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

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Yan Zhai <yan@cloudflare.com>, Eric Dumazet <edumazet@google.com>,
	netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Jiri Pirko <jiri@resnulli.us>, Simon Horman <horms@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Lorenzo Bianconi <lorenzo@kernel.org>,
	Coco Li <lixiaoyan@google.com>, Wei Wang <weiwan@google.com>,
	Alexander Duyck <alexanderduyck@fb.com>,
	Hannes Frederic Sowa <hannes@stressinduktion.org>,
	linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
	bpf@vger.kernel.org, kernel-team@cloudflare.com,
	rostedt@goodmis.org, mark.rutland@arm.com
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll
Date: Wed, 28 Feb 2024 13:13:05 -0800	[thread overview]
Message-ID: <5b74968d-fe14-48b4-bb16-6cf098a04ca5@paulmck-laptop> (raw)
In-Reply-To: <CAEXW_YRfjhBjsMpBEdCoLd2S+=5YdFSs2AS07xwN72bgtW4sDQ@mail.gmail.com>

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@kernel.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@joelfernandes.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.

> 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?

							Thanx, Paul