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Date:   Thu, 31 Mar 2022 15:42:22 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Eric Dumazet <edumazet@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] rcu-tasks : should take care of sparse cpu masks
Message-ID: <20220331224222.GY4285@paulmck-ThinkPad-P17-Gen-1>
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References: <CANn89iKaNEwyNZ=L_PQnkH0LP_XjLYrr_dpyRKNNoDJaWKdrmg@mail.gmail.com>
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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;