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

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Message-ID: <d956c2e9-492d-4559-b9f9-400f37f523bf@joelfernandes.org>
Date: Thu, 7 Mar 2024 19:15:35 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: paulmck@...nel.org
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
 tglx@...utronix.de, peterz@...radead.org, torvalds@...ux-foundation.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, jpoimboe@...nel.org, mark.rutland@....com, jgross@...e.com,
 andrew.cooper3@...rix.com, bristot@...nel.org,
 mathieu.desnoyers@...icios.com, geert@...ux-m68k.org,
 glaubitz@...sik.fu-berlin.de, anton.ivanov@...bridgegreys.com,
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 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



On 3/7/2024 2:01 PM, Paul E. McKenney wrote:
> On Wed, Mar 06, 2024 at 03:42:10PM -0500, Joel Fernandes wrote:
>> Hi Ankur,
>>
>> On 3/5/2024 3:11 AM, Ankur Arora wrote:
>>>
>>> Joel Fernandes <joel@...lfernandes.org> writes:
>>>
>> [..]
>>>> IMO, just kill 'voluntary' if PREEMPT_AUTO is enabled. There is no
>>>> 'voluntary' business because
>>>> 1. The behavior vs =none is to allow higher scheduling class to preempt, it
>>>> is not about the old voluntary.
>>>
>>> What do you think about folding the higher scheduling class preemption logic
>>> into preempt=none? As Juri pointed out, prioritization of at least the leftmost
>>> deadline task needs to be done for correctness.
>>>
>>> (That'll get rid of the current preempt=voluntary model, at least until
>>> there's a separate use for it.)
>>
>> Yes I am all in support for that. Its less confusing for the user as well, and
>> scheduling higher priority class at the next tick for preempt=none sounds good
>> to me. That is still an improvement for folks using SCHED_DEADLINE for whatever
>> reason, with a vanilla CONFIG_PREEMPT_NONE=y kernel. :-P. If we want a new mode
>> that is more aggressive, it could be added in the future.
> 
> This would be something that happens only after removing cond_resched()
> might_sleep() functionality from might_sleep(), correct?

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. Not sure if that makes sense to me, I was asking not
to treat "higher class" any differently than "higher priority" for preempt=none.

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?

Re: cond_resched(), I did not follow you Paul, why does removing the proposed
preempt=voluntary mode (i.e. dropping this patch) have to happen only after
cond_resched()/might_sleep() modifications?

thanks,

 - Joel