[PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities

Dietmar Eggemann
2020-04-08T09:50:35Z

From: Luca Abeni <luca.abeni@santannapisa.it>

The current SCHED_DEADLINE (DL) admission control ensures that

sum of reserved CPU bandwidth < x * M

where

x = /proc/sys/kernel/sched_rt_{runtime,period}_us
M = # CPUs in root domain.

DL admission control works well for homogeneous systems where the
capacity of all CPUs are equal (1024). I.e. bounded tardiness for DL
and non-starvation of non-DL tasks is guaranteed.

But on heterogeneous systems where capacity of CPUs are different it
could fail by over-allocating CPU time on smaller capacity CPUs.

On an Arm big.LITTLE/DynamIQ system DL tasks can easily starve other
tasks making it unusable.

Fix this by explicitly considering the CPU capacity in the DL admission
test by replacing M with the root domain CPU capacity sum.

Signed-off-by: Luca Abeni <luca.abeni@santannapisa.it>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
kernel/sched/deadline.c | 23 +++++++++++------------
kernel/sched/sched.h | 7 +++++--
2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 504d2f51b0d6..53b34a95e29e 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2551,11 +2551,11 @@ void sched_dl_do_global(void)
int sched_dl_overflow(struct task_struct *p, int policy,
const struct sched_attr *attr)
{
- struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
u64 period = attr->sched_period ?: attr->sched_deadline;
u64 runtime = attr->sched_runtime;
u64 new_bw = dl_policy(policy) ? to_ratio(period, runtime) : 0;
- int cpus, err = -1;
+ int cpus, err = -1, cpu = task_cpu(p);
+ struct dl_bw *dl_b = dl_bw_of(cpu);

if (attr->sched_flags & SCHED_FLAG_SUGOV)
return 0;
@@ -2570,15 +2570,15 @@ int sched_dl_overflow(struct task_struct *p, int policy,
* allocated bandwidth of the container.
*/
raw_spin_lock(&dl_b->lock);
- cpus = dl_bw_cpus(task_cpu(p));
+ cpus = dl_bw_cpus(cpu);
if (dl_policy(policy) && !task_has_dl_policy(p) &&
- !__dl_overflow(dl_b, cpus, 0, new_bw)) {
+ !__dl_overflow(dl_b, cpu, 0, new_bw)) {
if (hrtimer_active(&p->dl.inactive_timer))
__dl_sub(dl_b, p->dl.dl_bw, cpus);
__dl_add(dl_b, new_bw, cpus);
err = 0;
} else if (dl_policy(policy) && task_has_dl_policy(p) &&
- !__dl_overflow(dl_b, cpus, p->dl.dl_bw, new_bw)) {
+ !__dl_overflow(dl_b, cpu, p->dl.dl_bw, new_bw)) {
/*
* XXX this is slightly incorrect: when the task
* utilization decreases, we should delay the total
@@ -2715,18 +2715,17 @@ bool dl_param_changed(struct task_struct *p, const struct sched_attr *attr)
int dl_task_can_attach(struct task_struct *p, const struct cpumask *cs_cpus_allowed)
{
unsigned int dest_cpu;
+ unsigned long flags;
struct dl_bw *dl_b;
bool overflow;
- int cpus, ret;
- unsigned long flags;
+ int ret;

dest_cpu = cpumask_any_and(cpu_active_mask, cs_cpus_allowed);

rcu_read_lock_sched();
dl_b = dl_bw_of(dest_cpu);
raw_spin_lock_irqsave(&dl_b->lock, flags);
- cpus = dl_bw_cpus(dest_cpu);
- overflow = __dl_overflow(dl_b, cpus, 0, p->dl.dl_bw);
+ overflow = __dl_overflow(dl_b, dest_cpu, 0, p->dl.dl_bw);
if (overflow) {
ret = -EBUSY;
} else {
@@ -2736,6 +2735,8 @@ int dl_task_can_attach(struct task_struct *p, const struct cpumask *cs_cpus_allo
* We will free resources in the source root_domain
* later on (see set_cpus_allowed_dl()).
*/
+ int cpus = dl_bw_cpus(dest_cpu);
+
__dl_add(dl_b, p->dl.dl_bw, cpus);
ret = 0;
}
@@ -2771,13 +2772,11 @@ bool dl_cpu_busy(unsigned int cpu)
unsigned long flags;
struct dl_bw *dl_b;
bool overflow;
- int cpus;

rcu_read_lock_sched();
dl_b = dl_bw_of(cpu);
raw_spin_lock_irqsave(&dl_b->lock, flags);
- cpus = dl_bw_cpus(cpu);
- overflow = __dl_overflow(dl_b, cpus, 0, 0);
+ overflow = __dl_overflow(dl_b, cpu, 0, 0);
raw_spin_unlock_irqrestore(&dl_b->lock, flags);
rcu_read_unlock_sched();

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 91bd0cb0c529..6cbdf7a342a6 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -304,11 +304,14 @@ void __dl_add(struct dl_bw *dl_b, u64 tsk_bw, int cpus)
__dl_update(dl_b, -((s32)tsk_bw / cpus));
}

+static inline unsigned long rd_capacity(int cpu);
+
static inline
-bool __dl_overflow(struct dl_bw *dl_b, int cpus, u64 old_bw, u64 new_bw)
+bool __dl_overflow(struct dl_bw *dl_b, int cpu, u64 old_bw, u64 new_bw)
{
return dl_b->bw != -1 &&
- dl_b->bw * cpus < dl_b->total_bw - old_bw + new_bw;
+ cap_scale(dl_b->bw, rd_capacity(cpu)) <
+ dl_b->total_bw - old_bw + new_bw;
}

extern void init_dl_bw(struct dl_bw *dl_b);
--
2.17.1