sched/deadline: Improve admission control for asymmetric CPU capacities

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

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 01f474a..9ebd0a9 100644 (file)
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -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();
 

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8d5d068..91b250f 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -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);