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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: Luca Abeni <luca.abeni@santannapisa.it>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200520134243.19352-5-dietmar.eggemann@arm.com
This commit is contained in:
Luca Abeni 2020-05-20 15:42:42 +02:00 committed by Peter Zijlstra
parent 60ffd5edc5
commit b4118988fd
3 changed files with 42 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
kernel/sched/cpudeadline.c
View File

@ -121,7 +121,19 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
if (later_mask &&
cpumask_and(later_mask, cp->free_cpus, p->cpus_ptr)) {
return 1;
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);

18
kernel/sched/deadline.c
View File

@ -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)) &&
(curr->nr_cpus_allowed < 2 ||
!dl_entity_preempt(&p->dl, &curr->dl)) &&
(p->nr_cpus_allowed > 1)) {
select_rq = unlikely(dl_task(curr)) &&
(curr->nr_cpus_allowed < 2 ||
!dl_entity_preempt(&p->dl, &curr->dl)) &&
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 &&

15
kernel/sched/sched.h
View File

@ -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);