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

Jan Kiszka <[email protected]> writes:

> On 15.06.26 10:09, 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).
>>>>
>>>
>>> Please avoid that for the final patch. I already had troubles telling
>>> substantial from cosmetical changes apart in your lazy-blocking commit
>>> (>50% unrelated changes in there).
>>>
>>>> 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.
>>>
>>> ...and why we need a separate queue now? Please provide reasoning for
>>> changes.
>>>
>>>>     
>>>>     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);
>>>>  	void (*sched_yield)(struct evl_thread *thread);
>>>>  	void (*sched_migrate)(struct evl_thread *thread,
>>>>  			      struct evl_rq *rq);
>>>> diff --git a/include/evl/sched/quota.h b/include/evl/sched/quota.h
>>>> index dfe3b7390958..dc8416645da8 100644
>>>> --- a/include/evl/sched/quota.h
>>>> +++ b/include/evl/sched/quota.h
>>>> @@ -40,6 +40,7 @@ struct evl_quota_group {
>>>>  
>>>>  struct evl_sched_quota {
>>>>  	ktime_t period;
>>>> +	struct evl_sched_queue runnable;
>>>>  	struct evl_timer refill_timer;
>>>>  	struct evl_timer limit_timer;
>>>>  	struct list_head groups;
>>>> diff --git a/kernel/evl/sched/core.c b/kernel/evl/sched/core.c
>>>> index d1d025e06a5d..7127d4e52df9 100644
>>>> --- a/kernel/evl/sched/core.c
>>>> +++ b/kernel/evl/sched/core.c
>>>> @@ -773,7 +773,8 @@ static inline void set_next_running(struct evl_rq *rq,
>>>>  		evl_stop_timer(&rq->rrbtimer);
>>>>  }
>>>>  
>>>> -static struct evl_thread *__pick_next_thread(struct evl_rq *rq)
>>>> +static __always_inline struct evl_thread *
>>>> +__pick_next_thread(struct evl_rq *rq)
>>>>  {
>>>>  	struct evl_sched_class *sched_class;
>>>>  	struct evl_thread *curr = rq->curr;
>>>> @@ -821,8 +822,14 @@ static struct evl_thread *__pick_next_thread(struct evl_rq *rq)
>>>>  /* rq->curr->lock + rq->lock held, hard irqs off. */
>>>>  static struct evl_thread *pick_next_thread(struct evl_rq *rq)
>>>>  {
>>>> -	struct evl_thread *next = __pick_next_thread(rq);
>>>> +	struct evl_thread *next, *prev = rq->curr;
>>>> +	struct evl_sched_class *prev_class = prev->sched_class;
>>>>  
>>>> +	if (prev_class->sched_out)
>>>> +		prev_class->sched_out(prev);
>>>> +
>>>> +	next = __pick_next_thread(rq);
>>>> +	trace_evl_pick_thread(next);
>>>>  	set_next_running(rq, next);
>>>>  
>>>>  	return next;
>>>> @@ -972,21 +979,20 @@ void __evl_schedule(void) /* oob or/and hard irqs off (CPU migration-safe) */
>>>>  		return;
>>>>  	}
>>>>  
>>>> +	prev = curr;
>>>>  	next = pick_next_thread(this_rq);
>>>> -	trace_evl_pick_thread(next);
>>>> -	if (next == curr) {
>>>> -		if (unlikely(next->state & EVL_T_ROOT)) {
>>>> +	if (next == prev) {
>>>> +		if (unlikely(prev->state & EVL_T_ROOT)) {
>>>>  			if (this_rq->local_flags & RQ_TPROXY)
>>>>  				evl_notify_proxy_tick(this_rq);
>>>>  			if (this_rq->local_flags & RQ_TDEFER)
>>>>  				evl_program_local_tick(&evl_mono_clock);
>>>>  		}
>>>>  		raw_spin_unlock(&this_rq->lock);
>>>> -		raw_spin_unlock_irqrestore(&curr->lock, flags);
>>>> +		raw_spin_unlock_irqrestore(&prev->lock, flags);
>>>>  		return;
>>>>  	}
>>>>  
>>>> -	prev = curr;
>>>>  	this_rq->curr = next;
>>>>  	leaving_inband = false;
>>>>  
>>>> diff --git a/kernel/evl/sched/quota.c b/kernel/evl/sched/quota.c
>>>> index 0829da711a66..14748e3a843e 100644
>>>> --- a/kernel/evl/sched/quota.c
>>>> +++ b/kernel/evl/sched/quota.c
>>>> @@ -12,45 +12,35 @@
>>>>  #include <uapi/evl/sched-abi.h>
>>>>  
>>>>  /*
>>>> - * With this policy, each per-CPU runqueue maintains a list of active
>>>> - * thread groups for the sched_fifo class.
>>>> - *
>>>> - * Each time a thread is picked from the runqueue, we check whether we
>>>> - * still have budget for running it, looking at the group it belongs
>>>> - * to. If so, a timer is armed to elapse when that group has no more
>>>> - * budget, would the incoming thread run unpreempted until then
>>>> - * (i.e. evl_quota->limit_timer).
>>>> + * Each time a thread is picked from the ->runnable queue, we check
>>>> + * whether the group it belongs to still has runtime budget.  If so, a
>>>> + * timer is armed to fire when that group has no more budget, would
>>>> + * the incoming thread run unpreempted until then
>>>> + * (i.e. quota->limit_timer).
>>>>   *
>>>>   * Otherwise, if no budget remains in the group for running the
>>>>   * candidate thread, we move the latter to a local expiry queue
>>>>   * maintained by the group. This process is done on the fly as we pull
>>>> - * from the runqueue.
>>>> + * from the ->runnable queue.
>>>>   *
>>>> - * Updating the remaining budget is done each time the EVL core asks
>>>> - * for replacing the current thread with the next runnable one,
>>>> - * i.e. evl_quota_pick(). There we charge the elapsed run time of the
>>>> - * outgoing thread to the relevant group, and conversely, we check
>>>> - * whether the incoming thread has budget.
>>>> + * Updating the remaining budget is done each time the EVL core
>>>> + * schedules out a thread undergoing the quota scheduling policy,
>>>>   *
>>>> - * Finally, a per-CPU timer (evl_quota->refill_timer) periodically
>>>> - * ticks in the background, in accordance to the defined quota
>>>> - * interval. Thread group budgets get replenished by its handler in
>>>> - * accordance to their respective share, pushing all expired threads
>>>> - * back to the run queue in the same move.
>>>> + * Finally, a per-CPU timer (quota->refill_timer) periodically ticks
>>>> + * in the background, in accordance to the defined quota interval,
>>>> + * replenishing per-group budgets, pushing all expired threads back to
>>>> + * the quota ->runqueue too.
>>>>   *
>>>> - * NOTE: since the core logic enforcing the budget entirely happens in
>>>> - * evl_quota_pick(), applying a budget change can be done as simply as
>>>> - * forcing the rescheduling procedure to be invoked asap. As a result
>>>> - * of this, the EVL core will ask for the next thread to run, which
>>>> - * means calling evl_quota_pick() eventually.
>>>> + * NOTE: forcing a call to the rescheduling procedure is enoiugh to
>>>> + * apply a budget change.
>>>>   *
>>>> - * CAUTION: evl_quota_group->nr_active does count both the threads
>>>> - * from that group linked to the sched_fifo runqueue, _and_ the
>>>> - * threads moved to the local expiry queue. As a matter of fact, the
>>>> - * expired threads - those for which we consumed all the per-group
>>>> - * budget - are still seen as runnable (i.e. not blocked/suspended) by
>>>> - * the EVL core. This only means that the SCHED_QUOTA policy won't
>>>> - * pick them until the corresponding budget is replenished.
>>>> + * CAUTION: quota_group->nr_active does count both the threads from
>>>> + * that group linked to the runnable queue, _and_ the threads moved to
>>>> + * the local expiry queue. As a matter of fact, the expired threads -
>>>> + * those for which we consumed all the per-group budget - are still
>>>> + * seen as runnable (i.e. not blocked/suspended) by the EVL core. This
>>>> + * only means that the SCHED_QUOTA policy won't pick them until the
>>>> + * corresponding budget is replenished.
>>>>   */
>>>>  
>>>>  #define MAX_QUOTA_GROUPS  1024
>>>> @@ -61,15 +51,19 @@ static DECLARE_BITMAP(group_map, MAX_QUOTA_GROUPS);
>>>>  
>>>>  static LIST_HEAD(group_list);
>>>>  
>>>> -static inline bool thread_on_quota(struct evl_thread *thread,
>>>> -				struct evl_quota_group *tg)
>>>> +static inline bool current_on_quota(struct evl_quota_group *tg)
>>>>  {
>>>> -	/*
>>>> -	 * Check whether @thread is running on some CPU, and belongs
>>>> -	 * to quota group @tg.
>>>> -	 */
>>>> -	return thread->quota == tg &&
>>>> -		!(thread->state & (EVL_T_READY|EVL_THREAD_BLOCK_MASK));
>>>> +	struct evl_rq *rq = tg->rq;
>>>> +	struct evl_thread *curr = rq->curr;
>>>> +	struct evl_sched_quota *qs = &rq->quota;
>>>> +
>>>> +	if (curr->quota != tg)
>>>> +		return false;
>>>> +
>>>> +	if (curr->state & (EVL_T_READY|EVL_T_KICKED|EVL_THREAD_BLOCK_MASK))
>>>> +		return false;
>>>> +
>>>> +	return evl_timer_is_running(&qs->limit_timer);
>>>>  }
>>>>  
>>>>  static inline bool group_is_active(struct evl_quota_group *tg)
>>>> @@ -82,7 +76,7 @@ static inline bool group_is_active(struct evl_quota_group *tg)
>>>>  	 * runqueue, in which case tg->nr_active already accounted for
>>>>  	 * it.
>>>>  	 */
>>>> -	return thread_on_quota(tg->rq->curr, tg);
>>>> +	return current_on_quota(tg);
>>>>  }
>>>>  
>>>>  static inline void replenish_budget(struct evl_sched_quota *qs,
>>>> @@ -134,10 +128,10 @@ static inline void replenish_budget(struct evl_sched_quota *qs,
>>>>  	} else if (tg->run_credit) {
>>>>  		credit = ktime_sub(tg->quota_peak, budget);
>>>>  		/* Consume the accumulated credit. */
>>>> -		if (tg->run_credit >= credit)
>>>> +		if (tg->run_credit >= credit) {
>>>>  			tg->run_credit =
>>>>  				ktime_sub(tg->run_credit, credit);
>>>> -		else {
>>>> +		} else {
>>>>  			credit = tg->run_credit;
>>>>  			tg->run_credit = 0;
>>>>  		}
>>>> @@ -150,8 +144,8 @@ static inline void replenish_budget(struct evl_sched_quota *qs,
>>>>  
>>>>  static void quota_refill_handler(struct evl_timer *timer) /* oob stage stalled */
>>>>  {
>>>> -	struct evl_quota_group *tg;
>>>>  	struct evl_thread *thread, *tmp;
>>>> +	struct evl_quota_group *tg;
>>>>  	struct evl_sched_quota *qs;
>>>>  	struct evl_rq *rq;
>>>>  
>>>> @@ -167,7 +161,7 @@ static void quota_refill_handler(struct evl_timer *timer) /* oob stage stalled *
>>>>  		if (tg->run_budget == 0 || list_empty(&tg->expired))
>>>>  			continue;
>>>>  		/*
>>>> -		 * For each group living on this CPU, move all expired
>>>> +		 * For each group pinned on this CPU, move all expired
>>>>  		 * threads back to the runqueue. Since those threads
>>>>  		 * were moved out of the runqueue as we were
>>>>  		 * considering them for execution, we push them back
>>>> @@ -178,7 +172,7 @@ static void quota_refill_handler(struct evl_timer *timer) /* oob stage stalled *
>>>>  		list_for_each_entry_safe_reverse(thread, tmp,
>>>>  						&tg->expired, quota_expired) {
>>>>  			list_del_init(&thread->quota_expired);
>>>> -			evl_add_schedq(&rq->fifo.runnable, thread);
>>>> +			evl_add_schedq(&qs->runnable, thread);
>>>>  		}
>>>>  	}
>>>>  
>>>> @@ -195,7 +189,7 @@ static void quota_limit_handler(struct evl_timer *timer) /* oob stage stalled */
>>>>  	/*
>>>>  	 * Force a rescheduling on the return path of the current
>>>>  	 * interrupt, so that the budget is re-evaluated for the
>>>> -	 * current group in evl_quota_pick().
>>>> +	 * current group in quota_pick().
>>>>  	 */
>>>>  	raw_spin_lock(&rq->lock);
>>>>  	evl_set_self_resched(rq);
>>>> @@ -221,6 +215,7 @@ static void quota_init(struct evl_rq *rq)
>>>>  {
>>>>  	struct evl_sched_quota *qs = &rq->quota;
>>>>  
>>>> +	evl_init_schedq(&qs->runnable);
>>>>  	qs->period = quota_period;
>>>>  	INIT_LIST_HEAD(&qs->groups);
>>>>  
>>>> @@ -337,8 +332,8 @@ static void quota_forget(struct evl_thread *thread)
>>>>  
>>>>  static void quota_kick(struct evl_thread *thread)
>>>>  {
>>>> +	struct evl_sched_quota *qs = &thread->rq->quota;
>>>>  	struct evl_quota_group *tg = thread->quota;
>>>> -	struct evl_rq *rq = thread->rq;
>>>>  
>>>>  	/*
>>>>  	 * Allow a kicked thread to be elected for running until it
>>>> @@ -347,7 +342,7 @@ static void quota_kick(struct evl_thread *thread)
>>>>  	 */
>>>>  	if (tg->run_budget == 0 && !list_empty(&thread->quota_expired)) {
>>>>  		list_del_init(&thread->quota_expired);
>>>> -		evl_add_schedq_tail(&rq->fifo.runnable, thread);
>>>> +		evl_add_schedq_tail(&qs->runnable, thread);
>>>>  	}
>>>>  }
>>>>  
>>>> @@ -358,79 +353,82 @@ static inline int thread_is_runnable(struct evl_thread *thread)
>>>>  
>>>>  static void quota_enqueue(struct evl_thread *thread)
>>>>  {
>>>> +	struct evl_sched_quota *qs = &thread->rq->quota;
>>>>  	struct evl_quota_group *tg = thread->quota;
>>>> -	struct evl_rq *rq = thread->rq;
>>>>  
>>>>  	if (!thread_is_runnable(thread))
>>>>  		list_add_tail(&thread->quota_expired, &tg->expired);
>>>>  	else
>>>> -		evl_add_schedq_tail(&rq->fifo.runnable, thread);
>>>> +		evl_add_schedq_tail(&qs->runnable, thread);
>>>>  
>>>>  	tg->nr_active++;
>>>>  }
>>>>  
>>>>  static void quota_dequeue(struct evl_thread *thread)
>>>>  {
>>>> +	struct evl_sched_quota *qs = &thread->rq->quota;
>>>>  	struct evl_quota_group *tg = thread->quota;
>>>> -	struct evl_rq *rq = thread->rq;
>>>>  
>>>>  	if (!list_empty(&thread->quota_expired))
>>>>  		list_del_init(&thread->quota_expired);
>>>>  	else
>>>> -		evl_del_schedq(&rq->fifo.runnable, thread);
>>>> +		evl_del_schedq(&qs->runnable, thread);
>>>>  
>>>>  	tg->nr_active--;
>>>>  }
>>>>  
>>>>  static void quota_requeue(struct evl_thread *thread)
>>>>  {
>>>> +	struct evl_sched_quota *qs = &thread->rq->quota;
>>>>  	struct evl_quota_group *tg = thread->quota;
>>>> -	struct evl_rq *rq = thread->rq;
>>>>  
>>>>  	if (!thread_is_runnable(thread))
>>>>  		list_add(&thread->quota_expired, &tg->expired);
>>>>  	else
>>>> -		evl_add_schedq(&rq->fifo.runnable, thread);
>>>> +		evl_add_schedq(&qs->runnable, thread);
>>>>  
>>>>  	tg->nr_active++;
>>>>  }
>>>>  
>>>> -static struct evl_thread *quota_pick(struct evl_rq *rq)
>>>> +static void quota_out(struct evl_thread *thread)
>>>>  {
>>>> -	struct evl_thread *next, *curr = rq->curr;
>>>> -	struct evl_sched_quota *qs = &rq->quota;
>>>> -	struct evl_quota_group *otg, *tg;
>>>> -	ktime_t now, elapsed;
>>>> +	struct evl_sched_quota *qs = &thread->rq->quota;
>>>> +	struct evl_quota_group *tg = thread->quota;
>>>> +	ktime_t now, consumed;
>>>> +
>>>> +	/* Timer off means that we are not tracking quota. */
>>>> +	if (!evl_timer_is_running(&qs->limit_timer))
>>>> +		return;
>>>>  
>>>> -	now = evl_ktime_monotonic();
>>>> -	otg = curr->quota;
>>>> -	if (otg == NULL)
>>>> -		goto pick;
>>>>  	/*
>>>>  	 * Charge the time consumed by the outgoing thread to the
>>>>  	 * group it belongs to.
>>>>  	 */
>>>> -	elapsed = ktime_sub(now, otg->run_start);
>>>> -	if (elapsed < otg->run_budget)
>>>> -		otg->run_budget = ktime_sub(otg->run_budget, elapsed);
>>>> -	else
>>>> -		otg->run_budget = 0;
>>>> +	now = evl_ktime_monotonic();
>>>
>>> Thing brins some, though minor, glitch when doing the run_start update
>>> for the next group already in pick. That's why I'm trying to reuse that
>>> stamp when available.
>>>
>>>> +	consumed = ktime_sub(now, tg->run_start);
>>>> +	if (consumed < tg->run_budget) {
>>>> +		tg->run_budget = ktime_sub(tg->run_budget, consumed);
>>>> +	} else {
>>>> +		tg->run_budget = 0;
>>>> +		evl_stop_timer(&qs->limit_timer);
>>>> +	}
>>>> +}
>>>> +
>>>> +static struct evl_thread *quota_pick(struct evl_rq *rq)
>>>> +{
>>>> +	struct evl_thread *next, *curr = rq->curr;
>>>> +	struct evl_sched_quota *qs = &rq->quota;
>>>> +	struct evl_quota_group *tg;
>>>> +
>>>>  pick:
>>>> -	next = evl_get_schedq(&rq->fifo.runnable);
>>>> +	next = evl_get_schedq(&qs->runnable);
>>>>  	if (next == NULL) {
>>>>  		evl_stop_timer(&qs->limit_timer);
>>>>  		return NULL;
>>>>  	}
>>>>  
>>>> -	/*
>>>> -	 * As we basically piggyback on the SCHED_FIFO runqueue, make
>>>> -	 * sure to detect non-quota threads.
>>>> -	 */
>>>>  	tg = next->quota;
>>>> -	if (tg == NULL)
>>>> -		return next;
>>>> -
>>>> -	tg->run_start = now;
>>>> +	tg->nr_active--;
>>>>  
>>>>  	/*
>>>>  	 * Don't consider budget if kicked, we have to allow this
>>>> @@ -439,25 +437,29 @@ static struct evl_thread *quota_pick(struct evl_rq *rq)
>>>>  	 */
>>>>  	if (next->info & EVL_T_KICKED) {
>>>>  		evl_stop_timer(&qs->limit_timer);
>>>> -		goto out;
>>>> +		return next;
>>>>  	}
>>>>  
>>>> +	/*
>>>> +	 * __pick_next_thread() might have requeued the current
>>>> +	 * thread which is still leading the pack.
>>>> +	 */
>>>> +	if (curr == next)
>>>> +		return next;
>>>> +
>>>>  	if (ktime_to_ns(tg->run_budget) == 0) {
>>>> -		/* Flush expired group members as we go. */
>>>> +		/* Park expired group members as we go. */
>>>>  		list_add_tail(&next->quota_expired, &tg->expired);
>>>>  		goto pick;
>>>>  	}
>>>>  
>>>> -	if (otg == tg && evl_timer_is_running(&qs->limit_timer))
>>>> -		/* Same group, leave the running timer untouched. */
>>>> -		goto out;
>>>> -
>>>> -	/* Arm limit timer for the new running group. */
>>>> -	evl_start_timer(&qs->limit_timer,
>>>> -			ktime_add(now, tg->run_budget),
>>>> -			EVL_INFINITE);
>>>> -out:
>>>> -	tg->nr_active--;
>>>> +	/* Arm new limit timer on change of running group. */
>>>> +	if (curr->quota != tg || !evl_timer_is_running(&qs->limit_timer)) {
>>>> +		tg->run_start = evl_ktime_monotonic();
>>>> +		evl_start_timer(&qs->limit_timer,
>>>> +				ktime_add(tg->run_start, tg->run_budget),
>>>> +				EVL_INFINITE);
>>>> +	}
>>>>  
>>>>  	return next;
>>>>  }
>>>> @@ -542,7 +544,7 @@ static int quota_destroy_group(struct evl_quota_group *tg,
>>>>  	 * Unregister the group before we drop rq->lock. As a result,
>>>>  	 * it won't accept threads anymore while we are busy moving
>>>>  	 * the current members to the fifo class, and concurrent
>>>> -	 * evl_quota_remove requests would receive -EINVAL.
>>>> +	 * quota_remove requests would receive -EINVAL.
>>>>  	 */
>>>>  	__clear_bit(tg->tgid, group_map);
>>>>  	list_del(&tg->next);
>>>> @@ -556,7 +558,7 @@ static int quota_destroy_group(struct evl_quota_group *tg,
>>>>  	 * hold rq->lock on entry, we do a trylock dance to prevent an
>>>>  	 * ABBA issue. No livelock is possible since we unregistered
>>>>  	 * that group already, so &tg->members can only be depleted
>>>> -	 * (by this loop specifically).
>>>> +	 * (by this loop exclusively).
>>>>  	 */
>>>>  
>>>>  	while (!list_empty(&tg->members)) {
>>>> @@ -583,10 +585,10 @@ static void quota_set_limit(struct evl_quota_group *tg,
>>>>  			int *quota_sum_r)
>>>>  {
>>>>  	struct evl_rq *rq = tg->rq;
>>>> -	struct evl_thread *thread, *tmp, *curr = rq->curr;
>>>> +	struct evl_thread *thread, *tmp;
>>>>  	struct evl_sched_quota *qs = &rq->quota;
>>>> -	ktime_t now, elapsed, consumed;
>>>>  	ktime_t old_quota = tg->quota;
>>>> +	ktime_t consumed;
>>>>  	u64 n;
>>>>  
>>>>  	assert_hard_lock(&rq->lock);
>>>> @@ -615,16 +617,8 @@ static void quota_set_limit(struct evl_quota_group *tg,
>>>>  	tg->quota_percent = quota_percent;
>>>>  	tg->quota_peak_percent = quota_peak_percent;
>>>>  
>>>> -	if (thread_on_quota(curr, tg)) {
>>>> -		now = evl_ktime_monotonic();
>>>> -
>>>> -		elapsed = now - tg->run_start;
>>>> -		if (elapsed < tg->run_budget)
>>>> -			tg->run_budget -= elapsed;
>>>> -		else
>>>> -			tg->run_budget = 0;
>>>> -
>>>> -		tg->run_start = now;
>>>> +	if (current_on_quota(tg)) {
>>>> +		quota_out(rq->curr);
>>>>  		evl_stop_timer(&qs->limit_timer);
>>>>  	}
>>>>  
>>>> @@ -646,7 +640,7 @@ static void quota_set_limit(struct evl_quota_group *tg,
>>>>  		list_for_each_entry_safe_reverse(thread, tmp, &tg->expired,
>>>>  						quota_expired) {
>>>>  			list_del_init(&thread->quota_expired);
>>>> -			evl_add_schedq(&rq->fifo.runnable, thread);
>>>> +			evl_add_schedq(&qs->runnable, thread);
>>>>  		}
>>>>  	}
>>>>  
>>>> @@ -786,6 +780,7 @@ struct evl_sched_class evl_sched_quota = {
>>>>  	.sched_dequeue		=	quota_dequeue,
>>>>  	.sched_requeue		=	quota_requeue,
>>>>  	.sched_pick		=	quota_pick,
>>>> +	.sched_out		=	quota_out,
>>>>  	.sched_migrate		=	quota_migrate,
>>>>  	.sched_chkparam		=	quota_chkparam,
>>>>  	.sched_setparam		=	quota_setparam,
>>>>
>>>
>>> You also seem to miss the case I discovered only yesterday as well: When
>>> starting the current thread on a quota via setparam, we also need to set
>>> its run_start stamp at that point.
>>>
>>> Please have a closer look at both my test case (libevl) as well as my
>>> patch 2. Currently, I don't see remaining issues with both, but I'm all
>>> ears to learn what I missed.
>>>
>> 
>> AFAIU, the logic of your changes is as follows:
>> 
>> 1. provide a sched_out() handler which charges the previous/current
>>    group with the budget consumed by the outgoing thread, dropping such
>>    accounting from sched_pick() in the same move. As a consequence,
>>    charge the consumed budget in quota_limit_handler() too since you
>>    can't expect sched_pick() to do so (i.e. turning on the resched bit
>>    on curr->rq in quota_limit_handler() would not help in this case).
>
> Yep.
>
>> 
>> 2. invoke sched_out() after sched_pick(), unless prev == next.
>> 
>> Since sched_pick() checks next->quota->run_budget to make sure that next
>> may run, what if curr and next belong to the same quota group given that
>> curr->quota->run_budget won't be updated by sched_out() until
>> sched_pick() has returned?
>
> Nothing is changed in case final next->quota == curr->quota. It's indeed
> true that the run_budget == 0 check is not going to trigger anymore -
> can be removed, the quota-timer will take care anyway.
>

For this to work, every place stopping the timer would have to call
charge_usage() then.

-- 
Philippe.