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

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Date: Wed, 28 Feb 2024 15:14:34 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: paulmck@...nel.org
Cc: 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, rostedt@...dmis.org, mark.rutland@....com
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

On Wed, Feb 28, 2024 at 12:18 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Wed, Feb 28, 2024 at 10:37:51AM -0600, Yan Zhai wrote:
> > On Wed, Feb 28, 2024 at 9:37 AM Joel Fernandes <joel@...lfernandes.org> wrote:
> > > Also optionally, I wonder if calling rcu_tasks_qs() directly is better
> > > (for documentation if anything) since the issue is Tasks RCU specific Also
> > > code comment above the rcu_softirq_qs() call about cond_resched() not taking
> > > care of Tasks RCU would be great!
> > >
> > Yes it's quite surprising to me that cond_resched does not help here,
>
> In theory, it would be possible to make cond_resched() take care of
> Tasks RCU.  In practice, the lazy-preemption work is looking to get rid
> of cond_resched().  But if for some reason cond_resched() needs to stay
> around, doing that work might make sense.

In my opinion, cond_resched() doing Tasks-RCU QS does not make sense
(to me), because cond_resched() is to inform the scheduler to run
something else possibly of higher priority while the current task is
still runnable. On the other hand, what's not permitted in a Tasks RCU
reader is a voluntary sleep. So IMO even though cond_resched() is a
voluntary call, it is still not a sleep but rather a preemption point.

So a Tasks RCU reader should perfectly be able to be scheduled out in
the middle of a read-side critical section (in current code) by
calling cond_resched(). It is just like involuntary preemption in the
middle of a RCU reader, in disguise, Right?

thanks,

 - Joel