Commit b4118988 authored by Luca Abeni's avatar Luca Abeni Committed by Peter Zijlstra
Browse files

sched/deadline: Make DL capacity-aware 



The current SCHED_DEADLINE (DL) scheduler uses a global EDF scheduling
algorithm w/o considering CPU capacity or task utilization.
This works well on homogeneous systems where DL tasks are guaranteed
to have a bounded tardiness but presents issues on heterogeneous
systems.

A DL task can migrate to a CPU which does not have enough CPU capacity
to correctly serve the task (e.g. a task w/ 70ms runtime and 100ms
period on a CPU w/ 512 capacity).

Add the DL fitness function dl_task_fits_capacity() for DL admission
control on heterogeneous systems. A task fits onto a CPU if:

    CPU original capacity / 1024 >= task runtime / task deadline

Use this function on heterogeneous systems to try to find a CPU which
meets this criterion during task wakeup, push and offline migration.

On homogeneous systems the original behavior of the DL admission
control should be retained.

Signed-off-by: default avatarLuca Abeni <luca.abeni@santannapisa.it>
Signed-off-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarJuri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200520134243.19352-5-dietmar.eggemann@arm.com
parent 60ffd5ed

kernel/sched/cpudeadline.c

+13 −1
Original line number Diff line number Diff line
@@ -121,6 +121,18 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p, 


	if (later_mask &&

	    cpumask_and(later_mask, cp->free_cpus, p->cpus_ptr)) {

		int cpu;



		if (!static_branch_unlikely(&sched_asym_cpucapacity))

			return 1;



		/* Ensure the capacity of the CPUs fits the task. */

		for_each_cpu(cpu, later_mask) {

			if (!dl_task_fits_capacity(p, cpu))

				cpumask_clear_cpu(cpu, later_mask);

		}



		if (!cpumask_empty(later_mask))

			return 1;

	} else {

		int best_cpu = cpudl_maximum(cp);

kernel/sched/deadline.c

+14 −4
Original line number Diff line number Diff line
@@ -1643,6 +1643,7 @@ static int 
select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)

{

	struct task_struct *curr;

	bool select_rq;

	struct rq *rq;



	if (sd_flag != SD_BALANCE_WAKE)
@@ -1662,10 +1663,19 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags) 
	 * other hand, if it has a shorter deadline, we

	 * try to make it stay here, it might be important.

	 */

	if (unlikely(dl_task(curr)) &&

	select_rq = unlikely(dl_task(curr)) &&

		    (curr->nr_cpus_allowed < 2 ||

		     !dl_entity_preempt(&p->dl, &curr->dl)) &&

	    (p->nr_cpus_allowed > 1)) {

		    p->nr_cpus_allowed > 1;



	/*

	 * Take the capacity of the CPU into account to

	 * ensure it fits the requirement of the task.

	 */

	if (static_branch_unlikely(&sched_asym_cpucapacity))

		select_rq |= !dl_task_fits_capacity(p, cpu);



	if (select_rq) {

		int target = find_later_rq(p);



		if (target != -1 &&

kernel/sched/sched.h

+15 −0
Original line number Diff line number Diff line
@@ -317,6 +317,21 @@ static inline bool __dl_overflow(struct dl_bw *dl_b, unsigned long cap, 
	       cap_scale(dl_b->bw, cap) < dl_b->total_bw - old_bw + new_bw;

}



/*

 * Verify the fitness of task @p to run on @cpu taking into account the

 * CPU original capacity and the runtime/deadline ratio of the task.

 *

 * The function will return true if the CPU original capacity of the

 * @cpu scaled by SCHED_CAPACITY_SCALE >= runtime/deadline ratio of the

 * task and false otherwise.

 */

static inline bool dl_task_fits_capacity(struct task_struct *p, int cpu)

{

	unsigned long cap = arch_scale_cpu_capacity(cpu);



	return cap_scale(p->dl.dl_deadline, cap) >= p->dl.dl_runtime;

}



extern void init_dl_bw(struct dl_bw *dl_b);

extern int  sched_dl_global_validate(void);

extern void sched_dl_do_global(void);

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