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

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Date: Wed, 28 Feb 2024 13:52:47 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joel@...lfernandes.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 04:27:47PM -0500, Joel Fernandes wrote:
> 
> 
> > On Feb 28, 2024, at 4:13 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> > 
> > On Wed, Feb 28, 2024 at 03:14:34PM -0500, Joel Fernandes wrote:
> >>> 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.
> > 
> > From the viewpoint of Task RCU's users, the point is to figure out
> > when it is OK to free an already-removed tracing trampoline.  The
> > current Task RCU implementation relies on the fact that tracing
> > trampolines do not do voluntary context switches.
> 
> Yes.
> 
> > 
> >> 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?
> > 
> > You lost me on this one.  This for example is not permitted:
> > 
> >    rcu_read_lock();
> >    cond_resched();
> >    rcu_read_unlock();
> > 
> > But in a CONFIG_PREEMPT=y kernel, that RCU reader could be preempted.
> > 
> > So cond_resched() looks like a voluntary context switch to me.  Recall
> > that vanilla non-preemptible RCU will treat them as quiescent states if
> > the grace period extends long enough.
> > 
> > What am I missing here?
> 
> That we are discussing Tasks-RCU read side section? Sorry I should have been more clear. I thought sleeping was not permitted in Tasks RCU reader, but non-sleep context switches (example involuntarily getting preempted were).

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.

						Thanx, Paul