SCHED_DEADLINE server infrastructure

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

To prevent this issue, implement a starvation monitor mechanism that starts the deadline server only if a (fair in this case) task hasn't been scheduled for some interval of time after it has been enqueued. Use pick/put functions to manage starvation monitor status.

Signed-off-by Juri Lelli <[email protected]>

Signed-off-by Daniel Bristot de Oliveira <[email protected]>

---
 kernel/sched/fair.c  | 57 ++++++++++++++++++++++++++++++++++++++++++--
 kernel/sched/sched.h |  4 ++++
 2 files changed, 59 insertions(+), 2 deletions(-)

kernel/sched/fair.c +55 -2

index f493f05c1f84..75eadd85e2b3 100644
@@ -6315,6 +6315,53 @@ static int sched_idle_cpu(int cpu)
 }
 #endif
 
+
+static void fair_server_watchdog(struct timer_list *list)
+{
+       struct rq *rq = container_of(list, struct rq, fair_server_wd);
+       struct rq_flags rf;
+
+       rq_lock_irqsave(rq, &rf);
+       rq->fair_server_wd_running = 0;
+
+       if (!rq->cfs.h_nr_running)
+               goto out;
+
+       update_rq_clock(rq);
+       dl_server_start(&rq->fair_server);
+       rq->fair_server_active = 1;
+       resched_curr(rq);
+
+out:
+       rq_unlock_irqrestore(rq, &rf);
+}
+
+static inline void fair_server_watchdog_start(struct rq *rq)
+{
+       if (rq->fair_server_wd_running || rq->fair_server_active)
+               return;
+
+       timer_setup(&rq->fair_server_wd, fair_server_watchdog, 0);
+       rq->fair_server_wd.expires = jiffies + FAIR_SERVER_WATCHDOG_INTERVAL;
+       add_timer_on(&rq->fair_server_wd, cpu_of(rq));
+       rq->fair_server_active = 0;
+       rq->fair_server_wd_running = 1;
+}
+
+static inline void fair_server_watchdog_stop(struct rq *rq, bool stop_server)
+{
+       if (!rq->fair_server_wd_running && !stop_server)
+               return;
+
+       del_timer(&rq->fair_server_wd);
+       rq->fair_server_wd_running = 0;
+
+       if (stop_server && rq->fair_server_active) {
+               dl_server_stop(&rq->fair_server);
+               rq->fair_server_active = 0;
+       }
+}
+
 /*
  * The enqueue_task method is called before nr_running is
  * increased. Here we update the fair scheduling stats and

@@ -6337,7 +6384,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
        util_est_enqueue(&rq->cfs, p);
 
        if (!rq->cfs.h_nr_running)
-               dl_server_start(&rq->fair_server);
+               fair_server_watchdog_start(rq);
 
        /*
         * If in_iowait is set, the code below may not trigger any cpufreq

@@ -6484,7 +6531,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 
 dequeue_throttle:
        if (!rq->cfs.h_nr_running)
-               dl_server_stop(&rq->fair_server);
+               fair_server_watchdog_stop(rq, true);
 
        util_est_update(&rq->cfs, p, task_sleep);
        hrtick_update(rq);

@@ -8193,6 +8240,7 @@ done: __maybe_unused;
                hrtick_start_fair(rq, p);
 
        update_misfit_status(p, rq);
+       fair_server_watchdog_stop(rq, false);
 
        return p;
 

@@ -8248,6 +8296,8 @@ void fair_server_init(struct rq *rq)
        dl_se->dl_period = 20 * TICK_NSEC;
 
        dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick);
+
+       rq->fair_server_wd_running = 0;
 }
 
 /*

@@ -8262,6 +8312,9 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
                cfs_rq = cfs_rq_of(se);
                put_prev_entity(cfs_rq, se);
        }
+
+       if (rq->cfs.h_nr_running)
+               fair_server_watchdog_start(rq);
 }
 
 /*

kernel/sched/sched.h +4 -1

index d4a7c0823c53..cab5d2b1e71f 100644
@@ -353,6 +353,7 @@ extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
                    dl_server_has_tasks_f has_tasks,
                    dl_server_pick_f pick);
 
+#define FAIR_SERVER_WATCHDOG_INTERVAL (HZ >> 1)
 extern void fair_server_init(struct rq *);
 
 #ifdef CONFIG_CGROUP_SCHED

@@ -1018,6 +1019,9 @@ struct rq {
        struct dl_rq            dl;
 
        struct sched_dl_entity  fair_server;
+       int                     fair_server_active;
+       struct timer_list       fair_server_wd;
+       int                     fair_server_wd_running;
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
        /* list of leaf cfs_rq on this CPU: */
--

2.40.1

Hello,

Quoted 10 lines · Daniel Bristot de Oliveira

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

Quoted 4 lines

To prevent this issue, implement a starvation monitor mechanism that starts the deadline server only if a (fair in this case) task hasn't been scheduled for some interval of time after it has been enqueued. Use pick/put functions to manage starvation monitor status.

Me and Vineeth were discussing that another way of resolving this issue is to use a DL-server for RT as well, and then using a smaller deadline for RT. That way the RT is more likely to be selected due to its earlier deadline/period.

Another approach could be to implement the 0-laxity scheduling as a general SCHED_DEADLINE feature, perhaps through a flag. And allow DL tasks to opt-in to 0-laxity scheduling unless there are idle cycles. And then opt-in the feature for the CFS deadline server task.

Lastly, if the goal is to remove RT throttling code eventually, are you also planning to remove RT group scheduling as well? Are there users of RT group scheduling that might be impacted? On the other hand, RT throttling / group scheduling code can be left as it is (perhaps documenting it as deprecated) and the server stuff can be implemented via a CONFIG option.

- Joel

Quoted 2 lines

Signed-off-by Juri Lelli <[email protected]>

Signed-off-by Daniel Bristot de Oliveira <[email protected]>

Quoted 17 lines

Quoted 10 lines · Daniel Bristot de Oliveira

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

Quoted 4 lines

To prevent this issue, implement a starvation monitor mechanism that starts the deadline server only if a (fair in this case) task hasn't been scheduled for some interval of time after it has been enqueued. Use pick/put functions to manage starvation monitor status.

Quoted 4 lines

Me and Vineeth were discussing that another way of resolving this issue is to use a DL-server for RT as well, and then using a smaller deadline for RT. That way the RT is more likely to be selected due to its earlier deadline/period.

It would not be that different from what we have now.

One of the problems of throttling nowadays is that it accounts for a large window of time, and any "imprecision" can cause the mechanism not to work as expected.

For example, we work on a fully-isolated CPU scenario, where some very sporadic workload can be placed on the isolated CPU because of per-cpu kernel activities, e.g., kworkers... We need to let them run, but for a minimal amount of time, for instance, 20 us, to bound the interference.

The current mechanism does not give this precision because the IRQ accounting does not account for runtime for the rt throttling (which makes sense).

So the
RT throttling has the 20 us stolen from IRQs and keeps running. The same will happen if we swap the current mechanism with a DL server for the RT.

Also, thinking about short deadlines to fake a fixed priority is... not starting well. A fixed-priority higher instance is not a property of a deadline-based scheduler, and Linux has a fixed-priority hierarchy: STOP -> DL -> RT -> CFS... It is simple, and that is good.

That is why it is better to boost CFS instead of throttling RT. By boosting CFS, you do not need a server for RT, and we account for anything on top of CFS for free (IRQ/DL/FIFO...).

Quoted 4 lines

Another approach could be to implement the 0-laxity scheduling as a general SCHED_DEADLINE feature, perhaps through a flag. And allow DL tasks to opt-in to 0-laxity scheduling unless there are idle cycles. And then opt-in the feature for the CFS deadline server task.

A 0-laxity scheduler is not as simple as it sounds, as the priority also depends on the "C" (runtime, generally WCET), which is hard to find and embeds pessimism. Also, having such a feature would make other mechanisms harder, as well as debugging things. For example, proxy-execution or a more precise schedulability test...

In a paper, the scheduler alone is the solution. In real life, the solution for problems like locking is as fundamental as the scheduler. We need to keep things simple to expand on these other topics as well.

So, I do not think we need all the drawbacks of a mixed solution to just fix the throttling problem, and EDF is more capable and explored for the general case.

With this patch's idea (and expansions), we can fix the throttling problem without breaking other behaviors like scheduling order...

Quoted 6 lines

Lastly, if the goal is to remove RT throttling code eventually, are you also planning to remove RT group scheduling as well? Are there users of RT group scheduling that might be impacted? On the other hand, RT throttling / group scheduling code can be left as it is (perhaps documenting it as deprecated) and the server stuff can be implemented via a CONFIG option.

I think that the idea is to have the DL servers eventually replace the group schedule. But I also believe that it is better to start by solving the throttling and then moving to other constructions on top of the mechanism.

From: Peter Zijlstra <[email protected]>

Low priority tasks (e.g., SCHED_OTHER) can suffer starvation if tasks with higher priority (e.g., SCHED_FIFO) monopolize CPU(s).

RT Throttling has been introduced a while ago as a (mostly debug) countermeasure one can utilize to reserve some CPU time for low priority tasks (usually background type of work, e.g. workqueues, timers, etc.). It however has its own problems (see documentation) and the undesired effect of unconditionally throttling FIFO tasks even when no lower priority activity needs to run (there are mechanisms to fix this issue as well, but, again, with their own problems).

Introduce deadline servers to service low priority tasks needs under starvation conditions. Deadline servers are built extending SCHED_DEADLINE implementation to allow 2-level scheduling (a sched_deadline entity becomes a container for lower priority scheduling entities).

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Signed-off-by Daniel Bristot de Oliveira <[email protected]>

---
 include/linux/sched.h   |  22 ++-
 kernel/sched/core.c     |  17 ++
 kernel/sched/deadline.c | 350 +++++++++++++++++++++++++++-------------
 kernel/sched/fair.c     |   4 +
 kernel/sched/sched.h    |  29 ++++
 5 files changed, 309 insertions(+), 113 deletions(-)

include/linux/sched.h +20 -1

index 26b1925a702a..4c90d7693a75 100644
@@ -64,12 +64,14 @@ struct robust_list_head;
 struct root_domain;
 struct rq;
 struct sched_attr;
+struct sched_dl_entity;
 struct sched_param;
 struct seq_file;
 struct sighand_struct;
 struct signal_struct;
 struct task_delay_info;
 struct task_group;
+struct task_struct;
 struct user_event_mm;
 
 /*

@@ -600,6 +602,9 @@ struct sched_rt_entity {
 #endif
 } __randomize_layout;
 
+typedef bool (*dl_server_has_tasks_f)(struct sched_dl_entity *);
+typedef struct task_struct *(*dl_server_pick_f)(struct sched_dl_entity *);
+
 struct sched_dl_entity {
 	struct rb_node			rb_node;
 

@@ -647,6 +652,7 @@ struct sched_dl_entity {
 	unsigned int			dl_yielded        : 1;
 	unsigned int			dl_non_contending : 1;
 	unsigned int			dl_overrun	  : 1;
+	unsigned int			dl_server         : 1;
 
 	/*
 	 * Bandwidth enforcement timer. Each -deadline task has its

@@ -661,7 +667,20 @@ struct sched_dl_entity {
 	 * timer is needed to decrease the active utilization at the correct
 	 * time.
 	 */
-	struct hrtimer inactive_timer;
+	struct hrtimer			inactive_timer;
+
+	/*
+	 * Bits for DL-server functionality. Also see the comment near
+	 * dl_server_update().
+	 *
+	 * @rq the runqueue this server is for
+	 *
+	 * @server_has_tasks() returns true if @server_pick return a
+	 * runnable task.
+	 */
+	struct rq			*rq;
+	dl_server_has_tasks_f		server_has_tasks;
+	dl_server_pick_f		server_pick;
 
 #ifdef CONFIG_RT_MUTEXES
 	/*

include/linux/sched.h +0 -0

 #ifdef CONFIG_SCHED_CORE

kernel/sched/core.c +17 -0

index e34b02cbe41f..5b88b822ec89 100644
@@ -3803,6 +3803,8 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
 		rq->idle_stamp = 0;
 	}
 #endif
+
+	p->server = NULL;
 }
 
 /*

@@ -6013,12 +6015,27 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 			p = pick_next_task_idle(rq);
 		}
 
+		/*
+		 * This is the fast path; it cannot be a DL server pick;
+		 * therefore even if @p == @prev, ->server must be NULL.
+		 */
+		if (p->server)
+			p->server = NULL;
+
 		return p;
 	}
 
 restart:
 	put_prev_task_balance(rq, prev, rf);
 
+	/*
+	 * We've updated @prev and no longer need the server link, clear it.
+	 * Must be done before ->pick_next_task() because that can (re)set
+	 * ->server.
+	 */
+	if (prev->server)
+		prev->server = NULL;
+
 	for_each_class(class) {
 		p = class->pick_next_task(rq);
 		if (p)

kernel/sched/deadline.c +238 -112

index 869734eecb2c..c67056ff5749 100644
@@ -52,8 +52,14 @@ static int __init sched_dl_sysctl_init(void)
 late_initcall(sched_dl_sysctl_init);
 #endif
 
+static bool dl_server(struct sched_dl_entity *dl_se)
+{
+	return dl_se->dl_server;
+}
+
 static inline struct task_struct *dl_task_of(struct sched_dl_entity *dl_se)
 {
+	BUG_ON(dl_server(dl_se));
 	return container_of(dl_se, struct task_struct, dl);
 }
 

@@ -62,14 +68,22 @@ static inline struct rq *rq_of_dl_rq(struct dl_rq *dl_rq)
 	return container_of(dl_rq, struct rq, dl);
 }
 
-static inline struct dl_rq *dl_rq_of_se(struct sched_dl_entity *dl_se)
+static inline struct rq *rq_of_dl_se(struct sched_dl_entity *dl_se)
 {
-	struct task_struct *p = dl_task_of(dl_se);
-	struct rq *rq = task_rq(p);
+	struct rq *rq = dl_se->rq;
+
+	if (!dl_server(dl_se))
+		rq = task_rq(dl_task_of(dl_se));
 
-	return &rq->dl;
+	return rq;
 }
 
+static inline struct dl_rq *dl_rq_of_se(struct sched_dl_entity *dl_se)
+{
+	return &rq_of_dl_se(dl_se)->dl;
+}
+
+
 static inline int on_dl_rq(struct sched_dl_entity *dl_se)
 {
 	return !RB_EMPTY_NODE(&dl_se->rb_node);

@@ -392,9 +406,8 @@ static void __dl_clear_params(struct sched_dl_entity *dl_se);
 static void task_non_contending(struct sched_dl_entity *dl_se)
 {
 	struct hrtimer *timer = &dl_se->inactive_timer;
-	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
-	struct rq *rq = rq_of_dl_rq(dl_rq);
-	struct task_struct *p = dl_task_of(dl_se);
+	struct rq *rq = rq_of_dl_se(dl_se);
+	struct dl_rq *dl_rq = &rq->dl;
 	s64 zerolag_time;
 
 	/*

@@ -424,25 +437,33 @@ static void task_non_contending(struct sched_dl_entity *dl_se)
 	 * utilization now, instead of starting a timer
 	 */
 	if ((zerolag_time < 0) || hrtimer_active(&dl_se->inactive_timer)) {
-		if (dl_task(p))
+		if (dl_server(dl_se)) {
 			sub_running_bw(dl_se, dl_rq);
+		} else {
+			struct task_struct *p = dl_task_of(dl_se);
+
+			if (dl_task(p))
+				sub_running_bw(dl_se, dl_rq);
 
-		if (!dl_task(p) || READ_ONCE(p->__state) == TASK_DEAD) {
-			struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
+			if (!dl_task(p) || READ_ONCE(p->__state) == TASK_DEAD) {
+				struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
 
-			if (READ_ONCE(p->__state) == TASK_DEAD)
-				sub_rq_bw(dl_se, &rq->dl);
-			raw_spin_lock(&dl_b->lock);
-			__dl_sub(dl_b, dl_se->dl_bw, dl_bw_cpus(task_cpu(p)));
-			raw_spin_unlock(&dl_b->lock);
-			__dl_clear_params(dl_se);
+				if (READ_ONCE(p->__state) == TASK_DEAD)
+					sub_rq_bw(dl_se, &rq->dl);
+				raw_spin_lock(&dl_b->lock);
+				__dl_sub(dl_b, dl_se->dl_bw, dl_bw_cpus(task_cpu(p)));
+				raw_spin_unlock(&dl_b->lock);
+				__dl_clear_params(dl_se);
+			}
 		}
 
 		return;
 	}
 
 	dl_se->dl_non_contending = 1;
-	get_task_struct(p);
+	if (!dl_server(dl_se))
+		get_task_struct(dl_task_of(dl_se));
+
 	hrtimer_start(timer, ns_to_ktime(zerolag_time), HRTIMER_MODE_REL_HARD);
 }
 

@@ -469,8 +490,10 @@ static void task_contending(struct sched_dl_entity *dl_se, int flags)
 		 * will not touch the rq's active utilization,
 		 * so we are still safe.
 		 */
-		if (hrtimer_try_to_cancel(&dl_se->inactive_timer) == 1)
-			put_task_struct(dl_task_of(dl_se));
+		if (hrtimer_try_to_cancel(&dl_se->inactive_timer) == 1) {
+			if (!dl_server(dl_se))
+				put_task_struct(dl_task_of(dl_se));
+		}
 	} else {
 		/*
 		 * Since "dl_non_contending" is not set, the

@@ -483,10 +506,8 @@ static void task_contending(struct sched_dl_entity *dl_se, int flags)
 	}
 }
 
-static inline int is_leftmost(struct task_struct *p, struct dl_rq *dl_rq)
+static inline int is_leftmost(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
 {
-	struct sched_dl_entity *dl_se = &p->dl;
-
 	return rb_first_cached(&dl_rq->root) == &dl_se->rb_node;
 }
 

@@ -573,8 +594,6 @@ static void inc_dl_migration(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
 
 	if (p->nr_cpus_allowed > 1)
 		dl_rq->dl_nr_migratory++;
-
-	update_dl_migration(dl_rq);
 }
 
 static void dec_dl_migration(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)

@@ -583,8 +602,6 @@ static void dec_dl_migration(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
 
 	if (p->nr_cpus_allowed > 1)
 		dl_rq->dl_nr_migratory--;
-
-	update_dl_migration(dl_rq);
 }
 
 #define __node_2_pdl(node) \

@@ -762,8 +779,10 @@ static inline void deadline_queue_pull_task(struct rq *rq)
 }
 #endif /* CONFIG_SMP */
 
+static void
+enqueue_dl_entity(struct sched_dl_entity *dl_se, int flags);
 static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags);
-static void __dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags);
+static void dequeue_dl_entity(struct sched_dl_entity *dl_se, int flags);
 static void check_preempt_curr_dl(struct rq *rq, struct task_struct *p, int flags);
 
 static inline void replenish_dl_new_period(struct sched_dl_entity *dl_se,

@@ -1011,8 +1030,7 @@ static inline bool dl_is_implicit(struct sched_dl_entity *dl_se)
  */
 static void update_dl_entity(struct sched_dl_entity *dl_se)
 {
-	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
-	struct rq *rq = rq_of_dl_rq(dl_rq);
+	struct rq *rq = rq_of_dl_se(dl_se);
 
 	if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
 	    dl_entity_overflow(dl_se, rq_clock(rq))) {

@@ -1043,11 +1061,11 @@ static inline u64 dl_next_period(struct sched_dl_entity *dl_se)
  * actually started or not (i.e., the replenishment instant is in
  * the future or in the past).
  */
-static int start_dl_timer(struct task_struct *p)
+static int start_dl_timer(struct sched_dl_entity *dl_se)
 {
-	struct sched_dl_entity *dl_se = &p->dl;
 	struct hrtimer *timer = &dl_se->dl_timer;
-	struct rq *rq = task_rq(p);
+	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
+	struct rq *rq = rq_of_dl_rq(dl_rq);
 	ktime_t now, act;
 	s64 delta;
 

@@ -1081,13 +1099,33 @@ static int start_dl_timer(struct task_struct *p)
 	 * and observe our state.
 	 */
 	if (!hrtimer_is_queued(timer)) {
-		get_task_struct(p);
+		if (!dl_server(dl_se))
+			get_task_struct(dl_task_of(dl_se));
 		hrtimer_start(timer, act, HRTIMER_MODE_ABS_HARD);
 	}
 
 	return 1;
 }
 
+static void __push_dl_task(struct rq *rq, struct rq_flags *rf)
+{
+#ifdef CONFIG_SMP
+	/*
+	 * Queueing this task back might have overloaded rq, check if we need
+	 * to kick someone away.
+	 */
+	if (has_pushable_dl_tasks(rq)) {
+		/*
+		 * Nothing relies on rq->lock after this, so its safe to drop
+		 * rq->lock.
+		 */
+		rq_unpin_lock(rq, rf);
+		push_dl_task(rq);
+		rq_repin_lock(rq, rf);
+	}
+#endif
+}
+
 /*
  * This is the bandwidth enforcement timer callback. If here, we know
  * a task is not on its dl_rq, since the fact that the timer was running

@@ -1106,10 +1144,34 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	struct sched_dl_entity *dl_se = container_of(timer,
 						     struct sched_dl_entity,
 						     dl_timer);
-	struct task_struct *p = dl_task_of(dl_se);
+	struct task_struct *p;
 	struct rq_flags rf;
 	struct rq *rq;
 
+	if (dl_server(dl_se)) {
+		struct rq *rq = rq_of_dl_se(dl_se);
+		struct rq_flags rf;
+
+		rq_lock(rq, &rf);
+		if (dl_se->dl_throttled) {
+			sched_clock_tick();
+			update_rq_clock(rq);
+
+			if (dl_se->server_has_tasks(dl_se)) {
+				enqueue_dl_entity(dl_se, ENQUEUE_REPLENISH);
+				resched_curr(rq);
+				__push_dl_task(rq, &rf);
+			} else {
+				replenish_dl_entity(dl_se);
+			}
+
+		}
+		rq_unlock(rq, &rf);
+
+		return HRTIMER_NORESTART;
+	}
+
+	p = dl_task_of(dl_se);
 	rq = task_rq_lock(p, &rf);
 
 	/*

@@ -1180,21 +1242,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	else
 		resched_curr(rq);
 
-#ifdef CONFIG_SMP
-	/*
-	 * Queueing this task back might have overloaded rq, check if we need
-	 * to kick someone away.
-	 */
-	if (has_pushable_dl_tasks(rq)) {
-		/*
-		 * Nothing relies on rq->lock after this, so its safe to drop
-		 * rq->lock.
-		 */
-		rq_unpin_lock(rq, &rf);
-		push_dl_task(rq);
-		rq_repin_lock(rq, &rf);
-	}
-#endif
+	__push_dl_task(rq, &rf);
 
 unlock:
 	task_rq_unlock(rq, p, &rf);

@@ -1236,12 +1284,11 @@ static void init_dl_task_timer(struct sched_dl_entity *dl_se)
  */
 static inline void dl_check_constrained_dl(struct sched_dl_entity *dl_se)
 {
-	struct task_struct *p = dl_task_of(dl_se);
-	struct rq *rq = rq_of_dl_rq(dl_rq_of_se(dl_se));
+	struct rq *rq = rq_of_dl_se(dl_se);
 
 	if (dl_time_before(dl_se->deadline, rq_clock(rq)) &&
 	    dl_time_before(rq_clock(rq), dl_next_period(dl_se))) {
-		if (unlikely(is_dl_boosted(dl_se) || !start_dl_timer(p)))
+		if (unlikely(is_dl_boosted(dl_se) || !start_dl_timer(dl_se)))
 			return;
 		dl_se->dl_throttled = 1;
 		if (dl_se->runtime > 0)

@@ -1296,29 +1343,13 @@ static u64 grub_reclaim(u64 delta, struct rq *rq, struct sched_dl_entity *dl_se)
 	return (delta * u_act) >> BW_SHIFT;
 }
 
-/*
- * Update the current task's runtime statistics (provided it is still
- * a -deadline task and has not been removed from the dl_rq).
- */
-static void update_curr_dl(struct rq *rq)
+static inline void
+update_stats_dequeue_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se,
+                        int flags);
+static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 delta_exec)
 {
-	struct task_struct *curr = rq->curr;
-	struct sched_dl_entity *dl_se = &curr->dl;
-	s64 delta_exec, scaled_delta_exec;
-	int cpu = cpu_of(rq);
-
-	if (!dl_task(curr) || !on_dl_rq(dl_se))
-		return;
+	s64 scaled_delta_exec;
 
-	/*
-	 * Consumed budget is computed considering the time as
-	 * observed by schedulable tasks (excluding time spent
-	 * in hardirq context, etc.). Deadlines are instead
-	 * computed using hard walltime. This seems to be the more
-	 * natural solution, but the full ramifications of this
-	 * approach need further study.
-	 */
-	delta_exec = update_curr_common(rq);
 	if (unlikely(delta_exec <= 0)) {
 		if (unlikely(dl_se->dl_yielded))
 			goto throttle;

@@ -1336,10 +1367,9 @@ static void update_curr_dl(struct rq *rq)
 	 * according to current frequency and CPU maximum capacity.
 	 */
 	if (unlikely(dl_se->flags & SCHED_FLAG_RECLAIM)) {
-		scaled_delta_exec = grub_reclaim(delta_exec,
-						 rq,
-						 &curr->dl);
+		scaled_delta_exec = grub_reclaim(delta_exec, rq, dl_se);
 	} else {
+		int cpu = cpu_of(rq);
 		unsigned long scale_freq = arch_scale_freq_capacity(cpu);
 		unsigned long scale_cpu = arch_scale_cpu_capacity(cpu);
 

@@ -1358,11 +1388,21 @@ static void update_curr_dl(struct rq *rq)
 		    (dl_se->flags & SCHED_FLAG_DL_OVERRUN))
 			dl_se->dl_overrun = 1;
 
-		__dequeue_task_dl(rq, curr, 0);
-		if (unlikely(is_dl_boosted(dl_se) || !start_dl_timer(curr)))
-			enqueue_task_dl(rq, curr, ENQUEUE_REPLENISH);
+		dequeue_dl_entity(dl_se, 0);
+		if (!dl_server(dl_se)) {
+			/* XXX: After v2, from __dequeue_task_dl() */
+			update_stats_dequeue_dl(&rq->dl, dl_se, 0);
+			dequeue_pushable_dl_task(rq, dl_task_of(dl_se));
+		}
+
+		if (unlikely(is_dl_boosted(dl_se) || !start_dl_timer(dl_se))) {
+			if (dl_server(dl_se))
+				enqueue_dl_entity(dl_se, ENQUEUE_REPLENISH);
+			else
+				enqueue_task_dl(rq, dl_task_of(dl_se), ENQUEUE_REPLENISH);
+		}
 
-		if (!is_leftmost(curr, &rq->dl))
+		if (!is_leftmost(dl_se, &rq->dl))
 			resched_curr(rq);
 	}
 

@@ -1392,20 +1432,82 @@ static void update_curr_dl(struct rq *rq)
 	}
 }
 
+void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
+{
+	update_curr_dl_se(dl_se->rq, dl_se, delta_exec);
+}
+
+void dl_server_start(struct sched_dl_entity *dl_se)
+{
+	if (!dl_server(dl_se)) {
+		dl_se->dl_server = 1;
+		setup_new_dl_entity(dl_se);
+	}
+	enqueue_dl_entity(dl_se, ENQUEUE_WAKEUP);
+}
+
+void dl_server_stop(struct sched_dl_entity *dl_se)
+{
+	dequeue_dl_entity(dl_se, DEQUEUE_SLEEP);
+}
+
+void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
+		    dl_server_has_tasks_f has_tasks,
+		    dl_server_pick_f pick)
+{
+	dl_se->rq = rq;
+	dl_se->server_has_tasks = has_tasks;
+	dl_se->server_pick = pick;
+}
+
+/*
+ * Update the current task's runtime statistics (provided it is still
+ * a -deadline task and has not been removed from the dl_rq).
+ */
+static void update_curr_dl(struct rq *rq)
+{
+	struct task_struct *curr = rq->curr;
+	struct sched_dl_entity *dl_se = &curr->dl;
+	s64 delta_exec;
+
+	if (!dl_task(curr) || !on_dl_rq(dl_se))
+		return;
+
+	/*
+	 * Consumed budget is computed considering the time as
+	 * observed by schedulable tasks (excluding time spent
+	 * in hardirq context, etc.). Deadlines are instead
+	 * computed using hard walltime. This seems to be the more
+	 * natural solution, but the full ramifications of this
+	 * approach need further study.
+	 */
+	delta_exec = update_curr_common(rq);
+	update_curr_dl_se(rq, dl_se, delta_exec);
+}
+
 static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 {
 	struct sched_dl_entity *dl_se = container_of(timer,
 						     struct sched_dl_entity,
 						     inactive_timer);
-	struct task_struct *p = dl_task_of(dl_se);
+	struct task_struct *p = NULL;
 	struct rq_flags rf;
 	struct rq *rq;
 
-	rq = task_rq_lock(p, &rf);
+	if (!dl_server(dl_se)) {
+		p = dl_task_of(dl_se);
+		rq = task_rq_lock(p, &rf);
+	} else {
+		rq = dl_se->rq;
+		rq_lock(rq, &rf);
+	}
 
 	sched_clock_tick();
 	update_rq_clock(rq);
 
+	if (dl_server(dl_se))
+		goto no_task;
+
 	if (!dl_task(p) || READ_ONCE(p->__state) == TASK_DEAD) {
 		struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
 

@@ -1422,14 +1524,21 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 
 		goto unlock;
 	}
+
+no_task:
 	if (dl_se->dl_non_contending == 0)
 		goto unlock;
 
 	sub_running_bw(dl_se, &rq->dl);
 	dl_se->dl_non_contending = 0;
 unlock:
-	task_rq_unlock(rq, p, &rf);
-	put_task_struct(p);
+
+	if (!dl_server(dl_se)) {
+		task_rq_unlock(rq, p, &rf);
+		put_task_struct(p);
+	} else {
+		rq_unlock(rq, &rf);
+	}
 
 	return HRTIMER_NORESTART;
 }

@@ -1487,34 +1596,35 @@ static void dec_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
 static inline void inc_dl_deadline(struct dl_rq *dl_rq, u64 deadline) {}
 static inline void dec_dl_deadline(struct dl_rq *dl_rq, u64 deadline) {}
 
+static inline void update_dl_migration(struct dl_rq *dl_rq) {}
+
 #endif /* CONFIG_SMP */
 
 static inline
 void inc_dl_tasks(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
 {
-	int prio = dl_task_of(dl_se)->prio;
 	u64 deadline = dl_se->deadline;
 
-	WARN_ON(!dl_prio(prio));
 	dl_rq->dl_nr_running++;
 	add_nr_running(rq_of_dl_rq(dl_rq), 1);
 
 	inc_dl_deadline(dl_rq, deadline);
-	inc_dl_migration(dl_se, dl_rq);
+	if (!dl_server(dl_se))
+		inc_dl_migration(dl_se, dl_rq);
+	update_dl_migration(dl_rq);
 }
 
 static inline
 void dec_dl_tasks(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
 {
-	int prio = dl_task_of(dl_se)->prio;
-
-	WARN_ON(!dl_prio(prio));
 	WARN_ON(!dl_rq->dl_nr_running);
 	dl_rq->dl_nr_running--;
 	sub_nr_running(rq_of_dl_rq(dl_rq), 1);
 
 	dec_dl_deadline(dl_rq, dl_se->deadline);
-	dec_dl_migration(dl_se, dl_rq);
+	if (!dl_server(dl_se))
+		dec_dl_migration(dl_se, dl_rq);
+	update_dl_migration(dl_rq);
 }
 
 static inline bool __dl_less(struct rb_node *a, const struct rb_node *b)

@@ -1676,8 +1786,7 @@ enqueue_dl_entity(struct sched_dl_entity *dl_se, int flags)
 	} else if (flags & ENQUEUE_REPLENISH) {
 		replenish_dl_entity(dl_se);
 	} else if ((flags & ENQUEUE_RESTORE) &&
-		  dl_time_before(dl_se->deadline,
-				 rq_clock(rq_of_dl_rq(dl_rq_of_se(dl_se))))) {
+		   dl_time_before(dl_se->deadline, rq_clock(rq_of_dl_se(dl_se)))) {
 		setup_new_dl_entity(dl_se);
 	}
 

@@ -1762,14 +1871,6 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 		enqueue_pushable_dl_task(rq, p);
 }
 
-static void __dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
-{
-	update_stats_dequeue_dl(&rq->dl, &p->dl, flags);
-	dequeue_dl_entity(&p->dl, flags);
-
-	if (!p->dl.dl_throttled)
-		dequeue_pushable_dl_task(rq, p);
-}
 
 static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 {

@@ -1778,7 +1879,9 @@ static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 	if (p->on_rq == TASK_ON_RQ_MIGRATING)
 		flags |= DEQUEUE_MIGRATING;
 
-	__dequeue_task_dl(rq, p, flags);
+	dequeue_dl_entity(&p->dl, flags);
+	if (!p->dl.dl_throttled)
+		dequeue_pushable_dl_task(rq, p);
 }
 
 /*

@@ -1968,12 +2071,12 @@ static void check_preempt_curr_dl(struct rq *rq, struct task_struct *p,
 }
 
 #ifdef CONFIG_SCHED_HRTICK
-static void start_hrtick_dl(struct rq *rq, struct task_struct *p)
+static void start_hrtick_dl(struct rq *rq, struct sched_dl_entity *dl_se)
 {
-	hrtick_start(rq, p->dl.runtime);
+	hrtick_start(rq, dl_se->runtime);
 }
 #else /* !CONFIG_SCHED_HRTICK */
-static void start_hrtick_dl(struct rq *rq, struct task_struct *p)
+static void start_hrtick_dl(struct rq *rq, struct sched_dl_entity *dl_se)
 {
 }
 #endif

@@ -1993,9 +2096,6 @@ static void set_next_task_dl(struct rq *rq, struct task_struct *p, bool first)
 	if (!first)
 		return;
 
-	if (hrtick_enabled_dl(rq))
-		start_hrtick_dl(rq, p);
-
 	if (rq->curr->sched_class != &dl_sched_class)
 		update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 0);
 

@@ -2018,12 +2118,26 @@ static struct task_struct *pick_task_dl(struct rq *rq)
 	struct dl_rq *dl_rq = &rq->dl;
 	struct task_struct *p;
 
+again:
 	if (!sched_dl_runnable(rq))
 		return NULL;
 
 	dl_se = pick_next_dl_entity(dl_rq);
 	WARN_ON_ONCE(!dl_se);
-	p = dl_task_of(dl_se);
+
+
+	if (dl_server(dl_se)) {
+		p = dl_se->server_pick(dl_se);
+		if (!p) {
+			// XXX should not happen, warn?!
+			dl_se->dl_yielded = 1;
+			update_curr_dl_se(rq, dl_se, 0);
+			goto again;
+		}
+		p->server = dl_se;
+	} else {
+		p = dl_task_of(dl_se);
+	}
 
 	return p;
 }

@@ -2033,9 +2147,20 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
 	struct task_struct *p;
 
 	p = pick_task_dl(rq);
-	if (p)
+	if (!p)
+		return p;
+
+	/*
+	 * XXX: re-check !dl_server, changed from v2 because of
+	 * pick_next_task_dl change
+	 */
+	if (!dl_server(&p->dl))
 		set_next_task_dl(rq, p, true);
 
+	/* XXX not quite right */
+	if (hrtick_enabled(rq))
+		start_hrtick_dl(rq, &p->dl);
+
 	return p;
 }
 

@@ -2073,8 +2198,8 @@ static void task_tick_dl(struct rq *rq, struct task_struct *p, int queued)
 	 * be set and schedule() will start a new hrtick for the next task.
 	 */
 	if (hrtick_enabled_dl(rq) && queued && p->dl.runtime > 0 &&
-	    is_leftmost(p, &rq->dl))
-		start_hrtick_dl(rq, p);
+	    is_leftmost(&p->dl, &rq->dl))
+		start_hrtick_dl(rq, &p->dl);
 }
 
 static void task_fork_dl(struct task_struct *p)

@@ -3003,6 +3128,7 @@ static void __dl_clear_params(struct sched_dl_entity *dl_se)
 	dl_se->dl_yielded		= 0;
 	dl_se->dl_non_contending	= 0;
 	dl_se->dl_overrun		= 0;
+	dl_se->dl_server		= 0;
 
 #ifdef CONFIG_RT_MUTEXES
 	dl_se->pi_se			= dl_se;

kernel/sched/fair.c +4 -0

index fda67f05190d..0c58d8e55b69 100644
@@ -930,6 +930,8 @@ s64 update_curr_common(struct rq *rq)
 
 	account_group_exec_runtime(curr, delta_exec);
 	cgroup_account_cputime(curr, delta_exec);
+	if (curr->server)
+		dl_server_update(curr->server, delta_exec);
 
 	return delta_exec;
 }

@@ -958,6 +960,8 @@ static void update_curr(struct cfs_rq *cfs_rq)
 		trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
 		cgroup_account_cputime(curtask, delta_exec);
 		account_group_exec_runtime(curtask, delta_exec);
+		if (curtask->server)
+			dl_server_update(curtask->server, delta_exec);
 	}
 
 	account_cfs_rq_runtime(cfs_rq, delta_exec);

kernel/sched/sched.h +29 -0

index aaf163695c2e..390c99e2f8a8 100644
@@ -324,6 +324,35 @@ extern bool dl_param_changed(struct task_struct *p, const struct sched_attr *att
 extern int  dl_cpuset_cpumask_can_shrink(const struct cpumask *cur, const struct cpumask *trial);
 extern int  dl_cpu_busy(int cpu, struct task_struct *p);
 
+/*
+ * SCHED_DEADLINE supports servers (nested scheduling) with the following
+ * interface:
+ *
+ *   dl_se::rq -- runqueue we belong to.
+ *
+ *   dl_se::server_has_tasks() -- used on bandwidth enforcement; we 'stop' the
+ *                                server when it runs out of tasks to run.
+ *
+ *   dl_se::server_pick() -- nested pick_next_task(); we yield the period if this
+ *                           returns NULL.
+ *
+ *   dl_server_update() -- called from update_curr_common(), propagates runtime
+ *                         to the server.
+ *
+ *   dl_server_start()
+ *   dl_server_stop()  -- start/stop the server when it has (no) tasks
+ *
+ *   dl_server_init()
+ *
+ * XXX
+ */
+extern void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec);
+extern void dl_server_start(struct sched_dl_entity *dl_se);
+extern void dl_server_stop(struct sched_dl_entity *dl_se);
+extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
+		    dl_server_has_tasks_f has_tasks,
+		    dl_server_pick_f pick);
+
 #ifdef CONFIG_CGROUP_SCHED
 
 struct cfs_rq;

Signature · 1 line

2.40.1

From: Peter Zijlstra <[email protected]>

All classes use sched_entity::exec_start to track runtime and have copies of the exact same code around to compute runtime.

Collapse all that.

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Signed-off-by Daniel Bristot de Oliveira <[email protected]>

---
 include/linux/sched.h    |  2 +-
 kernel/sched/deadline.c  | 15 +++--------
 kernel/sched/fair.c      | 57 ++++++++++++++++++++++++++++++----------
 kernel/sched/rt.c        | 15 +++--------
 kernel/sched/sched.h     | 12 ++-------
 kernel/sched/stop_task.c | 13 +--------
 6 files changed, 53 insertions(+), 61 deletions(-)

include/linux/sched.h +1 -1

index 1292d38d66cc..26b1925a702a 100644
@@ -521,7 +521,7 @@ struct sched_statistics {
 	u64				block_max;
 	s64				sum_block_runtime;
 
-	u64				exec_max;
+	s64				exec_max;
 	u64				slice_max;
 
 	u64				nr_migrations_cold;

kernel/sched/deadline.c +3 -12

index f827067ad03b..030e7c11607f 100644
@@ -1301,9 +1301,8 @@ static void update_curr_dl(struct rq *rq)
 {
 	struct task_struct *curr = rq->curr;
 	struct sched_dl_entity *dl_se = &curr->dl;
-	u64 delta_exec, scaled_delta_exec;
+	s64 delta_exec, scaled_delta_exec;
 	int cpu = cpu_of(rq);
-	u64 now;
 
 	if (!dl_task(curr) || !on_dl_rq(dl_se))
 		return;

@@ -1316,21 +1315,13 @@ static void update_curr_dl(struct rq *rq)
 	 * natural solution, but the full ramifications of this
 	 * approach need further study.
 	 */
-	now = rq_clock_task(rq);
-	delta_exec = now - curr->se.exec_start;
-	if (unlikely((s64)delta_exec <= 0)) {
+	delta_exec = update_curr_common(rq);
+	if (unlikely(delta_exec <= 0)) {
 		if (unlikely(dl_se->dl_yielded))
 			goto throttle;
 		return;
 	}
 
-	schedstat_set(curr->stats.exec_max,
-		      max(curr->stats.exec_max, delta_exec));
-
-	trace_sched_stat_runtime(curr, delta_exec, 0);
-
-	update_current_exec_runtime(curr, now, delta_exec);
-
 	if (dl_entity_is_special(dl_se))
 		return;
 

kernel/sched/fair.c +43 -14

index 6189d1a45635..fda67f05190d 100644
@@ -891,23 +891,17 @@ static void update_tg_load_avg(struct cfs_rq *cfs_rq)
 }
 #endif /* CONFIG_SMP */
 
-/*
- * Update the current task's runtime statistics.
- */
-static void update_curr(struct cfs_rq *cfs_rq)
+static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
 {
-	struct sched_entity *curr = cfs_rq->curr;
-	u64 now = rq_clock_task(rq_of(cfs_rq));
-	u64 delta_exec;
-
-	if (unlikely(!curr))
-		return;
+	u64 now = rq_clock_task(rq);
+	s64 delta_exec;
 
 	delta_exec = now - curr->exec_start;
-	if (unlikely((s64)delta_exec <= 0))
-		return;
+	if (unlikely(delta_exec <= 0))
+		return delta_exec;
 
 	curr->exec_start = now;
+	curr->sum_exec_runtime += delta_exec;
 
 	if (schedstat_enabled()) {
 		struct sched_statistics *stats;

@@ -917,8 +911,43 @@ static void update_curr(struct cfs_rq *cfs_rq)
 				max(delta_exec, stats->exec_max));
 	}
 
-	curr->sum_exec_runtime += delta_exec;
-	schedstat_add(cfs_rq->exec_clock, delta_exec);
+	return delta_exec;
+}
+
+/*
+ * Used by other classes to account runtime.
+ */
+s64 update_curr_common(struct rq *rq)
+{
+	struct task_struct *curr = rq->curr;
+	s64 delta_exec;
+
+	delta_exec = update_curr_se(rq, &curr->se);
+	if (unlikely(delta_exec <= 0))
+		return delta_exec;
+
+	trace_sched_stat_runtime(curr, delta_exec, 0);
+
+	account_group_exec_runtime(curr, delta_exec);
+	cgroup_account_cputime(curr, delta_exec);
+
+	return delta_exec;
+}
+
+/*
+ * Update the current task's runtime statistics.
+ */
+static void update_curr(struct cfs_rq *cfs_rq)
+{
+	struct sched_entity *curr = cfs_rq->curr;
+	s64 delta_exec;
+
+	if (unlikely(!curr))
+		return;
+
+	delta_exec = update_curr_se(rq_of(cfs_rq), curr);
+	if (unlikely(delta_exec <= 0))
+		return;
 
 	curr->vruntime += calc_delta_fair(delta_exec, curr);
 	update_min_vruntime(cfs_rq);

kernel/sched/rt.c +3 -12

index 00e0e5074115..efec4f3fef83 100644
@@ -1046,24 +1046,15 @@ static void update_curr_rt(struct rq *rq)
 {
 	struct task_struct *curr = rq->curr;
 	struct sched_rt_entity *rt_se = &curr->rt;
-	u64 delta_exec;
-	u64 now;
+	s64 delta_exec;
 
 	if (curr->sched_class != &rt_sched_class)
 		return;
 
-	now = rq_clock_task(rq);
-	delta_exec = now - curr->se.exec_start;
-	if (unlikely((s64)delta_exec <= 0))
+	delta_exec = update_curr_common(rq);
+	if (unlikely(delta_exec <= 0))
 		return;
 
-	schedstat_set(curr->stats.exec_max,
-		      max(curr->stats.exec_max, delta_exec));
-
-	trace_sched_stat_runtime(curr, delta_exec, 0);
-
-	update_current_exec_runtime(curr, now, delta_exec);
-
 	if (!rt_bandwidth_enabled())
 		return;
 

kernel/sched/sched.h +2 -10

index 556496c77dc2..da0cec2fc63a 100644
@@ -2176,6 +2176,8 @@ struct affinity_context {
 	unsigned int flags;
 };
 
+extern s64 update_curr_common(struct rq *rq);
+
 struct sched_class {
 
 #ifdef CONFIG_UCLAMP_TASK

@@ -3207,16 +3209,6 @@ extern int sched_dynamic_mode(const char *str);
 extern void sched_dynamic_update(int mode);
 #endif
 
-static inline void update_current_exec_runtime(struct task_struct *curr,
-						u64 now, u64 delta_exec)
-{
-	curr->se.sum_exec_runtime += delta_exec;
-	account_group_exec_runtime(curr, delta_exec);
-
-	curr->se.exec_start = now;
-	cgroup_account_cputime(curr, delta_exec);
-}
-
 #ifdef CONFIG_SCHED_MM_CID
 
 #define SCHED_MM_CID_PERIOD_NS	(100ULL * 1000000)	/* 100ms */

kernel/sched/stop_task.c +1 -12

index 85590599b4d6..7595494ceb6d 100644
@@ -70,18 +70,7 @@ static void yield_task_stop(struct rq *rq)
 
 static void put_prev_task_stop(struct rq *rq, struct task_struct *prev)
 {
-	struct task_struct *curr = rq->curr;
-	u64 now, delta_exec;
-
-	now = rq_clock_task(rq);
-	delta_exec = now - curr->se.exec_start;
-	if (unlikely((s64)delta_exec < 0))
-		delta_exec = 0;
-
-	schedstat_set(curr->stats.exec_max,
-		      max(curr->stats.exec_max, delta_exec));
-
-	update_current_exec_runtime(curr, now, delta_exec);
+	update_curr_common(rq);
 }
 
 /*

Signature · 1 line

2.40.1

From: Peter Zijlstra <[email protected]>

Create a single function that initializes a sched_dl_entity.

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Signed-off-by Daniel Bristot de Oliveira <[email protected]>

---
 kernel/sched/core.c     |  5 +----
 kernel/sched/deadline.c | 22 +++++++++++++++-------
 kernel/sched/sched.h    |  5 +----
 3 files changed, 17 insertions(+), 15 deletions(-)

kernel/sched/core.c +1 -4

index ac38225e6d09..e34b02cbe41f 100644
@@ -4511,10 +4511,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 	memset(&p->stats, 0, sizeof(p->stats));
 #endif
 
-	RB_CLEAR_NODE(&p->dl.rb_node);
-	init_dl_task_timer(&p->dl);
-	init_dl_inactive_task_timer(&p->dl);
-	__dl_clear_params(p);
+	init_dl_entity(&p->dl);
 
 	INIT_LIST_HEAD(&p->rt.run_list);
 	p->rt.timeout		= 0;

kernel/sched/deadline.c +15 -7

index 030e7c11607f..22e5e64812c9 100644
@@ -333,6 +333,8 @@ static void dl_change_utilization(struct task_struct *p, u64 new_bw)
 	__add_rq_bw(new_bw, &rq->dl);
 }
 
+static void __dl_clear_params(struct sched_dl_entity *dl_se);
+
 /*
  * The utilization of a task cannot be immediately removed from
  * the rq active utilization (running_bw) when the task blocks.

@@ -432,7 +434,7 @@ static void task_non_contending(struct task_struct *p)
 			raw_spin_lock(&dl_b->lock);
 			__dl_sub(dl_b, p->dl.dl_bw, dl_bw_cpus(task_cpu(p)));
 			raw_spin_unlock(&dl_b->lock);
-			__dl_clear_params(p);
+			__dl_clear_params(dl_se);
 		}
 
 		return;

@@ -1205,7 +1207,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
-void init_dl_task_timer(struct sched_dl_entity *dl_se)
+static void init_dl_task_timer(struct sched_dl_entity *dl_se)
 {
 	struct hrtimer *timer = &dl_se->dl_timer;
 

@@ -1415,7 +1417,7 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 		raw_spin_lock(&dl_b->lock);
 		__dl_sub(dl_b, p->dl.dl_bw, dl_bw_cpus(task_cpu(p)));
 		raw_spin_unlock(&dl_b->lock);
-		__dl_clear_params(p);
+		__dl_clear_params(dl_se);
 
 		goto unlock;
 	}

@@ -1431,7 +1433,7 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
-void init_dl_inactive_task_timer(struct sched_dl_entity *dl_se)
+static void init_dl_inactive_task_timer(struct sched_dl_entity *dl_se)
 {
 	struct hrtimer *timer = &dl_se->inactive_timer;
 

@@ -2974,10 +2976,8 @@ bool __checkparam_dl(const struct sched_attr *attr)
 /*
  * This function clears the sched_dl_entity static params.
  */
-void __dl_clear_params(struct task_struct *p)
+static void __dl_clear_params(struct sched_dl_entity *dl_se)
 {
-	struct sched_dl_entity *dl_se = &p->dl;
-
 	dl_se->dl_runtime		= 0;
 	dl_se->dl_deadline		= 0;
 	dl_se->dl_period		= 0;

@@ -2995,6 +2995,14 @@ void __dl_clear_params(struct task_struct *p)
 #endif
 }
 
+void init_dl_entity(struct sched_dl_entity *dl_se)
+{
+	RB_CLEAR_NODE(&dl_se->rb_node);
+	init_dl_task_timer(dl_se);
+	init_dl_inactive_task_timer(dl_se);
+	__dl_clear_params(dl_se);
+}
+
 bool dl_param_changed(struct task_struct *p, const struct sched_attr *attr)
 {
 	struct sched_dl_entity *dl_se = &p->dl;

kernel/sched/sched.h +1 -4

index da0cec2fc63a..fa6512070fa7 100644
@@ -284,8 +284,6 @@ struct rt_bandwidth {
 	unsigned int		rt_period_active;
 };
 
-void __dl_clear_params(struct task_struct *p);
-
 static inline int dl_bandwidth_enabled(void)
 {
 	return sysctl_sched_rt_runtime >= 0;

@@ -2390,8 +2388,7 @@ extern struct rt_bandwidth def_rt_bandwidth;
 extern void init_rt_bandwidth(struct rt_bandwidth *rt_b, u64 period, u64 runtime);
 extern bool sched_rt_bandwidth_account(struct rt_rq *rt_rq);
 
-extern void init_dl_task_timer(struct sched_dl_entity *dl_se);
-extern void init_dl_inactive_task_timer(struct sched_dl_entity *dl_se);
+extern void init_dl_entity(struct sched_dl_entity *dl_se);
 
 #define BW_SHIFT		20
 #define BW_UNIT			(1 << BW_SHIFT)

Signature · 1 line

2.40.1

Hello Daniel!

On Mon, Jun 12, 2023 at 1:21 PM Daniel Bristot de Oliveira <[email protected]> wrote:
[...]

Quoted 32 lines

Quoted 17 lines

Quoted 10 lines · Daniel Bristot de Oliveira

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

Quoted 4 lines

To prevent this issue, implement a starvation monitor mechanism that starts the deadline server only if a (fair in this case) task hasn't been scheduled for some interval of time after it has been enqueued. Use pick/put functions to manage starvation monitor status.

Quoted 4 lines

Me and Vineeth were discussing that another way of resolving this issue is to use a DL-server for RT as well, and then using a smaller deadline for RT. That way the RT is more likely to be selected due to its earlier deadline/period.

It would not be that different from what we have now.

One of the problems of throttling nowadays is that it accounts for a large window of time, and any "imprecision" can cause the mechanism not to work as expected.

For example, we work on a fully-isolated CPU scenario, where some very sporadic workload can be placed on the isolated CPU because of per-cpu kernel activities, e.g., kworkers... We need to let them run, but for a minimal amount of time, for instance, 20 us, to bound the interference.

Quoted 2 lines

The current mechanism does not give this precision because the IRQ accounting does not account for runtime for the rt throttling (which makes sense).

I lost you here, "Runtime for the rt throttling" does not make much sense to me as a statement.

Quoted 3 lines

So the
RT throttling has the 20 us stolen from IRQs and keeps running. The same will happen if we swap the current mechanism with a DL server for the RT.

I read this about 10 times to learn that *maybe* you mean that IRQs stole time from the "Well behaved running time" of the RT class.

not seeing how that is related to creation of a DL-server for the RT class. Maybe describe your point a bit more clearly?

Quoted 6 lines

Also, thinking about short deadlines to fake a fixed priority is... not starting well. A fixed-priority higher instance is not a property of a deadline-based scheduler, and Linux has a fixed-priority hierarchy: STOP -> DL -> RT -> CFS... It is simple, and that is good.

Quoted 3 lines

That is why it is better to boost CFS instead of throttling RT. By boosting CFS, you do not need a server for RT, and we account for anything on top of CFS for free (IRQ/DL/FIFO...).

I did mention in my last email that it is not ideal. I just brought it up as an option. It might reduce the problem being seen and is better than not having it.

Quoted 11 lines

Quoted 5 lines

Another approach could be to implement the 0-laxity scheduling as a general SCHED_DEADLINE feature, perhaps through a flag. And allow DL tasks to opt-in to 0-laxity scheduling unless there are idle cycles. And then opt-in the feature for the CFS deadline server task.

A 0-laxity scheduler is not as simple as it sounds, as the priority also depends on the "C" (runtime, generally WCET), which is hard to find and embeds pessimism. Also, having such a feature would make other mechanisms harder, as well as debugging things. For example, proxy-execution or a more precise schedulability test...

I think you did not read my email properly, I was saying make the 0-laxity default-off and the opt-in for certain DL tasks. That may work perfectly well for a system like ChromeOS where likely we will use the DL server as the sole deadline task and opt-in for the 0-laxity. Then we don't need watchdog hacks at all and it all cleanly works within the DL class itself. There are the drawbacks of the pessimism/locking etc (I already knew that by the way as the obvious drawbacks of 0-laxity) but I am not immediately seeing how this CFS-watchdog with 0-laxity is any different from the DL-server itself having such a property. If you really have a concrete point on why that won't work, and if you could clarify that more clearly why a watchdog is better than it, that would be great.

Quoted 7 lines

In a paper, the scheduler alone is the solution. In real life, the solution for problems like locking is as fundamental as the scheduler. We need to keep things simple to expand on these other topics as well.

So, I do not think we need all the drawbacks of a mixed solution to just fix the throttling problem, and EDF is more capable and explored for the general case.

Again, I was saying making it opt-in seems like a reasonable approach and just enabling such property for the DL server.

Quoted 2 lines

With this patch's idea (and expansions), we can fix the throttling problem without breaking other behaviors like scheduling order...

I don't mind the watchdog patch as such, of course. I presented its mechanics at OSSNA and I know how it works, but I feel the DL server opting-in for 0-laxity would be cleaner while keeping such behavior as default-off for regular DL uses, that's my opinion -- but what else am I missing? Either way, no harm in discussing alternate approaches as well even if we are settling for the watchdog.

Quoted 10 lines

Quoted 6 lines

Lastly, if the goal is to remove RT throttling code eventually, are you also planning to remove RT group scheduling as well? Are there users of RT group scheduling that might be impacted? On the other hand, RT throttling / group scheduling code can be left as it is (perhaps documenting it as deprecated) and the server stuff can be implemented via a CONFIG option.

I think that the idea is to have the DL servers eventually replace the group schedule. But I also believe that it is better to start by solving the throttling and then moving to other constructions on top of the mechanism.

Hmm. For throttling at the root level yes, but I am not seeing how you can replace the group scheduling code for existing users of RT Cgroups with this. The throttling in the RT group scheduling code is not exactly only about "not starving CFS", it is more related to letting RT groups run with certain bandwidth. So you cannot really delete it if there are real users of that code -- you'll have to migrate those users away first (to an alternate implementation like DL). If there are no users of RT group scheduling, that's lovely though. We don't use it in ChromeOS fwiw.

- Joel

Quoted 38 lines

On Mon, Jun 12, 2023 at 1:21 PM Daniel Bristot de Oliveira <[email protected]> wrote:
[...]

Quoted 34 lines

Quoted 17 lines

Quoted 10 lines · Daniel Bristot de Oliveira

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

Quoted 4 lines

To prevent this issue, implement a starvation monitor mechanism that starts the deadline server only if a (fair in this case) task hasn't been scheduled for some interval of time after it has been enqueued. Use pick/put functions to manage starvation monitor status.

Quoted 4 lines

Me and Vineeth were discussing that another way of resolving this issue is to use a DL-server for RT as well, and then using a smaller deadline for RT. That way the RT is more likely to be selected due to its earlier deadline/period.

It would not be that different from what we have now.

One of the problems of throttling nowadays is that it accounts for a large window of time, and any "imprecision" can cause the mechanism not to work as expected.

For example, we work on a fully-isolated CPU scenario, where some very sporadic workload can be placed on the isolated CPU because of per-cpu kernel activities, e.g., kworkers... We need to let them run, but for a minimal amount of time, for instance, 20 us, to bound the interference.

The current mechanism does not give this precision because the IRQ accounting does not account for runtime for the rt throttling (which makes sense).

Quoted 2 lines

I lost you here, "Runtime for the rt throttling" does not make much sense to me as a statement.

Both update_curr_rt() and update_curr_dl() use rq_clock_task() as clock. update_rq_clock_task() reduces the irq_delta from task's clock (inside #ifdef CONFIG_IRQ_TIME_ACCOUNTING), and this clock is used to throttling.

Quoted 6 lines

Quoted 3 lines

So the
RT throttling has the 20 us stolen from IRQs and keeps running. The same will happen if we swap the current mechanism with a DL server for the RT.

I read this about 10 times to learn that *maybe* you mean that IRQs stole time from the "Well behaved running time" of the RT class.

I also read emails many times :-)

Quoted 2 lines

not seeing how that is related to creation of a DL-server for the RT class. Maybe describe your point a bit more clearly?

This patch is targeting a better way to avoid SCHED_OTHER starvation. Having a DL server for RT class does not help on that. We need to boost SCHED_OTHER.

Quoted 9 lines

Quoted 8 lines

Also, thinking about short deadlines to fake a fixed priority is... not starting well. A fixed-priority higher instance is not a property of a deadline-based scheduler, and Linux has a fixed-priority hierarchy: STOP -> DL -> RT -> CFS... It is simple, and that is good.

That is why it is better to boost CFS instead of throttling RT. By boosting CFS, you do not need a server for RT, and we account for anything on top of CFS for free (IRQ/DL/FIFO...).

Quoted 3 lines

I did mention in my last email that it is not ideal. I just brought it up as an option. It might reduce the problem being seen and is better than not having it.

We have thought about it, but boosting SCHED_OTHER is the way to go.

Quoted 11 lines

Quoted 10 lines

Quoted 4 lines

Another approach could be to implement the 0-laxity scheduling as a general SCHED_DEADLINE feature, perhaps through a flag. And allow DL tasks to opt-in to 0-laxity scheduling unless there are idle cycles. And then opt-in the feature for the CFS deadline server task.

A 0-laxity scheduler is not as simple as it sounds, as the priority also depends on the "C" (runtime, generally WCET), which is hard to find and embeds pessimism. Also, having such a feature would make other mechanisms harder, as well as debugging things. For example, proxy-execution or a more precise schedulability test...

Quoted 12 lines

I think you did not read my email properly, I was saying make the 0-laxity default-off and the opt-in for certain DL tasks. That may work perfectly well for a system like ChromeOS where likely we will use the DL server as the sole deadline task and opt-in for the 0-laxity. Then we don't need watchdog hacks at all and it all cleanly works within the DL class itself. There are the drawbacks of the pessimism/locking etc (I already knew that by the way as the obvious drawbacks of 0-laxity) but I am not immediately seeing how this CFS-watchdog with 0-laxity is any different from the DL-server itself having such a property. If you really have a concrete point on why that won't work, and if you could clarify that more clearly why a watchdog is better than it, that would be great.

I think you are overloading a term and a goal, and this makes your thoughts ambiguous.

0-laxity is a point in time. What do you want to do at 0-laxity? Do you want to run or start/replenish?

In the previous discussions, we mentioned using a scheduler that uses it as a way to prioritize the task (to run). That is an overkill, as it would be another scheduler. That is the first interpretation for 0-laxity in this thread, mainly associated with the word "scheduling" (not only I read that way).

In this patch, Juri's PoC shows that if we defer the DL server start (replenish) for a point in the future, we can keep the fixed-priority order of the schedulers, boosting SCHED_OTHER if it starves, without breaking EDF.

If you see the cover, I mentioned using the 0-laxity point in time to activate the DL server under EDF. In that way, at the 0-laxity point, the DL server is replenished with runtime and deadline as "now" + period. With that implemented...

In the base case:
it is never activated.

In the Busy-loop FIFO case:
Busy-loop FIFO task run starving OTHER for (period - runtime):
SCHED_OTHER server will be started at 0-laxity and get the processor for its runtime immediately because there are no DL tasks.

In the presence of DL & RT tasks:
DL and RT Starving OTHER for (period - runtime):
SCHED_OTHER server will be started & scheduled under EDF, before or after the other DL tasks, following EDF. Anyways, before returning to the SCHED_RT.

So, in this way, the OTHER will be boosted over SCHED_RT without breaking SCHED_DEADLINE tasks.

In an 0-laxity scheduler, the server would run at 0-laxity, jumping in front of DL tasks... that would break EDF. It would be mixing two schedulers in one. It is not required and likely not a good idea either.

In the cover, I mentioned improving this patch, so maybe watchdog is not the appropriate name. 0-laxity server is not a good name either because it might induce people to think that the server will RUN at 0-laxity while it will actually be replenished at 0-laxity. Maybe a deferred server might be a better name.

Quoted 10 lines

Quoted 7 lines

In a paper, the scheduler alone is the solution. In real life, the solution for problems like locking is as fundamental as the scheduler. We need to keep things simple to expand on these other topics as well.

So, I do not think we need all the drawbacks of a mixed solution to just fix the throttling problem, and EDF is more capable and explored for the general case.

Again, I was saying making it opt-in seems like a reasonable approach and just enabling such property for the DL server.

Can we have a "deferred DL server?" is that your question?

If so, I think so. But we have other points to look first. DL servers are per-cpu, so they break global. We need to think about an interface... and there are other points that we need to understand before trying some other more optimized cases.

Quoted 11 lines

Quoted 10 lines

Quoted 6 lines

Lastly, if the goal is to remove RT throttling code eventually, are you also planning to remove RT group scheduling as well? Are there users of RT group scheduling that might be impacted? On the other hand, RT throttling / group scheduling code can be left as it is (perhaps documenting it as deprecated) and the server stuff can be implemented via a CONFIG option.

I think that the idea is to have the DL servers eventually replace the group schedule. But I also believe that it is better to start by solving the throttling and then moving to other constructions on top of the mechanism.

Quoted 9 lines

Hmm. For throttling at the root level yes, but I am not seeing how you can replace the group scheduling code for existing users of RT Cgroups with this. The throttling in the RT group scheduling code is not exactly only about "not starving CFS", it is more related to letting RT groups run with certain bandwidth. So you cannot really delete it if there are real users of that code -- you'll have to migrate those users away first (to an alternate implementation like DL). If there are no users of RT group scheduling, that's lovely though. We don't use it in ChromeOS fwiw.

The idea behind the base patchset from Peter is solid and is the best way we can start, and starting with avoiding OTHER starvation is an easy starting point. Many people will benefit from it - like all the people that ping me because of the RT_RUNTIME_GREED (including Google in the past)... which is the starting point of all this work.

Generalizing it requires time, but it will happen... and you know that Juri and I care about Chromeos' use case, as I have been discussing this with you all and even participating in Google/chrome focused meetings about sched... at 6 pm our time ;-).

On Tue, Jun 13, 2023 at 9:43 AM Daniel Bristot de Oliveira <[email protected]> wrote:
[...]

Quoted 38 lines

Quoted 38 lines

On Mon, Jun 12, 2023 at 1:21 PM Daniel Bristot de Oliveira <[email protected]> wrote:
[...]

Quoted 34 lines

Quoted 17 lines

Quoted 10 lines · Daniel Bristot de Oliveira

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

Quoted 4 lines

To prevent this issue, implement a starvation monitor mechanism that starts the deadline server only if a (fair in this case) task hasn't been scheduled for some interval of time after it has been enqueued. Use pick/put functions to manage starvation monitor status.

Quoted 4 lines

Me and Vineeth were discussing that another way of resolving this issue is to use a DL-server for RT as well, and then using a smaller deadline for RT. That way the RT is more likely to be selected due to its earlier deadline/period.

It would not be that different from what we have now.

One of the problems of throttling nowadays is that it accounts for a large window of time, and any "imprecision" can cause the mechanism not to work as expected.

For example, we work on a fully-isolated CPU scenario, where some very sporadic workload can be placed on the isolated CPU because of per-cpu kernel activities, e.g., kworkers... We need to let them run, but for a minimal amount of time, for instance, 20 us, to bound the interference.

The current mechanism does not give this precision because the IRQ accounting does not account for runtime for the rt throttling (which makes sense).

Quoted 6 lines

Quoted 2 lines

I lost you here, "Runtime for the rt throttling" does not make much sense to me as a statement.

Both update_curr_rt() and update_curr_dl() use rq_clock_task() as clock. update_rq_clock_task() reduces the irq_delta from task's clock (inside #ifdef CONFIG_IRQ_TIME_ACCOUNTING), and this clock is used to throttling.

That was a much better description. You're basically saying that since the running time of the RT class is not accounted for in the clock, it affects the throttling and unthrottling times. I actually ran into a similar issue on Android I recall, where the RT time was showing up as CFS load if I recall.

For RT throttling though, in our testing the time scales are large enough (for our usecase) that such time stealing wasn't an issue. I am going for something that is practical and that works, does not have to be perfect since it has been several years now with these problems and leaving RT throttling broken is probably not a good thing.

Quoted 6 lines

Quoted 2 lines

not seeing how that is related to creation of a DL-server for the RT class. Maybe describe your point a bit more clearly?

This patch is targeting a better way to avoid SCHED_OTHER starvation. Having a DL server for RT class does not help on that. We need to boost SCHED_OTHER.

Oh, actually the problem of boosting SCHED_OTHER is a bit orthogonal to what I said. I was not saying not to boost SCHED_OTHER, I was talking more about this particular patch and using an DL-based RT server to mitigate that issue. The boosting is already handled in previous patches with the DL-server.

Quoted 25 lines

Quoted 23 lines

Quoted 10 lines

Quoted 4 lines

Another approach could be to implement the 0-laxity scheduling as a general SCHED_DEADLINE feature, perhaps through a flag. And allow DL tasks to opt-in to 0-laxity scheduling unless there are idle cycles. And then opt-in the feature for the CFS deadline server task.

A 0-laxity scheduler is not as simple as it sounds, as the priority also depends on the "C" (runtime, generally WCET), which is hard to find and embeds pessimism. Also, having such a feature would make other mechanisms harder, as well as debugging things. For example, proxy-execution or a more precise schedulability test...

I think you did not read my email properly, I was saying make the 0-laxity default-off and the opt-in for certain DL tasks. That may work perfectly well for a system like ChromeOS where likely we will use the DL server as the sole deadline task and opt-in for the 0-laxity. Then we don't need watchdog hacks at all and it all cleanly works within the DL class itself. There are the drawbacks of the pessimism/locking etc (I already knew that by the way as the obvious drawbacks of 0-laxity) but I am not immediately seeing how this CFS-watchdog with 0-laxity is any different from the DL-server itself having such a property. If you really have a concrete point on why that won't work, and if you could clarify that more clearly why a watchdog is better than it, that would be great.

Quoted 2 lines

I think you are overloading a term and a goal, and this makes your thoughts ambiguous.

Well the term was mentioned in this series cover letter as well. ;-)

Quoted 3 lines

0-laxity is a point in time. What do you want to do at 0-laxity? Do you want to run or start/replenish?

You could do either.

could you just push it out to run in the next period if it has the flag.

Here is the definition of 0-laxity as I understand it. Please correct me as I have not done a phD in these things like you ;-)

Laxity, also known as slack time,

can be delayed without causing a missed deadline.

one that has no more time to spare and must be executed immediately

avoid missing its deadline.

And here's where I need your input: If we take all admitted DL tasks and run it at their respective 0-laxity times or slightly earlier, then in-theory, they should all meet their deadlines correctly?

Now, I don't really mean to run it exactly at 0-laxity. It could be run a bit earlier to factor in sleep times, locking time, preemptions etc.

anyway -- so even if you run the DL task as soon as possible, you might still miss your deadline.

to the next period and consider it "activated". I am just thinking out loud.

That could break EDF the way it is now. However, it could be an interesting idea that could be developed into a better idea. A DL task does not have to run immediately to meet its deadline (it could be run later as well)and that I know for a fact -- so why not add this flexibility within SCHED_DEADLINE itself rather than inventing a hack (and by hack I mean only this patch, not the other patches from Peter or the idea of CFS boosting).

My impression is the other (DL tasks without flag) should still have their promised bandwidth so it is not mixing 2 schedulers.If this series gets stalled for some reason, I would probably explore such an idea in my own time later.

Quoted 34 lines

In the previous discussions, we mentioned using a scheduler that uses it as a way to prioritize the task (to run). That is an overkill, as it would be another scheduler. That is the first interpretation for 0-laxity in this thread, mainly associated with the word "scheduling" (not only I read that way).

In this patch, Juri's PoC shows that if we defer the DL server start (replenish) for a point in the future, we can keep the fixed-priority order of the schedulers, boosting SCHED_OTHER if it starves, without breaking EDF.

If you see the cover, I mentioned using the 0-laxity point in time to activate the DL server under EDF. In that way, at the 0-laxity point, the DL server is replenished with runtime and deadline as "now" + period. With that implemented...

In the base case:
it is never activated.

In the Busy-loop FIFO case:
Busy-loop FIFO task run starving OTHER for (period - runtime):
SCHED_OTHER server will be started at 0-laxity and get the processor for its runtime immediately because there are no DL tasks.

In the presence of DL & RT tasks:
DL and RT Starving OTHER for (period - runtime):
SCHED_OTHER server will be started & scheduled under EDF, before or after the other DL tasks, following EDF. Anyways, before returning to the SCHED_RT.

So, in this way, the OTHER will be boosted over SCHED_RT without breaking SCHED_DEADLINE tasks.

Quoted 3 lines

In an 0-laxity scheduler, the server would run at 0-laxity, jumping in front of DL tasks... that would break EDF. It would be mixing two schedulers in one. It is not required and likely not a good idea either.

I am still missing why some tasks cannot be run at close to 0-laxity time, and as opt-in.And if the DL-server is the sole task running, then there's nothing else to break.

In fact, I am not immediately seeing how this can break SCHED_DEADLINE if you allow at most 1-task to run at close to 0-laxity. The others should still have their promised bandwidth so it is not mixing 2 schedulers, you just delay the DL-server till it's close to the 0-laxity. What am I missing?

Perhaps you mean the algorithm needs to push the new period/deadline to a later time at the 0-laxity.

variation of the idea is possible AFAICS (again could be missing something that mathematically makes this impossible).

Quoted 6 lines

In the cover, I mentioned improving this patch, so maybe watchdog is not the appropriate name. 0-laxity server is not a good name either because it might induce people to think that the server will RUN at 0-laxity while it will actually be replenished at 0-laxity. Maybe a deferred server might be a better name.

Yeah maybe a deferred server could be a better name.

Quoted 11 lines

Quoted 10 lines

Quoted 7 lines

In a paper, the scheduler alone is the solution. In real life, the solution for problems like locking is as fundamental as the scheduler. We need to keep things simple to expand on these other topics as well.

So, I do not think we need all the drawbacks of a mixed solution to just fix the throttling problem, and EDF is more capable and explored for the general case.

Again, I was saying making it opt-in seems like a reasonable approach and just enabling such property for the DL server.

Quoted 6 lines

Can we have a "deferred DL server?" is that your question?

If so, I think so. But we have other points to look first. DL servers are per-cpu, so they break global. We need to think about an interface... and there are other points that we need to understand before trying some other more optimized cases.

You mean an interface for the DL server params? Those can be similar to other sched knobs. And then boot with a CONFIG option and boost CFS things by default if RT is active. Would that work or did you mean something else by interface?

Quoted 21 lines

Quoted 11 lines

Quoted 10 lines

Quoted 6 lines

Lastly, if the goal is to remove RT throttling code eventually, are you also planning to remove RT group scheduling as well? Are there users of RT group scheduling that might be impacted? On the other hand, RT throttling / group scheduling code can be left as it is (perhaps documenting it as deprecated) and the server stuff can be implemented via a CONFIG option.

I think that the idea is to have the DL servers eventually replace the group schedule. But I also believe that it is better to start by solving the throttling and then moving to other constructions on top of the mechanism.

Quoted 9 lines

Hmm. For throttling at the root level yes, but I am not seeing how you can replace the group scheduling code for existing users of RT Cgroups with this. The throttling in the RT group scheduling code is not exactly only about "not starving CFS", it is more related to letting RT groups run with certain bandwidth. So you cannot really delete it if there are real users of that code -- you'll have to migrate those users away first (to an alternate implementation like DL). If there are no users of RT group scheduling, that's lovely though. We don't use it in ChromeOS fwiw.

Quoted 5 lines

The idea behind the base patchset from Peter is solid and is the best way we can start, and starting with avoiding OTHER starvation is an easy starting point. Many people will benefit from it - like all the people that ping me because of the RT_RUNTIME_GREED (including Google in the past)... which is the starting point of all this work.

Right, you know I am on the same page about that. I presented exactly the same stuff at 2 conferences in 2 countries this year.

Quoted 1 line

Quoted 4 lines

Generalizing it requires time, but it will happen... and you know that Juri and I care about Chromeos' use case, as I have been discussing this with you all and even participating in Google/chrome focused meetings about sched... at 6 pm our time ;-).

I totally appreciate that, please don't get offended, we go a long way back as friends ;-)

prove I am an expert compared to you. I just want to get it *done* and not have to wait for more years. You can see my 2 presentations this year on this topic alone -- I travelled to 2 countries leaving my family behind to discuss these.

Many thanks,

- Joel

By postponing the enqueue/replanishment of the DL server here, we are fixing the problem in a practical way, that works without breaking existing useful properties & use-cases.

Quoted 54 lines · Joel Fernandes

On Tue, Jun 13, 2023 at 9:43 AM Daniel Bristot de Oliveira <[email protected]> wrote:
[...]

Quoted 38 lines

Quoted 38 lines

On Mon, Jun 12, 2023 at 1:21 PM Daniel Bristot de Oliveira <[email protected]> wrote:
[...]

Quoted 34 lines

Quoted 17 lines

Quoted 10 lines · Daniel Bristot de Oliveira

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

Quoted 4 lines

To prevent this issue, implement a starvation monitor mechanism that starts the deadline server only if a (fair in this case) task hasn't been scheduled for some interval of time after it has been enqueued. Use pick/put functions to manage starvation monitor status.

Quoted 4 lines

Me and Vineeth were discussing that another way of resolving this issue is to use a DL-server for RT as well, and then using a smaller deadline for RT. That way the RT is more likely to be selected due to its earlier deadline/period.

It would not be that different from what we have now.

One of the problems of throttling nowadays is that it accounts for a large window of time, and any "imprecision" can cause the mechanism not to work as expected.

For example, we work on a fully-isolated CPU scenario, where some very sporadic workload can be placed on the isolated CPU because of per-cpu kernel activities, e.g., kworkers... We need to let them run, but for a minimal amount of time, for instance, 20 us, to bound the interference.

The current mechanism does not give this precision because the IRQ accounting does not account for runtime for the rt throttling (which makes sense).

Quoted 6 lines

Quoted 2 lines

I lost you here, "Runtime for the rt throttling" does not make much sense to me as a statement.

Both update_curr_rt() and update_curr_dl() use rq_clock_task() as clock. update_rq_clock_task() reduces the irq_delta from task's clock (inside #ifdef CONFIG_IRQ_TIME_ACCOUNTING), and this clock is used to throttling.

That was a much better description. You're basically saying that since the running time of the RT class is not accounted for in the clock, it affects the throttling and unthrottling times. I actually ran into a similar issue on Android I recall, where the RT time was showing up as CFS load if I recall.

Quoted 5 lines

For RT throttling though, in our testing the time scales are large enough (for our usecase) that such time stealing wasn't an issue. I am going for something that is practical and that works, does not have to be perfect since it has been several years now with these problems and leaving RT throttling broken is probably not a good thing.

By postponing the enqueue/replanishment of the DL server here, we are fixing the problem in a practical way, that works without breaking existing useful properties & use-cases.

[...]

Quoted 7 lines

Quoted 6 lines

Quoted 2 lines

not seeing how that is related to creation of a DL-server for the RT class. Maybe describe your point a bit more clearly?

This patch is targeting a better way to avoid SCHED_OTHER starvation. Having a DL server for RT class does not help on that. We need to boost SCHED_OTHER.

Quoted 5 lines

Oh, actually the problem of boosting SCHED_OTHER is a bit orthogonal to what I said. I was not saying not to boost SCHED_OTHER, I was talking more about this particular patch and using an DL-based RT server to mitigate that issue. The boosting is already handled in previous patches with the DL-server.

The boosting of the previous patch is breaking FIFO priority. This patch fixes that single point without touching and or breaking SCHED_DEADLINE/EDF properties. With these things in place we do not mitigate, we fix the problem.

Quoted 2 lines

could you just push it out to run in the next period if it has the flag.

The improvements on top of this patch is to postpone the enqueue/replenish to the 0-laxity time. By doing so, the task receives a new period (and so deadline) a period ahead.

Quoted 5 lines

Here is the definition of 0-laxity as I understand it. Please correct me as I have not done a phD in these things like you ;-)

Laxity, also known as slack time,

laxity = absolute deadline - activation time - runtime.

is the amount of time that a task

Quoted 1 line

can be delayed without causing a missed deadline.

If you look at the task alone! e.g, if it does not face preemption! A 0-laxity task is

Quoted 1 line

one that has no more time to spare and must be executed immediately

and not be preempted!

to

Quoted 1 line

avoid missing its deadline.

Again, you are looking at the task alone.

Quoted 3 lines

And here's where I need your input: If we take all admitted DL tasks and run it at their respective 0-laxity times or slightly earlier, then in-theory, they should all meet their deadlines correctly?

For all tasksets, no!

There might be a taskset here o there that creates such timeline under EDF, but it is not always true that a task under EDF will wait until the 0-laxity time for them to run. EDF is working conserving.

For a working conserving scheduler to build such a timeline, it needs to have no idle time. Then, lets get the classical single core assumptions (these servers are per-cpu).

- Assuming single-core/partitioned scheduler.
- Assuming periodic tasks with deadline = period
- Assuming a task set with the sum of each task utilization = 1
- Assuming all tasks are dispatched at the same time (critical instant)
- Assuming all tasks will run for their entire runtime, without blocking

(so... the thing that EDF is optimum... fully loaded...)

Even so you will not have EDF always creating such timeline because the task with the earliest deadline will run first, still deadlines will be met.

For example:

t1 = 5/10
t2 = 5/10

Each task you pick first will run 5 unities of time before the "0-laxity time".

If there is a scheduler that always build a timeline like you want, it will not schedule the taskset I mentioned... thus.. it will schedule less than EDF.

Quoted 2 lines

Perhaps you mean the algorithm needs to push the new period/deadline to a later time at the 0-laxity.

This is the idea behind this patch ^^^^ This is the different between running and replenishing I mention on previous emails.

That's also fine with me. But some

Quoted 2 lines

variation of the idea is possible AFAICS (again could be missing something that mathematically makes this impossible).

you are looking for a fragment of the information... "0-laxity time," with a single task in mind - not in the context of a scheduler.

Quoted 26 lines

Quoted 5 lines

In the cover, I mentioned improving this patch, so maybe watchdog is not the appropriate name. 0-laxity server is not a good name either because it might induce people to think that the server will RUN at 0-laxity while it will actually be replenished at 0-laxity. Maybe a deferred server might be a better name.

Yeah maybe a deferred server could be a better name.

Quoted 17 lines

Quoted 10 lines

Quoted 7 lines

In a paper, the scheduler alone is the solution. In real life, the solution for problems like locking is as fundamental as the scheduler. We need to keep things simple to expand on these other topics as well.

So, I do not think we need all the drawbacks of a mixed solution to just fix the throttling problem, and EDF is more capable and explored for the general case.

Again, I was saying making it opt-in seems like a reasonable approach and just enabling such property for the DL server.

Can we have a "deferred DL server?" is that your question?

If so, I think so. But we have other points to look first. DL servers are per-cpu, so they break global. We need to think about an interface... and there are other points that we need to understand before trying some other more optimized cases.

Quoted 4 lines

You mean an interface for the DL server params? Those can be similar to other sched knobs. And then boot with a CONFIG option and boost CFS things by default if RT is active. Would that work or did you mean something else by interface?

Quoted 7 lines

Quoted 4 lines

Generalizing it requires time, but it will happen... and you know that Juri and I care about Chromeos' use case, as I have been discussing this with you all and even participating in Google/chrome focused meetings about sched... at 6 pm our time ;-).

I totally appreciate that, please don't get offended, we go a long way back as friends ;-)

I did not get offended, and nothing changes on our friendship :-). I am just clarifying you things we know - even before this rebase... We are aware of Chrome needs, as well as general RT Linux needs.

The basic idea behind this patch works for all cases and is unlocking this situation. The code will be improved in the next version.

Hey,

Quoted 3 lines

By postponing the enqueue/replanishment of the DL server here, we are fixing the problem in a practical way, that works without breaking existing useful properties & use-cases.

In my understanding, if we simply postpone actual activation of the DL server up to the point it really needs to boost/run for giving CFS tasks some breath (the infamous 0-laxity :), we save RT tasks from useless interruptions and still can keep EDF/CBS working w/o much changes.

It looks like a low hanging fruit, small improvement on what we have today than doesn't prevent us for implementing more complex features (i.e., full blown hierarchical scheduling, alternative schedulers) in the future if the need arises.

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

To prevent this issue, implement a starvation monitor mechanism that

Quoted 6 lines · Joel Fernandes

Lastly, if the goal is to remove RT throttling code eventually, are you also planning to remove RT group scheduling as well? Are there users of RT group scheduling that might be impacted? On the other hand, RT throttling / group scheduling code can be left as it is (perhaps documenting it as deprecated) and the server stuff can be implemented via a CONFIG option.

I think that the idea is to have the DL servers eventually replace the group schedule. But I also believe that it is better to start by solving the throttling and then moving to other constructions on top of the mechanism.

In an 0-laxity scheduler, the server would run at 0-laxity, jumping in front of DL tasks... that would break EDF. It would be mixing two schedulers in one. It is not required and likely not a good idea either.

Quoted 3 lines · Daniel Bristot de Oliveira

In an 0-laxity scheduler, the server would run at 0-laxity, jumping in front of DL tasks... that would break EDF. It would be mixing two schedulers in one. It is not required and likely not a good idea either.

I did consider a hybrid 0-laxity and EDF scheduler for mixed criticality, as have others like Ted Baker IIRC. IIRC it can be done using an augmented tree, but none of that solves the problems 0-laxity has (like over preemption and the general problem of playing chicken by doing things at the *VERY* last possible moment).

I think I did a talk at OSPERT on this at some point many years ago. Luckily some bright fellow had this semi-partitioned stuff that would make live much simpler :-)

kernel/sched/fair.c static struct task_struct *__pick_next_task_fair(struct rq *rq) · line 8251

Quoted 12 lines · Daniel Bristot de Oliveira

kernel/sched/fair.c +12 -0

+void fair_server_init(struct rq *rq)
+{
+	struct sched_dl_entity *dl_se = &rq->fair_server;
+
+	init_dl_entity(dl_se);
+
+	dl_se->dl_runtime = TICK_NSEC;
+	dl_se->dl_deadline = 20 * TICK_NSEC;
+	dl_se->dl_period = 20 * TICK_NSEC;
+
+	dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick);
+}

These here numbers should obviously become a tunable somewhere... :-)

Quoted 15 lines · Peter Zijlstra

kernel/sched/fair.c static struct task_struct *__pick_next_task_fair(struct rq *rq) · line 8251

Quoted 12 lines · Daniel Bristot de Oliveira

kernel/sched/fair.c +12 -0

+void fair_server_init(struct rq *rq)
+{
+	struct sched_dl_entity *dl_se = &rq->fair_server;
+
+	init_dl_entity(dl_se);
+
+	dl_se->dl_runtime = TICK_NSEC;
+	dl_se->dl_deadline = 20 * TICK_NSEC;
+	dl_se->dl_period = 20 * TICK_NSEC;
+
+	dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick);
+}

These here numbers should obviously become a tunable somewhere... :-)

From sched_rt_runtime and sched_rt_period, no?

Quoted 17 lines · Daniel Bristot de Oliveira

Quoted 15 lines · Peter Zijlstra

kernel/sched/fair.c static struct task_struct *__pick_next_task_fair(struct rq *rq) · line 8251

Quoted 12 lines · Daniel Bristot de Oliveira

kernel/sched/fair.c +12 -0

+void fair_server_init(struct rq *rq)
+{
+	struct sched_dl_entity *dl_se = &rq->fair_server;
+
+	init_dl_entity(dl_se);
+
+	dl_se->dl_runtime = TICK_NSEC;
+	dl_se->dl_deadline = 20 * TICK_NSEC;
+	dl_se->dl_period = 20 * TICK_NSEC;
+
+	dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick);
+}

These here numbers should obviously become a tunable somewhere... :-)

Quoted 1 line

From sched_rt_runtime and sched_rt_period, no?

Well, probably new names. IIRC those are also limits on the whole rt-cgroup mess, so e can't trivially change it without also ripping all that out, and that's a little bit more work.

Ripping out the basic throttle is much simpler than replacing all that and should be done earlier.

Quoted 9 lines · Daniel Bristot de Oliveira

From: Juri Lelli <[email protected]>

Starting deadline server for lower priority classes right away when first task is enqueued might break guarantees, as tasks belonging to intermediate priority classes could be uselessly preempted. E.g., a well behaving (non hog) FIFO task can be preempted by NORMAL tasks even if there are still CPU cycles available for NORMAL tasks to run, as they'll be running inside the fair deadline server for some period of time.

Quoted 1 line

To prevent this issue, implement a starvation monitor mechanism that

Why should this be prevented? FIFO can be preempted by a higher priority FIFO/RR, or in this case by DEADLINE, and always by stop_task.

Just accept this.

Anything that's build around FIFO-99 not getting preempted is just plain broken. Don't try and pander to broken

From: Peter Zijlstra <[email protected]>

All classes use sched_entity::exec_start to track runtime and have copies of the exact same code around to compute runtime.

Collapse all that.

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Signed-off-by Daniel Bristot de Oliveira <[email protected]>

From: Peter Zijlstra <[email protected]>

Create a single function that initializes a sched_dl_entity.

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Signed-off-by Daniel Bristot de Oliveira <[email protected]>

From: Peter Zijlstra <[email protected]>

In preparation of introducing !task sched_dl_entity; move the bandwidth accounting into {en.de}queue_dl_entity().

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Signed-off-by Daniel Bristot de Oliveira <[email protected]>

Quoted 3 lines · Daniel Bristot de Oliveira

In an 0-laxity scheduler, the server would run at 0-laxity, jumping in front of DL tasks... that would break EDF. It would be mixing two schedulers in one. It is not required and likely not a good idea either.

I did consider a hybrid 0-laxity and EDF scheduler for mixed criticality, as have others like Ted Baker IIRC. IIRC it can be done using an augmented tree, but none of that solves the problems 0-laxity has (like over preemption and the general problem of playing chicken by doing things at the *VERY* last possible moment).

I think I did a talk at OSPERT on this at some point many years ago. Luckily some bright fellow had this semi-partitioned stuff that would make live much simpler :-)

Quoted 12 lines · Peter Zijlstra

Quoted 3 lines · Daniel Bristot de Oliveira

In an 0-laxity scheduler, the server would run at 0-laxity, jumping in front of DL tasks... that would break EDF. It would be mixing two schedulers in one. It is not required and likely not a good idea either.

I did consider a hybrid 0-laxity and EDF scheduler for mixed criticality, as have others like Ted Baker IIRC. IIRC it can be done using an augmented tree, but none of that solves the problems 0-laxity has (like over preemption and the general problem of playing chicken by doing things at the *VERY* last possible moment).

Quoted 3 lines

I think I did a talk at OSPERT on this at some point many years ago. Luckily some bright fellow had this semi-partitioned stuff that would make live much simpler :-)

I must clarify; I was thinking Least-Laxity-First, which is ofcourse not the same as a 0-laxity scheduler.

kernel/sched/deadline.c +0 -1

@@ -2033,9 +2147,20 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
 	struct task_struct *p;
 
 	p = pick_task_dl(rq);
-	if (p)

kernel/sched/core.c +0 -0

 	return p;
 }
 

Quoted 5 lines · Daniel Bristot de Oliveira

kernel/sched/deadline.c +0 -1

@@ -2033,9 +2147,20 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
 	struct task_struct *p;
 
 	p = pick_task_dl(rq);
-	if (p)

kernel/sched/deadline.c static struct task_struct *pick_next_task_dl(struct rq *rq) · line 2159

Quoted 10 lines

kernel/sched/deadline.c +8 -0

+	if (!p)
+		return p;
+
+	/*
+	 * XXX: re-check !dl_server, changed from v2 because of
+	 * pick_next_task_dl change
+	 */
+	if (!dl_server(&p->dl))
 		set_next_task_dl(rq, p, true);
 

Should this be

       if (!p->server)

instead? AFAICT dl_server(&p->dl) can never be true since there's no pi_se-like link to the server via the dl_se, only via the task_struct, and the server pick cannot return the server itself (as it's a pure sched_entity).

kernel/sched/deadline.c static struct task_struct *pick_next_task_dl(struct rq *rq) · line 2163

Quoted 5 lines

kernel/sched/deadline.c +4 -0

+	/* XXX not quite right */
+	if (hrtick_enabled(rq))
+		start_hrtick_dl(rq, &p->dl);
+

IIUC that got hauled out of set_next_task_dl() to cover the case where we pick the server (+ the server pick) and want to more thoroughly enforce the server's bandwidth. If so, what's the issue with starting the hrtick here?

Quoted 4 lines

kernel/sched/core.c +0 -0

 	return p;
 }
 

Back from EOSS...

Quoted 19 lines · Valentin Schneider

Quoted 5 lines · Daniel Bristot de Oliveira

kernel/sched/deadline.c +0 -1

@@ -2033,9 +2147,20 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
     struct task_struct *p;
 
     p = pick_task_dl(rq);
-    if (p)

kernel/sched/deadline.c static struct task_struct *pick_next_task_dl(struct rq *rq) · line 2159

Quoted 10 lines

kernel/sched/deadline.c +8 -0

+    if (!p)
+            return p;
+
+    /*
+     * XXX: re-check !dl_server, changed from v2 because of
+     * pick_next_task_dl change
+     */
+    if (!dl_server(&p->dl))
             set_next_task_dl(rq, p, true);
 

Should this be

Quoted 5 lines

       if (!p->server)

instead? AFAICT dl_server(&p->dl) can never be true since there's no pi_se-like link to the server via the dl_se, only via the task_struct, and the server pick cannot return the server itself (as it's a pure sched_entity).

makes sense... I will check that in the v4.

Quoted 10 lines

kernel/sched/deadline.c static struct task_struct *pick_next_task_dl(struct rq *rq) · line 2163

Quoted 6 lines

kernel/sched/deadline.c +4 -0

+    /* XXX not quite right */
+    if (hrtick_enabled(rq))
+            start_hrtick_dl(rq, &p->dl);
+

IIUC that got hauled out of set_next_task_dl() to cover the case where we pick the server (+ the server pick) and want to more thoroughly enforce the server's bandwidth. If so, what's the issue with starting the hrtick here?

I think that the commend was added more as a check if it is correct... it seems it is.

Thanks Vale!
-- Daniel

Quoted 5 lines

Quoted 4 lines

kernel/sched/core.c +0 -0

     return p;
 }
 

Quoted 30 lines · Daniel Bristot de Oliveira

Back from EOSS...

Quoted 24 lines · Valentin Schneider

Quoted 5 lines · Daniel Bristot de Oliveira

kernel/sched/deadline.c +0 -1

@@ -2033,9 +2147,20 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
     struct task_struct *p;
 
     p = pick_task_dl(rq);
-    if (p)

kernel/sched/deadline.c static struct task_struct *pick_next_task_dl(struct rq *rq) · line 2159

Quoted 10 lines

kernel/sched/deadline.c +8 -0

+    if (!p)
+            return p;
+
+    /*
+     * XXX: re-check !dl_server, changed from v2 because of
+     * pick_next_task_dl change
+     */
+    if (!dl_server(&p->dl))
             set_next_task_dl(rq, p, true);
 

Should this be

       if (!p->server)

instead? AFAICT dl_server(&p->dl) can never be true since there's no pi_se-like link to the server via the dl_se, only via the task_struct, and the server pick cannot return the server itself (as it's a pure sched_entity).

Quoted 1 line

makes sense... I will check that in the v4.

Makes sense to me too. Since p is either a real DL task or a CFS task, "if (dl_server(&p->dl))" is incorrect. "if (p->server)" is the right check.