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

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Date: Tue, 27 Feb 2024 20:42:24 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Eric Dumazet <edumazet@...gle.com>, Yan Zhai <yan@...udflare.com>,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
	Simon Horman <horms@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Lorenzo Bianconi <lorenzo@...nel.org>,
	Coco Li <lixiaoyan@...gle.com>, Wei Wang <weiwan@...gle.com>,
	Alexander Duyck <alexanderduyck@...com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
	bpf@...r.kernel.org, kernel-team@...udflare.com
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

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