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Commit bbb138bd authored by Dietmar Eggemann's avatar Dietmar Eggemann Committed by Dmitry Shmidt
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ANDROID: 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: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: default avatarAndres Oportus <andresoportus@google.com>
parent 4eb92977

kernel/sched/core.c

+2 −10
Original line number Diff line number Diff line
@@ -5885,6 +5885,8 @@ 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:
@@ -7131,20 +7133,10 @@ static int build_sched_domains(const struct cpumask *cpu_map, 
	for_each_cpu(i, cpu_map) {

		rq = cpu_rq(i);

		sd = *per_cpu_ptr(d.sd, i);



		/* Use READ_ONCE()/WRITE_ONCE() to avoid load/store tearing: */

		if (rq->cpu_capacity_orig > READ_ONCE(d.rd->max_cpu_capacity))

			WRITE_ONCE(d.rd->max_cpu_capacity, rq->cpu_capacity_orig);



		cpu_attach_domain(sd, d.rd, i);

	}

	rcu_read_unlock();



	if (rq && sched_debug_enabled) {

		pr_info("span: %*pbl (max cpu_capacity = %lu)\n",

			cpumask_pr_args(cpu_map), rq->rd->max_cpu_capacity);

	}



	ret = 0;

error:

	__free_domain_allocs(&d, alloc_state, cpu_map);

kernel/sched/fair.c

+31 −1
Original line number Diff line number Diff line
@@ -5534,7 +5534,7 @@ 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;
@@ -7246,13 +7246,43 @@ 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;

kernel/sched/sched.h

+10 −1
Original line number Diff line number Diff line
@@ -539,6 +539,12 @@ 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
@@ -576,7 +582,8 @@ struct root_domain { 
	cpumask_var_t rto_mask;

	struct cpupri cpupri;



	unsigned long max_cpu_capacity;

	/* Maximum cpu capacity in the system. */

	struct max_cpu_capacity max_cpu_capacity;

};



extern struct root_domain def_root_domain;
@@ -1414,6 +1421,8 @@ static inline void sched_update_tick_dependency(struct rq *rq) 
static inline void sched_update_tick_dependency(struct rq *rq) { }

#endif



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;