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: Wed, 28 Feb 2024 07:35:44 -0800	[thread overview]
Message-ID: <20240228073544.791ae897@kernel.org> (raw)
In-Reply-To: <9a0052f9-b022-42c9-a5da-1d6ca3b00885@paulmck-laptop>

On Wed, 28 Feb 2024 07:15:42 -0800 Paul E. McKenney wrote:
> > > 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?

Right, we can do that. Yan Zhai, have you measured the performance
impact / time spent in the call?