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sched/deadline: Improve admission control for asymmetric CPU capacities

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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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200520134243.19352-4-dietmar.eggemann@arm.com
This commit is contained in:
Luca Abeni 2020-05-20 15:42:41 +02:00 committed by Peter Zijlstra
parent fc9dc69847
commit 60ffd5edc5
2 changed files with 20 additions and 16 deletions
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30
kernel/sched/deadline.c
View File

@ -2590,11 +2590,12 @@ 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);
unsigned long cap;
if (attr->sched_flags & SCHED_FLAG_SUGOV)
return 0;
@ -2609,15 +2610,17 @@ 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);
cap = dl_bw_capacity(cpu);
if (dl_policy(policy) && !task_has_dl_policy(p) &&
!__dl_overflow(dl_b, cpus, 0, new_bw)) {
!__dl_overflow(dl_b, cap, 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, cap, p->dl.dl_bw, new_bw)) {
/*
* XXX this is slightly incorrect: when the task
* utilization decreases, we should delay the total
@ -2772,19 +2775,19 @@ bool dl_param_changed(struct task_struct *p, const struct sched_attr *attr)
#ifdef CONFIG_SMP
int dl_task_can_attach(struct task_struct *p, const struct cpumask *cs_cpus_allowed)
{
unsigned long flags, cap;
unsigned int dest_cpu;
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);
cap = dl_bw_capacity(dest_cpu);
overflow = __dl_overflow(dl_b, cap, 0, p->dl.dl_bw);
if (overflow) {
ret = -EBUSY;
} else {
@ -2794,6 +2797,8 @@ 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;
}
@ -2826,16 +2831,15 @@ int dl_cpuset_cpumask_can_shrink(const struct cpumask *cur,
bool dl_cpu_busy(unsigned int cpu)
{
unsigned long flags;
unsigned long flags, cap;
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);
cap = dl_bw_capacity(cpu);
overflow = __dl_overflow(dl_b, cap, 0, 0);
raw_spin_unlock_irqrestore(&dl_b->lock, flags);
rcu_read_unlock_sched();

6
kernel/sched/sched.h
View File

@ -310,11 +310,11 @@ void __dl_add(struct dl_bw *dl_b, u64 tsk_bw, int cpus)
__dl_update(dl_b, -((s32)tsk_bw / cpus));
}
static inline
bool __dl_overflow(struct dl_bw *dl_b, int cpus, u64 old_bw, u64 new_bw)
static inline bool __dl_overflow(struct dl_bw *dl_b, unsigned long cap,
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, cap) < dl_b->total_bw - old_bw + new_bw;
}
extern void init_dl_bw(struct dl_bw *dl_b);