linux-kernel - Re: [BUG] rcu-tasks : should take care of sparse cpu masks

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Message-ID: <CANn89iJjyp7s1fYB6VCqLhUnF+mmEXyw8GMpFC9Vi22usBsgAQ@mail.gmail.com>
Date:   Thu, 31 Mar 2022 15:54:02 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] rcu-tasks : should take care of sparse cpu masks

On Thu, Mar 31, 2022 at 3:42 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> 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.)

Code review really...

>
> > 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.)

If I understand correctly, cblist_init_generic() could setup
percpu_enqueue_shift
to something smaller than order_base_2(nr_cpu_ids)

Meaning that we could reach a non zero idx in (smp_processor_id() >>
percpu_enqueue_shift)

So even if CPU0 is always present (I am not sure this is guaranteed,
but this seems reasonable),
we could still attempt a per_cpu_ptr(PTR,  not_present_cpu), and get garbage.



> > 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;