Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

[Yan Zhai]

On Wed, Feb 28, 2024 at 9:37 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> Also optionally, I wonder if calling rcu_tasks_qs() directly is better
> (for documentation if anything) since the issue is Tasks RCU specific. Also
> code comment above the rcu_softirq_qs() call about cond_resched() not taking
> care of Tasks RCU would be great!
>
Yes it's quite surprising to me that cond_resched does not help here,
I will include that comment. Raising just the task RCU QS seems
sufficient to the problem we encountered. But according to commit
d28139c4e967 ("rcu: Apply RCU-bh QSes to RCU-sched and RCU-preempt
when safe"), there might be additional threat factor in __do_softirq
that also applies to threaded poll.

Yan


> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
> thanks,
>
>  - Joel
> [1]
> @@ -381,8 +553,10 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>                 pending >>= softirq_bit;
>         }
>
> -       if (__this_cpu_read(ksoftirqd) == current)
> +       if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
> +           __this_cpu_read(ksoftirqd) == current)
>                 rcu_softirq_qs();
> +
>         local_irq_disable();