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Commit 9c83d843 authored by Dietmar Eggemann's avatar Dietmar Eggemann Committed by Quentin Perret
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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.

Signed-off-by: default avatarIonela Voinescu <ionela.voinescu@arm.com&gt;*>
Signed-off-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
( Fixed cherry-pick issues )
Signed-off-by: default avatarQuentin Perret <quentin.perret@arm.com>
Change-Id: Idaa7a16723001e222e476de34df332558e48dd13
parent 95247bf7

kernel/sched/fair.c

+30 −1
Original line number Diff line number Diff line
@@ -6787,7 +6787,7 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu) 
		return 0;



	min_cap = min(capacity_orig_of(prev_cpu), capacity_orig_of(cpu));

	max_cap = cpu_rq(cpu)->rd->max_cpu_capacity;

	max_cap = cpu_rq(cpu)->rd->max_cpu_capacity.val;



	/* Minimum capacity is close to max, no need to abort wake_affine */

	if (max_cap - min_cap < max_cap >> 3)
@@ -8379,16 +8379,45 @@ static unsigned long scale_rt_capacity(int cpu, unsigned long max) 
	return scale_irq_capacity(free, irq, max);

}



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;



	capacity *= arch_scale_max_freq_capacity(sd, cpu);

	capacity >>= SCHED_CAPACITY_SHIFT;



	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);



	if (!capacity)

kernel/sched/sched.h

+9 −1
Original line number Diff line number Diff line
@@ -711,6 +711,12 @@ struct perf_domain { 
	struct rcu_head rcu;

};



struct max_cpu_capacity {

	raw_spinlock_t lock;

	unsigned long val;

	int cpu;

};



/* Scheduling group status flags */

#define SG_OVERLOAD		0x1 /* More than one runnable task on a CPU. */

#define SG_OVERUTILIZED		0x2 /* One or more CPUs are over-utilized. */
@@ -769,7 +775,8 @@ 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;



	/*

	 * NULL-terminated list of performance domains intersecting with the
@@ -782,6 +789,7 @@ extern struct root_domain def_root_domain; 
extern struct mutex sched_domains_mutex;



extern void init_defrootdomain(void);

extern void init_max_cpu_capacity(struct max_cpu_capacity *mcc);

extern int sched_init_domains(const struct cpumask *cpu_map);

extern void rq_attach_root(struct rq *rq, struct root_domain *rd);

extern void sched_get_rd(struct root_domain *rd);

kernel/sched/topology.c

+3 −12
Original line number Diff line number Diff line
@@ -524,6 +524,9 @@ static int init_rootdomain(struct root_domain *rd) 


	if (cpupri_init(&rd->cpupri) != 0)

		goto free_cpudl;



	init_max_cpu_capacity(&rd->max_cpu_capacity);



	return 0;



free_cpudl:
@@ -1950,7 +1953,6 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att 
	enum s_alloc alloc_state;

	struct sched_domain *sd;

	struct s_data d;

	struct rq *rq = NULL;

	int i, ret = -ENOMEM;

	struct sched_domain_topology_level *tl_asym;

	bool has_asym = false;
@@ -2013,13 +2015,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att 
	/* Attach the domains */

	rcu_read_lock();

	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();
@@ -2027,11 +2023,6 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att 
	if (has_asym)

		static_branch_enable_cpuslocked(&sched_asym_cpucapacity);



	if (rq && sched_debug_enabled) {

		pr_info("root domain 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);