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SubjectRe: [BUG] rcu-tasks : should take care of sparse cpu masks
On Thu, Mar 31, 2022 at 02:45:25PM -0700, Eric Dumazet wrote:
> Hi Paul
>
> It seems you assume per cpu ptr for arbitrary indexes (< nr_cpu_ids) are valid.

Gah! I knew I was forgetting something...

But just to check, is this a theoretical problem or something you hit
on real hardware? (For the rest of this email, I am assuming the latter.)

> What do you think of the (untested) following patch ?

One issue with this patch is that the contention could be unpredictable,
or worse, vary among CPU, especially if the cpu_possible_mask was oddly
distributed.

So might it be better to restrict this to all on CPU 0 on the one hand
and completely per-CPU on the other? (Or all on the boot CPU, in case
I am forgetting some misbegotten architecture that can run without a
CPU 0.)

Thanx, Paul

> Thanks.
>
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 99cf3a13954cfb17828fbbeeb884f11614a526a9..df3785be4022e903d9682dd403464aa9927aa5c2
> 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -273,13 +273,17 @@ static void call_rcu_tasks_generic(struct
> rcu_head *rhp, rcu_callback_t func,
> bool needadjust = false;
> bool needwake;
> struct rcu_tasks_percpu *rtpcp;
> + int ideal_cpu, chosen_cpu;
>
> rhp->next = NULL;
> rhp->func = func;
> local_irq_save(flags);
> rcu_read_lock();
> - rtpcp = per_cpu_ptr(rtp->rtpcpu,
> - smp_processor_id() >>
> READ_ONCE(rtp->percpu_enqueue_shift));
> +
> + ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift);
> + chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_online_mask);
> +
> + rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu);
> if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
> raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
> j = jiffies;

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