Messages in this thread | | | Date | Thu, 31 Mar 2022 15:42:22 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [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;
| |