor_id())) + swake_up_one_online(&rdp_gp->nocb_gp_wq); + else + wake_up_process(rdp_gp->nocb_gp_kthread); }
return needwake;