Messages in this thread | | | From | Eric Dumazet <> | Date | Thu, 31 Mar 2022 15:54:02 -0700 | 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@kernel.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;
| |