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

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Date: Wed, 28 Feb 2024 14:48:44 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, 
	Yan Zhai <yan@...udflare.com>, Eric Dumazet <edumazet@...gle.com>, 
	Network Development <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>, 
	LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org, 
	bpf <bpf@...r.kernel.org>, kernel-team <kernel-team@...udflare.com>, 
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

On Wed, Feb 28, 2024 at 2:31 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> 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 ]

I'm a bit lost in this thread :)
Just answering the above question.
bpf progs never call cond_resched() directly.
But there are sleepable (aka faultable) bpf progs that
can call some helper or kfunc that may call cond_resched()
in some path.
sleepable bpf progs are protected by rcu_tasks_trace.
That's a very different one vs rcu_tasks.