On Tue, Feb 27, 2024 at 07:10:01PM -0800, Jakub Kicinski wrote:
> 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.
I am not completely clear on what you are proposing, but one complication
is that We need preemption disabled across calls to rcu_softirq_qs()
and we cannot have preemption disabled across calls to cond_resched().
Another complication is that although CONFIG_PREEMPT_RT kernels are
built with CONFIG_PREEMPT_RCU, the reverse is not always the case.
And if we are not repolling, don't we have a high probability of doing
a voluntary context when we reach napi_thread_wait() at the beginning
of that loop?
All in all, I suspect that I am missing your point.
Thanx, Paul
