linux-kernel - Re: [PATCH 2/7] rcu: limit PREEMPT

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Message-ID: <0c04a986-91d0-4f6d-bd4e-ca00d1cd710a@paulmck-laptop>
Date: Fri, 18 Oct 2024 10:43:02 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...nel.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, frederic@...nel.org, efault@....de
Subject: Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations

On Thu, Oct 17, 2024 at 03:50:46PM -0700, Ankur Arora wrote:
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:
> > On 2024-10-15 15:13:46 [-0700], Ankur Arora wrote:
> >> Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:
> >>
> >> >>
> >> >> As for PREEMPT_LAZY, you seem to be suggesting a more intrusive change
> >> >> than just keeping non-preemptible RCU when the Kconfig options are
> >> >> consistent with this being expected.  If this is the case, what are the
> >> >> benefits of this more-intrusive change?
> >> >
> >> > As far as I understand you are only concerned about PREEMPT_LAZY and
> >> > everything else (PREEMPT_LAZY + PREEMPT_DYNAMIC or PREEMPT_DYNAMIC
> >> > without PREEMPT_LAZY) is fine.
> >> > In the PREEMPT_LAZY + !PREEMPT_DYNAMIC the suggested change
> >> >
> >> > | config PREEMPT_RCU
> >> > | 	bool
> >> > | 	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
> >> > | 	select TREE_RCU
> >> > | 	help
> >> >
> >> > would disable PREEMPT_RCU while the default model is PREEMPT. You argue
> >>
> >> With PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n, isn't the default model
> >> PREEMPT_LAZY, which has PREEMPTION=y, but PREEMPT=n?
> >
> > Correct.
> >
> >> > that only people on small embedded would do such a thing and they would
> >> > like to safe additional memory.
> >> >
> >> > I don't think this is always the case because the "preemptible" users
> >> > would also get this and this is an unexpected change for them.
> >>
> >> Can you clarify this? The intent with lazy is to be preemptible but
> >> preempt less often. In that it is meant to be quite different from
> >> CONFIG_PREEMPT.
> >
> > A wake up with PREEMPT may not always lead to a preemption but will lead
> > to preemption once the time slice is up. With LAZY this changes to the
> > point that a preemption point will be delayed to the sched tick. Tasks
> > which are not based on the fail class (SCHED_DL, FIFO, …) will receive a
> > wake up right away.
> 
> >> > I don't think this is always the case because the "preemptible" users
> >> > would also get this and this is an unexpected change for them.
> 
> Yes. My point was that "preemptible" is a mechanism.
> 
> The policy about how often preemption happens is determined by the
> chosen model PREEMPT_NONE/PREEMPT_VOLUNTARY/PREEMPT_LAZY/PREEMPT/
> PREEMPT_RT.
> 
> > If in the long term NONE and VOL goes away you could argue that everyone
> > using LAZY + !DYNAMIC is one of those.
> 
> > If additionally PREEMPT goes away then you can not.
> 
> Sure. But, that's just begging the question.
> 
> We want _NONE and _VOLUNTARY to go away because keeping cond_resched()
> around incurs a cost.

When you say "go away", do you mean for your use cases?  Or globally?

> > Therefore I would prefer to have the RCU model to be
> > selectable rather than forced. While !PREEMPT_RCU may save memory, it
> > also disable preemption for the read section.
> 
> When a user chooses one of PREEMPT_NONE/_VOLUNTARY/_LAZY, the implication
> is that on the throughput -- latency axis, they care about optimizing
> for throughput.
> 
> PREEMPT_RCU=n is clearly oriented towards that.

Agreed!

> That said, I'm agnostic about making the RCU model selectable. Paul
> is the best judge of that.

I am not seeing any reason to make it once again be selectable, and
plenty of reasons to keep it automatic.

If I am missing some motivation to bother users with the need to make
this decision, please let me know what that motivation might be.

							Thanx, Paul