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

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Date: Wed, 28 Feb 2024 17:33:07 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Joel Fernandes <joel@...lfernandes.org>, Yan Zhai <yan@...udflare.com>,
 Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org, "David S.
 Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 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,
 mark.rutland@....com, Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

On Wed, 28 Feb 2024 14:19:11 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> > > 
> > > Well, to your initial point, cond_resched() does eventually invoke
> > > preempt_schedule_common(), so you are quite correct that as far as
> > > Tasks RCU is concerned, cond_resched() is not a quiescent state.  
> > 
> >  Thanks for confirming. :-)  
> 
> However, given that the current Tasks RCU use cases wait for trampolines
> to be evacuated, Tasks RCU could make the choice that cond_resched()
> be a quiescent state, for example, by adjusting rcu_all_qs() and
> .rcu_urgent_qs accordingly.
> 
> But this seems less pressing given the chance that cond_resched() might
> go away in favor of lazy preemption.

Although cond_resched() is technically a "preemption point" and not truly a
voluntary schedule, I would be happy to state that it's not allowed to be
called from trampolines, or their callbacks. Now the question is, does BPF
programs ever call cond_resched()? I don't think they do.

[ Added Alexei ]

-- Steve