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

From: Jakub Kicinski <[email-masked]>
Date: Tue, 27 Feb 2024 19:10:01 -0800

On Tue, 27 Feb 2024 10:32:22 -0800 Paul E. McKenney wrote:
> > > +                       if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> > > +                               rcu_softirq_qs();
> > > +
> > >                         local_bh_enable();
> > >
> > >                         if (!repoll)
> >
> > Hmm....
> > Why napi_busy_loop() does not have a similar problem ?
> > 
> > It is unclear why rcu_all_qs() in __cond_resched() is guarded by
> > 
> > #ifndef CONFIG_PREEMPT_RCU
> >      rcu_all_qs();
> > #endif  
> 
> The theory is that PREEMPT_RCU kernels have preemption, and get their
> quiescent states that way.

But that doesn't work well enough?

Assuming that's the case why don't we add it with the inverse ifdef
condition next to the cond_resched() which follows a few lines down?

			skb_defer_free_flush(sd);
+
+			if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+				rcu_softirq_qs();
+
			local_bh_enable();

			if (!repoll)
				break;

			cond_resched();
		}

We won't repoll majority of the time.