linux-kernel - Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT

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Date: Sun, 10 Mar 2024 22:34:55 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>,
        Joel Fernandes
 <joel@...lfernandes.org>, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, peterz@...radead.org,
        torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
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        hpa@...or.com, mingo@...hat.com, vincent.guittot@...aro.org,
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        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO


Juri Lelli <juri.lelli@...hat.com> writes:

> On 07/03/24 19:49, Ankur Arora wrote:
>> Joel Fernandes <joel@...lfernandes.org> writes:
>
> ...
>
>> > Firstly, Maybe I misunderstood Ankur completely. Re-reading his comments above,
>> > he seems to be suggesting preempting instantly for higher scheduling CLASSES
>> > even for preempt=none mode, without having to wait till the next
>> > scheduling-clock interrupt.
>>
>> Yes, that's what I was suggesting.
>>
>> > Not sure if that makes sense to me, I was asking not
>> > to treat "higher class" any differently than "higher priority" for preempt=none.
>>
>> Ah. Understood.
>>
>> > And if SCHED_DEADLINE has a problem with that, then it already happens so with
>> > CONFIG_PREEMPT_NONE=y kernels, so no need special treatment for higher class any
>> > more than the treatment given to higher priority within same class. Ankur/Juri?
>>
>> No. I think that behaviour might be worse for PREEMPT_AUTO.
>>
>> PREEMPT_NONE=y (or PREEMPT_VOLUNTARY=y for that matter) don't
>> quite have a policy around when preemption happens. Preemption
>> might happen quickly, might happen slowly based on when the next
>> preemption point is found.
>>
>> The PREEMPT_AUTO, preempt=none policy in this series will always
>> cause preemption to be at user exit or the next tick. Seems like
>> it would be worse for higher scheduling classes more often.
>>
>> But, I wonder what Juri thinks about this.
>
> As I was saying in my last comment in the other discussion, I'm honestly
> not sure, mostly because I'm currently fail to see what type of users
> would choose preempt=none and have tasks scheduled with SCHED_DEADLINE
> (please suggest example usecases, as I'm pretty sure I'm missing
> something :). With that said, if the purpose of preempt=none is to have
> a model which is super conservative wrt preemptions, having to wait one
> tick to possibly schedule a DEADLINE task still seems kind of broken for
> DEADLINE, but at least is predictably broken (guess one needs to account
> for that somehow when coming up with parameters :).

True :). Let me do some performance comparisons between (preempt=none +
the leftmost logic) and whatever is left off in the preempt=voluntary
patch.

Thanks

--
ankur