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

[Paul E. McKenney]

On Wed, Feb 28, 2024 at 06:43:43AM -0800, Jakub Kicinski wrote:
> On Tue, 27 Feb 2024 20:42:24 -0800 Paul E. McKenney wrote:
> > 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:  
> > > > 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().
> 
> I was thinking of using rcu_all_qs(), like cond_resched() does.
> Not sure how it compares in terms of functionality and cost.

It is probably a bit cheaper, but it does nothing for Tasks RCU.  And that
"_all" in the name is a holdover from when there were separate mechanisms
for bh, sched, and preempt, so maybe we should change that name.

> > 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?
> 
> Very much so, which is why adding the cost of rcu_softirq_qs()
> for every NAPI run feels like an overkill.

Would it be better to do the rcu_softirq_qs() only once every 1000 times
or some such?  Or once every HZ jiffies?

Or is there a better way?

							Thanx, Paul