netdev - Re: [PATCH v5 net 1/3] rcu: add a helper to report consolidated flavor QS

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Date: Sat, 23 Mar 2024 16:53:21 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Yan Zhai <yan@...udflare.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	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>,
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
	bpf@...r.kernel.org, kernel-team@...udflare.com,
	Joel Fernandes <joel@...lfernandes.org>,
	Toke Høiland-Jørgensen <toke@...hat.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, mark.rutland@....com,
	Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH v5 net 1/3] rcu: add a helper to report consolidated
 flavor QS

On Fri, Mar 22, 2024 at 09:02:02PM -0500, Yan Zhai wrote:
> On Fri, Mar 22, 2024 at 4:31 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > On Fri, Mar 22, 2024 at 12:24:13PM +0100, Sebastian Andrzej Siewior wrote:
> > > On 2024-03-19 13:44:34 [-0700], Yan Zhai wrote:
> > > > + * The macro is not needed when CONFIG_PREEMPT_RT is defined. RT kernels would
> > > > + * have more chance to invoke schedule() calls and provide necessary quiescent
> > > > + * states. As a contrast, calling cond_resched() only won't achieve the same
> > > > + * effect because cond_resched() does not provide RCU-Tasks quiescent states.
> > > > + */
> > >
> > > Paul, so CONFIG_PREEMPTION is affected but CONFIG_PREEMPT_RT is not.
> > > Why does RT have more scheduling points?
> >
> > In RT, isn't BH-disabled code preemptible?  But yes, this would not help
> > RCU Tasks.
> >
> By "more chance to invoke schedule()", my thought was that
> cond_resched becomes no op on RT or PREEMPT kernel. So it will not
> call __schedule(SM_PEREEMPT), which clears the NEED_RESCHED flag. On a
> normal irq exit like timer, when NEED_RESCHED is on,
> schedule()/__schedule(0) can be called time by time then.
> __schedule(0) is good for RCU tasks, __schedule(SM_PREEMPT) is not.
> 
> But I think this code comment does not take into account frequent
> preempt_schedule and irqentry_exit_cond_resched on a PREEMPT kernel.
> When returning to these busy kthreads, irqentry_exit_cond_resched is
> in fact called now, not schedule(). So likely __schedule(PREEMPT) is
> still called frequently, or even more frequently. So the code comment
> looks incorrect on the RT argument part. We probably should remove the
> "IS_ENABLED" condition really. Paul and Sebastian, does this sound
> reasonable to you?

Removing the "IS_ENABLED(CONFIG_PREEMPT_RT)" condition makes a great deal
of sense to me, but I must defer to Sebastian for any RT implications.

							Thanx, Paul