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

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Message-ID: <b4c64bb3-5f2f-476d-bb67-8b45efb5aad1@paulmck-laptop>
Date: Fri, 18 Oct 2024 21:30:47 -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 Fri, Oct 18, 2024 at 06:07:52PM -0700, Ankur Arora wrote:
> 
> Paul E. McKenney <paulmck@...nel.org> writes:
> 
> > On Fri, Oct 18, 2024 at 12:18:04PM -0700, Ankur Arora wrote:
> >>
> >> Paul E. McKenney <paulmck@...nel.org> writes:
> >>
> >> > 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:
> >> >> >>
> >> [ ... ]
> >> >> 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?
> >>
> >> When I say "want _ to go away" I mean: cond_resched() is deleterious
> >> to performance since you are forced to have code which can do the
> >> rescheduling check synchronously -- when this could easily be done
> >> asynchronously (as the non voluntary models do).
> >>
> >> And this either means poor performance (ex. in the page zeroing code
> >> where it would be more optimal to work on continguous ranges) or
> >> gyrations like the ones that xen_pv_evtchn_do_upcall() and the
> >> Xen hypervisor have to go through.
> >>
> >> And, as we've discussed before, the cond_resched() interface, while it
> >> works, is not ideal.
> >
> > I would expect that many instances of cond_resched() could go away given
> > lazy preemption, but I would not be surprised if there were some that
> > needed to stay around.
> >
> > Your thought being that if *all* instance of cond_resched() go away,
> > then PREEMPT_NONE also goes away?
> 
> If *all* instances of cond_resched() go away, is there anything left of
> PREEMPT_NONE?

Yes, namely its relationship to PREEMPT_RCU.  Which, yes, can be adjusted,
perhaps even as Sebastian suggested.  But such an adjustment cannot be
applied suddenly without warning.

> > Given how long PREEMPT_NONE has been
> > around, this would need to be done (and communicated) quite carefully.
> 
> I don't think there's any question about that.

Whew!!!

> >> Also, a man can dream!
> >
> > Fair enough, just be very careful to distinguish dreams from reality.  ;-)
> 
> I've generally not found that to be a problem, but thanks for the warning.

"It is a service that I provide."  ;-)

							Thanx, Paul