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Subject[PATCH v3 4/5] sched/deadline: Make DL capacity-aware
Date
From: Luca Abeni <luca.abeni@santannapisa.it>

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>
---
kernel/sched/cpudeadline.c | 14 +++++++++++++-
kernel/sched/deadline.c | 18 ++++++++++++++----
kernel/sched/sched.h | 15 +++++++++++++++
3 files changed, 42 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 5cc4012572ec..8630f2a40a3f 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -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);

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index fa8566517715..f2e8f5a36707 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -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 &&
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 14cb6a97e2d2..6ebbb1f353c4 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -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);
--
2.17.1
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