Date: Sun, 10 Mar 2024 21:50:33 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: paulmck@...nel.org
Cc: Ankur Arora <ankur.a.arora@...cle.com>,
        Joel Fernandes
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Subject: Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO


Paul E. McKenney <paulmck@...nel.org> writes:

> On Thu, Mar 07, 2024 at 08:22:30PM -0800, Ankur Arora wrote:
>>
>> Paul E. McKenney <paulmck@...nel.org> writes:
>>
>> > On Thu, Mar 07, 2024 at 07:15:35PM -0500, Joel Fernandes wrote:
>> >>
>> >>
>> >> 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?
>> >
>> > Because right now, one large difference between CONFIG_PREEMPT_NONE
>> > an CONFIG_PREEMPT_VOLUNTARY is that for the latter might_sleep() is a
>> > preemption point, but not for the former.
>>
>> True. But, there is no difference between either of those with
>> PREEMPT_AUTO=y (at least right now).
>>
>> For (PREEMPT_AUTO=y, PREEMPT_VOLUNTARY=y, DEBUG_ATOMIC_SLEEP=y),
>> might_sleep() is:
>>
>> # define might_resched() do { } while (0)
>> # define might_sleep() \
>>         do { __might_sleep(__FILE__, __LINE__); might_resched(); } while (0)
>>
>> And, cond_resched() for (PREEMPT_AUTO=y, PREEMPT_VOLUNTARY=y,
>> DEBUG_ATOMIC_SLEEP=y):
>>
>> static inline int _cond_resched(void)
>> {
>>         klp_sched_try_switch();
>>         return 0;
>> }
>> #define cond_resched() ({                       \
>>         __might_resched(__FILE__, __LINE__, 0); \
>>         _cond_resched();                        \
>> })
>>
>> And, no change for (PREEMPT_AUTO=y, PREEMPT_NONE=y, DEBUG_ATOMIC_SLEEP=y).
>
> As long as it is easy to restore the prior cond_resched() functionality
> for testing in the meantime, I should be OK.  For example, it would
> be great to have the commit removing the old functionality from
> cond_resched() at the end of the series,

I would, of course, be happy to make any changes that helps testing,
but I think I'm missing something that you are saying wrt
cond_resched()/might_sleep().

There's no commit explicitly removing the core cond_reshed()
functionality: PREEMPT_AUTO explicitly selects PREEMPT_BUILD and selects
out PREEMPTION_{NONE,VOLUNTARY}_BUILD.
(That's patch-1 "preempt: introduce CONFIG_PREEMPT_AUTO".)

For the rest it just piggybacks on the CONFIG_PREEMPT_DYNAMIC work
and just piggybacks on (!CONFIG_PREEMPT_DYNAMIC && CONFIG_PREEMPTION):

#if !defined(CONFIG_PREEMPTION) || defined(CONFIG_PREEMPT_DYNAMIC)
	/* ... */
#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
	/* ... */
#elif defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
	/* ... */
#else /* !CONFIG_PREEMPTION */
	/* ... */
#endif /* PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */

#else /* CONFIG_PREEMPTION && !CONFIG_PREEMPT_DYNAMIC */
static inline int _cond_resched(void)
{
	klp_sched_try_switch();
	return 0;
}
#endif /* !CONFIG_PREEMPTION || CONFIG_PREEMPT_DYNAMIC */

Same for might_sleep() (which really amounts to might_resched()):

#ifdef CONFIG_PREEMPT_VOLUNTARY_BUILD
       /* ... */
#elif defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
      /* ... */
#elif defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
      /* ... */
#else
# define might_resched() do { } while (0)
#endif /* CONFIG_PREEMPT_* */

But, I doubt that I'm telling you anything new. So, what am I missing?

--
ankur

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