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

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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>
This commit is contained in:
Dietmar Eggemann 2015-09-26 18:19:54 +01:00 • committed by Leo Yan
parent b03f1ba3d5
commit 05a773bfc9
3 changed files with 42 additions and 8 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

8
kernel/sched/core.c
View file

@ -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);

32
kernel/sched/fair.c
View file

@ -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;

10
kernel/sched/sched.h
View file

@ -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;