Re: [PATCH 5.15 000/183] 5.15.134-rc1 review

[Frederic Weisbecker]

Le Tue, Oct 10, 2023 at 06:34:35PM -0700, Paul E. McKenney a écrit :
> If this problem is real, fixes include:
> 
> o     Revert Liam's patch and make Tiny RCU's call_rcu() deal with
>       the problem.  This is overhead and non-tinyness, but to Joel's
>       point, it might be best.

But what is calling call_rcu() or start_poll_synchronize_rcu() so
early that the CPU is not even online? (that's before boot_cpu_init() !)

Deferring PF_IDLE setting might pave the way for more issues like this one,
present or future. Though is_idle_task() returning true when the task is not
in the idle loop but is playing the init/0 role is debatable.

An alternative for tiny RCU is to force waking up ksoftirqd when call_rcu()
is in the idle task. Since rcu_qs() during the context switch raises a softirq
anyway. It's more overhead for start_poll_synchronize_rcu() though but do we
expect much RCU polling in idle?

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index a92bce40b04b..6ab15233e2be 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -604,6 +604,7 @@ extern void __raise_softirq_irqoff(unsigned int nr);
 
 extern void raise_softirq_irqoff(unsigned int nr);
 extern void raise_softirq(unsigned int nr);
+extern void raise_ksoftirqd_irqsoff(unsigned int nr);
 
 DECLARE_PER_CPU(struct task_struct *, ksoftirqd);
 
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index 42f7589e51e0..872dab8b8b53 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -189,12 +189,12 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
        local_irq_save(flags);
        *rcu_ctrlblk.curtail = head;
        rcu_ctrlblk.curtail = &head->next;
-       local_irq_restore(flags);
 
        if (unlikely(is_idle_task(current))) {
                /* force scheduling for rcu_qs() */
-               resched_cpu(0);
+               raise_ksoftirqd_irqsoff(RCU_SOFTIRQ);
        }
+       local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(call_rcu);
 
@@ -225,10 +225,13 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
 unsigned long start_poll_synchronize_rcu(void)
 {
        unsigned long gp_seq = get_state_synchronize_rcu();
+       unsigned long flags;
 
        if (unlikely(is_idle_task(current))) {
+               local_irq_save(flags);
                /* force scheduling for rcu_qs() */
-               resched_cpu(0);
+               raise_ksoftirqd_irqsoff(RCU_SOFTIRQ);
+               local_irq_restore(flags);
        }
        return gp_seq;
 }
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 210cf5f8d92c..ef105cbdc705 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -695,6 +695,14 @@ void __raise_softirq_irqoff(unsigned int nr)
        or_softirq_pending(1UL << nr);
 }
 
+#ifdef CONFIG_RCU_TINY
+void raise_ksoftirqd(unsigned int nr)
+{
+       __raise_softirq_irqoff(nr);
+       wakeup_softirqd();
+}
+#endif
+
 void open_softirq(int nr, void (*action)(struct softirq_action *))
 {
        softirq_vec[nr].action = action;