Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO

[Joel Fernandes]

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@xxxxxxxxxxxxxxxxx> 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