Re: [PATCH 1/2] evl/sched: Add sched_out handler to sched_class

On 15.06.26 09:14, Philippe Gerum wrote:
> Jan Kiszka <[email protected]> writes:
> 
>> On 15.06.26 08:42, Philippe Gerum wrote:
>>> Philippe Gerum <[email protected]> writes:
>>>
>>>> Jan Kiszka <[email protected]> writes:
>>>>
>>>>> From: Jan Kiszka <[email protected]>
>>>>>
>>>>> This shall be invoked before a thread switch, providing both the current
>>>>> and the next thread as arguments. Some scheduling classes may need it to
>>>>> correctly handle their state as the sched_pick may not be invoked when a
>>>>> higher-weighted class is providing the next thread.
>>>>>
>>>>> Signed-off-by: Jan Kiszka <[email protected]>
>>>>> ---
>>>>>  include/evl/sched.h     | 2 ++
>>>>>  kernel/evl/sched/core.c | 5 +++++
>>>>>  2 files changed, 7 insertions(+)
>>>>>
>>>>> diff --git a/include/evl/sched.h b/include/evl/sched.h
>>>>> index ae9690860146c..0b16f1b1cf626 100644
>>>>> --- a/include/evl/sched.h
>>>>> +++ b/include/evl/sched.h
>>>>> @@ -120,6 +120,8 @@ struct evl_sched_class {
>>>>>  	void (*sched_dequeue)(struct evl_thread *thread);
>>>>>  	void (*sched_requeue)(struct evl_thread *thread);
>>>>>  	struct evl_thread *(*sched_pick)(struct evl_rq *rq);
>>>>> +	void (*sched_out)(struct evl_thread *thread,
>>>>> +			  struct evl_thread *next);
>>>>>  	void (*sched_yield)(struct evl_thread *thread);
>>>>>  	void (*sched_migrate)(struct evl_thread *thread,
>>>>>  			      struct evl_rq *rq);
>>>>> diff --git a/kernel/evl/sched/core.c b/kernel/evl/sched/core.c
>>>>> index eb133e334d30f..0d49fc16bd67e 100644
>>>>> --- a/kernel/evl/sched/core.c
>>>>> +++ b/kernel/evl/sched/core.c
>>>>> @@ -910,6 +910,7 @@ static __always_inline bool test_resched(struct evl_rq *this_rq)
>>>>>   */
>>>>>  void __evl_schedule(void) /* oob or/and hard irqs off (CPU migration-safe) */
>>>>>  {
>>>>> +	struct evl_sched_class *prev_schedclass;
>>>>>  	struct evl_rq *this_rq = this_evl_rq();
>>>>>  	struct evl_thread *prev, *next, *curr;
>>>>>  	bool leaving_inband, inband_tail;
>>>>> @@ -990,6 +991,10 @@ void __evl_schedule(void) /* oob or/and hard irqs off (CPU migration-safe) */
>>>>>  	this_rq->curr = next;
>>>>>  	leaving_inband = false;
>>>>>  
>>>>> +	prev_schedclass = prev->sched_class;
>>>>> +	if (prev_schedclass->sched_out)
>>>>> +		prev_schedclass->sched_out(prev, next);
>>>>> +
>>>>>  	/*
>>>>>  	 * Careful: we _must_ have updated this_rq->curr before
>>>>>  	 * performing the rest of the context switch code
>>>>
>>>> I've been working on this lately too. It turns out that we need more
>>>> than this, although this is definitely part of the solution. I'll follow
>>>> up on this issue.
>>>
>>> This is still wip, I'm sharing this early to discuss details for
>>> reconciling both proposals (the hunk in __evl_schedule() is merely
>>> cosmetic, no functional change).
>>>
>>> commit f49dbd63c1389a6b99508ef0da852545b102d1cc (HEAD -> wip/fix-quota-sched)
>>> Author: Philippe Gerum <[email protected]>
>>> Date:   Sun Jun 14 12:08:59 2026 +0200
>>>
>>>     evl: sched/quota: fix budget tracking on preemption
>>>     
>>>     Upon preemption of a SCHED_QUOTA thread by a SCHED_FIFO one, the
>>>     runtime budget of the former is inaccurately tracked. This is due to
>>>     the fifo->pick() handler returning a valid thread, which prevents the
>>>     quota->pick() handler from being called. As a result, the last runtime
>>>     period of the outgoing thread is not accounted for.
>>>     
>>>     To fix this, we introduce a new sched_out() handler which is called
>>>     for the outgoing thread, which the quota policy uses to update the
>>>     remaining budget of preempted threads appropriately. In addition, the
>>>     implementation no longer shares the runnable thread queue with
>>>     SCHED_FIFO.
>>>     
>>>     Signed-off-by: Philippe Gerum <[email protected]>
>>>
>>> diff --git a/include/evl/sched.h b/include/evl/sched.h
>>> index ae9690860146..cc824c28004b 100644
>>> --- a/include/evl/sched.h
>>> +++ b/include/evl/sched.h
>>> @@ -120,6 +120,7 @@ struct evl_sched_class {
>>>  	void (*sched_dequeue)(struct evl_thread *thread);
>>>  	void (*sched_requeue)(struct evl_thread *thread);
>>>  	struct evl_thread *(*sched_pick)(struct evl_rq *rq);
>>> +	void (*sched_out)(struct evl_thread *thread);
>>
>> The out handler also needs the target thread in order to identify if
>> there is a tg change or not.
>>
> 
> Your implementation needs this because the sequence is pick() -> out(),
> mine is out() -> pick(), with the latter using the budget updated by the
> former to figure out what to do next.

But that will call out even on old == new - does not sound right.

Jan

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