sched: Update max cpu capacity in case of max frequency constraints

Wakeup balancing uses cpu capacity awareness and needs to know the
system-wide maximum cpu capacity.

Patch "sched: Store system-wide maximum cpu capacity in root domain"
finds the system-wide maximum cpu capacity during scheduler domain
hierarchy setup. This is sufficient as long as maximum frequency
invariance is not enabled.

If it is enabled, the system-wide maximum cpu capacity can change
between scheduler domain hierarchy setups due to frequency capping.

The cpu capacity is changed in update_cpu_capacity() which is called in
load balance on the lowest scheduler domain hierarchy level. To be able
to know if a change in cpu capacity for a certain cpu also has an effect
on the system-wide maximum cpu capacity it is normally necessary to
iterate over all cpus. This would be way too costly. That's why this
patch follows a different approach.

The unsigned long max_cpu_capacity value in struct root_domain is
replaced with a struct max_cpu_capacity, containing value (the
max_cpu_capacity) and cpu (the cpu index of the cpu providing the
maximum cpu_capacity).

Changes to the system-wide maximum cpu capacity and the cpu index are
made if:

 1 System-wide maximum cpu capacity < cpu capacity
 2 System-wide maximum cpu capacity > cpu capacity and cpu index == cpu

There are no changes to the system-wide maximum cpu capacity in all
other cases.

Atomic read and write access to the pair (max_cpu_capacity.val,
max_cpu_capacity.cpu) is enforced by max_cpu_capacity.lock.

The access to max_cpu_capacity.val in task_fits_max() is still performed
without taking the max_cpu_capacity.lock.

The code to set max cpu capacity in build_sched_domains() has been
removed because the whole functionality is now provided by
update_cpu_capacity() instead.

This approach can introduce errors temporarily, e.g. in case the cpu
currently providing the max cpu capacity has its cpu capacity lowered
due to frequency capping and calls update_cpu_capacity() before any cpu
which might provide the max cpu now.

There is also an outstanding question:

Should the cpu capacity of a cpu going idle be set to a very small
value?

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bc5bb26ccf3bc7ba90a14267237bb7ba2ea29e0d..3680e76b6beb5740175adcd03b48e5699aae48aa 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5888,6 +5888,8 @@ static int init_rootdomain(struct root_domain *rd)
 
        if (cpupri_init(&rd->cpupri) != 0)
                goto free_rto_mask;
+
+       init_max_cpu_capacity(&rd->max_cpu_capacity);
        return 0;
 
 free_rto_mask:
@@ -7105,15 +7107,9 @@ static int build_sched_domains(const struct cpumask *cpu_map,
                rq = cpu_rq(i);
                sd = *per_cpu_ptr(d.sd, i);
                cpu_attach_domain(sd, d.rd, i);
-
-               if (rq->cpu_capacity_orig > rq->rd->max_cpu_capacity)
-                       rq->rd->max_cpu_capacity = rq->cpu_capacity_orig;
        }
        rcu_read_unlock();
 
-       if (rq)
-               pr_info("max cpu_capacity %lu\n", rq->rd->max_cpu_capacity);
-
        ret = 0;
 error:
        __free_domain_allocs(&d, alloc_state, cpu_map);

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7327221c991ecbc7d710faddcb8cc2cc1cbde287..91dfc8aa8aff86e6b881ad76c2741a541e998fd2 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5078,7 +5078,7 @@ static inline bool __task_fits(struct task_struct *p, int cpu, int util)
 static inline bool task_fits_max(struct task_struct *p, int cpu)
 {
        unsigned long capacity = capacity_of(cpu);
-       unsigned long max_capacity = cpu_rq(cpu)->rd->max_cpu_capacity;
+       unsigned long max_capacity = cpu_rq(cpu)->rd->max_cpu_capacity.val;
 
        if (capacity == max_capacity)
                return true;
@@ -6564,13 +6564,43 @@ static unsigned long scale_rt_capacity(int cpu)
        return 1;
 }
 
+void init_max_cpu_capacity(struct max_cpu_capacity *mcc)
+{
+       raw_spin_lock_init(&mcc->lock);
+       mcc->val = 0;
+       mcc->cpu = -1;
+}
+
 static void update_cpu_capacity(struct sched_domain *sd, int cpu)
 {
        unsigned long capacity = arch_scale_cpu_capacity(sd, cpu);
        struct sched_group *sdg = sd->groups;
+       struct max_cpu_capacity *mcc;
+       unsigned long max_capacity;
+       int max_cap_cpu;
+       unsigned long flags;
 
        cpu_rq(cpu)->cpu_capacity_orig = capacity;
 
+       mcc = &cpu_rq(cpu)->rd->max_cpu_capacity;
+
+       raw_spin_lock_irqsave(&mcc->lock, flags);
+       max_capacity = mcc->val;
+       max_cap_cpu = mcc->cpu;
+
+       if ((max_capacity > capacity && max_cap_cpu == cpu) ||
+           (max_capacity < capacity)) {
+               mcc->val = capacity;
+               mcc->cpu = cpu;
+#ifdef CONFIG_SCHED_DEBUG
+               raw_spin_unlock_irqrestore(&mcc->lock, flags);
+               pr_info("CPU%d: update max cpu_capacity %lu\n", cpu, capacity);
+               goto skip_unlock;
+#endif
+       }
+       raw_spin_unlock_irqrestore(&mcc->lock, flags);
+
+skip_unlock: __attribute__ ((unused));
        capacity *= scale_rt_capacity(cpu);
        capacity >>= SCHED_CAPACITY_SHIFT;
 

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 2301b0476b90918b82f4c6686d3f07711ff48ff3..aac581932eff2c2fe6d0ed89cd4cd5dbbdd7ef2a 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -510,6 +510,12 @@ struct dl_rq {
 
 #ifdef CONFIG_SMP
 
+struct max_cpu_capacity {
+       raw_spinlock_t lock;
+       unsigned long val;
+       int cpu;
+};
+
 /*
  * We add the notion of a root-domain which will be used to define per-domain
  * variables. Each exclusive cpuset essentially defines an island domain by
@@ -548,7 +554,7 @@ struct root_domain {
        struct cpupri cpupri;
 
        /* Maximum cpu capacity in the system. */
-       unsigned long max_cpu_capacity;
+       struct max_cpu_capacity max_cpu_capacity;
 };
 
 extern struct root_domain def_root_domain;
@@ -1340,6 +1346,8 @@ unsigned long to_ratio(u64 period, u64 runtime);
 
 extern void init_entity_runnable_average(struct sched_entity *se);
 
+extern void init_max_cpu_capacity(struct max_cpu_capacity *mcc);
+
 static inline void add_nr_running(struct rq *rq, unsigned count)
 {
        unsigned prev_nr = rq->nr_running;