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

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Message-ID: <87zfv4fpkz.fsf@oracle.com>
Date: Mon, 11 Mar 2024 14:03:56 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, paulmck@...nel.org,
        Joel
 Fernandes <joel@...lfernandes.org>, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, peterz@...radead.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
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        mathieu.desnoyers@...icios.com, geert@...ux-m68k.org,
        glaubitz@...sik.fu-berlin.de, anton.ivanov@...bridgegreys.com,
        mattst88@...il.com, krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com,
        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


Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Mon, 11 Mar 2024 at 13:10, Ankur Arora <ankur.a.arora@...cle.com> wrote:
>>
>> Ah, I see your point. Basically, keep the lazy semantics but -- in
>> addition -- also provide the ability to dynamically toggle
>> cond_resched(), might_reshed() as a feature to help move this along
>> further.
>
> Please, let's not make up any random hypotheticals.
>
> Honestly, if we ever hit the hypothetical scenario that Paul outlined, let's
>
>  (a) deal with it THEN, when we actually know what the situation is
>
>  (b) learn and document what it is that actually causes the odd behavior
>
> IOW, instead of assuming that some "cond_resched()" case would even be
> the right thing to do, maybe there are other issues going on?  Let's
> not paper over them by keeping some hack around - and *if* some
> cond_resched() model is actually the right model in some individual
> place, let's make it the rule that *when* we hit that case, we
> document it.
>
> And we should absolutely not have some hypothetical case keep us from
> just doing the right thing and getting rid of the existing
> cond_resched().
>
> Because any potential future case is *not* going to be the same
> cond_resched() that the current case is anyway. It is going to have
> some very different cause.

Ack that. And, thanks that makes sense to me.

--
ankur