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

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Jakub Kicinski <kuba@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: Wed, 28 Feb 2024 07:15:42 -0800	[thread overview]
Message-ID: <9a0052f9-b022-42c9-a5da-1d6ca3b00885@paulmck-laptop> (raw)
In-Reply-To: <20240228064343.578a5363@kernel.org>

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