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

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Date: Wed, 28 Feb 2024 07:35:44 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: "Paul E. McKenney" <paulmck@...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 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?