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

From: Jakub Kicinski <kuba@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>, Yan Zhai <yan@cloudflare.com>,
	netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
	Paolo Abeni <pabeni@redhat.com>, Jiri Pirko <jiri@resnulli.us>,
	Simon Horman <horms@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Lorenzo Bianconi <lorenzo@kernel.org>,
	Coco Li <lixiaoyan@google.com>, Wei Wang <weiwan@google.com>,
	Alexander Duyck <alexanderduyck@fb.com>,
	Hannes Frederic Sowa <hannes@stressinduktion.org>,
	linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
	bpf@vger.kernel.org, kernel-team@cloudflare.com
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll
Date: Tue, 27 Feb 2024 19:10:01 -0800	[thread overview]
Message-ID: <20240227191001.0c521b03@kernel.org> (raw)
In-Reply-To: <d633c5b9-53a5-4cd6-9dbb-6623bb74c00b@paulmck-laptop>

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.