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

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Message-ID: <87iktokihj.fsf@oracle.com>
Date: Fri, 18 Oct 2024 18:07:52 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: paulmck@...nel.org
Cc: Ankur Arora <ankur.a.arora@...cle.com>,
        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


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?

> 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.

>> 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.

--
ankur