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Subject[PATCH v3 11/12] sched: replace capacity_factor by utilization
Date
The scheduler tries to compute how many tasks a group of CPUs can handle by
assuming that a task's load is SCHED_LOAD_SCALE and a CPU capacity is
SCHED_POWER_SCALE.
We can have a better idea of the capacity of a group of CPUs and of the
utilization of this group thanks to the rework of group_power_orig and
group_utilization. We can deduct how many capacity is still available.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/fair.c | 107 +++++++++++++++++++---------------------------------
1 file changed, 38 insertions(+), 69 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a6b4b25..cf65284 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5534,13 +5534,13 @@ struct sg_lb_stats {
unsigned long sum_weighted_load; /* Weighted load of group's tasks */
unsigned long load_per_task;
unsigned long group_capacity;
+ unsigned long group_capacity_orig;
unsigned long group_utilization; /* Total utilization of the group */
unsigned int sum_nr_running; /* Nr tasks running in the group */
- unsigned int group_capacity_factor;
unsigned int idle_cpus;
unsigned int group_weight;
int group_imb; /* Is there an imbalance in the group ? */
- int group_has_free_capacity;
+ int group_out_of_capacity;
#ifdef CONFIG_NUMA_BALANCING
unsigned int nr_numa_running;
unsigned int nr_preferred_running;
@@ -5781,31 +5781,6 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
}

/*
- * Try and fix up capacity for tiny siblings, this is needed when
- * things like SD_ASYM_PACKING need f_b_g to select another sibling
- * which on its own isn't powerful enough.
- *
- * See update_sd_pick_busiest() and check_asym_packing().
- */
-static inline int
-fix_small_capacity(struct sched_domain *sd, struct sched_group *group)
-{
- /*
- * Only siblings can have significantly less than SCHED_CAPACITY_SCALE
- */
- if (!(sd->flags & SD_SHARE_CPUCAPACITY))
- return 0;
-
- /*
- * If ~90% of the cpu_capacity is still there, we're good.
- */
- if (group->sgc->capacity * 32 > group->sgc->capacity_orig * 29)
- return 1;
-
- return 0;
-}
-
-/*
* Group imbalance indicates (and tries to solve) the problem where balancing
* groups is inadequate due to tsk_cpus_allowed() constraints.
*
@@ -5839,32 +5814,24 @@ static inline int sg_imbalanced(struct sched_group *group)
return group->sgc->imbalance;
}

-/*
- * Compute the group capacity factor.
- *
- * Avoid the issue where N*frac(smt_capacity) >= 1 creates 'phantom' cores by
- * first dividing out the smt factor and computing the actual number of cores
- * and limit unit capacity with that.
- */
-static inline int sg_capacity_factor(struct lb_env *env, struct sched_group *group)
+static inline int group_has_free_capacity(struct sg_lb_stats *sgs,
+ struct lb_env *env)
{
- unsigned int capacity_factor, smt, cpus;
- unsigned int capacity, capacity_orig;
-
- capacity = group->sgc->capacity;
- capacity_orig = group->sgc->capacity_orig;
- cpus = group->group_weight;
+ if ((sgs->group_capacity_orig * 100) > (sgs->group_utilization * env->sd->imbalance_pct)
+ || (sgs->sum_nr_running < sgs->group_weight))
+ return 1;

- /* smt := ceil(cpus / capacity), assumes: 1 < smt_capacity < 2 */
- smt = DIV_ROUND_UP(SCHED_CAPACITY_SCALE * cpus, capacity_orig);
- capacity_factor = cpus / smt; /* cores */
+ return 0;
+}

- capacity_factor = min_t(unsigned,
- capacity_factor, DIV_ROUND_CLOSEST(capacity, SCHED_CAPACITY_SCALE));
- if (!capacity_factor)
- capacity_factor = fix_small_capacity(env->sd, group);
+static inline int group_is_overloaded(struct sg_lb_stats *sgs,
+ struct lb_env *env)
+{
+ if ((sgs->group_capacity_orig * 100) < (sgs->group_utilization * env->sd->imbalance_pct)
+ && (sgs->sum_nr_running > sgs->group_weight))
+ return 1;

- return capacity_factor;
+ return 0;
}

/**
@@ -5905,6 +5872,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
sgs->idle_cpus++;
}

+ sgs->group_capacity_orig = group->sgc->capacity_orig;
/* Adjust by relative CPU capacity of the group */
sgs->group_capacity = group->sgc->capacity;
sgs->avg_load = (sgs->group_load*SCHED_CAPACITY_SCALE) / sgs->group_capacity;
@@ -5915,10 +5883,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
sgs->group_weight = group->group_weight;

sgs->group_imb = sg_imbalanced(group);
- sgs->group_capacity_factor = sg_capacity_factor(env, group);

- if (sgs->group_capacity_factor > sgs->sum_nr_running)
- sgs->group_has_free_capacity = 1;
+ sgs->group_out_of_capacity = group_is_overloaded(sgs, env);
}

/**
@@ -5942,7 +5908,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
if (sgs->avg_load <= sds->busiest_stat.avg_load)
return false;

- if (sgs->sum_nr_running > sgs->group_capacity_factor)
+ if (sgs->group_out_of_capacity)
return true;

if (sgs->group_imb)
@@ -6041,17 +6007,20 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd

/*
* In case the child domain prefers tasks go to siblings
- * first, lower the sg capacity factor to one so that we'll try
+ * first, lower the sg capacity to one so that we'll try
* and move all the excess tasks away. We lower the capacity
* of a group only if the local group has the capacity to fit
- * these excess tasks, i.e. nr_running < group_capacity_factor. The
+ * these excess tasks, i.e. group_capacity > 0. The
* extra check prevents the case where you always pull from the
* heaviest group when it is already under-utilized (possible
* with a large weight task outweighs the tasks on the system).
*/
if (prefer_sibling && sds->local &&
- sds->local_stat.group_has_free_capacity)
- sgs->group_capacity_factor = min(sgs->group_capacity_factor, 1U);
+ group_has_free_capacity(&sds->local_stat, env)) {
+ if (sgs->sum_nr_running > 1)
+ sgs->group_out_of_capacity = 1;
+ sgs->group_capacity = min(sgs->group_capacity, SCHED_CAPACITY_SCALE);
+ }

if (update_sd_pick_busiest(env, sds, sg, sgs)) {
sds->busiest = sg;
@@ -6223,11 +6192,11 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
* Except of course for the group_imb case, since then we might
* have to drop below capacity to reach cpu-load equilibrium.
*/
- load_above_capacity =
- (busiest->sum_nr_running - busiest->group_capacity_factor);
-
- load_above_capacity *= (SCHED_LOAD_SCALE * SCHED_CAPACITY_SCALE);
- load_above_capacity /= busiest->group_capacity;
+ load_above_capacity = busiest->sum_nr_running * SCHED_LOAD_SCALE;
+ if (load_above_capacity > busiest->group_capacity)
+ load_above_capacity -= busiest->group_capacity;
+ else
+ load_above_capacity = ~0UL;
}

/*
@@ -6290,6 +6259,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
local = &sds.local_stat;
busiest = &sds.busiest_stat;

+ /* ASYM feature bypasses nice load balance check */
if ((env->idle == CPU_IDLE || env->idle == CPU_NEWLY_IDLE) &&
check_asym_packing(env, &sds))
return sds.busiest;
@@ -6310,8 +6280,8 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
goto force_balance;

/* SD_BALANCE_NEWIDLE trumps SMP nice when underutilized */
- if (env->idle == CPU_NEWLY_IDLE && local->group_has_free_capacity &&
- !busiest->group_has_free_capacity)
+ if (env->idle == CPU_NEWLY_IDLE && group_has_free_capacity(local, env) &&
+ busiest->group_out_of_capacity)
goto force_balance;

/*
@@ -6369,7 +6339,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
int i;

for_each_cpu_and(i, sched_group_cpus(group), env->cpus) {
- unsigned long capacity, capacity_factor, wl;
+ unsigned long capacity, wl;
enum fbq_type rt;

rq = cpu_rq(i);
@@ -6398,9 +6368,6 @@ static struct rq *find_busiest_queue(struct lb_env *env,
continue;

capacity = capacity_of(i);
- capacity_factor = DIV_ROUND_CLOSEST(capacity, SCHED_CAPACITY_SCALE);
- if (!capacity_factor)
- capacity_factor = fix_small_capacity(env->sd, group);

wl = weighted_cpuload(i);

@@ -6408,7 +6375,9 @@ static struct rq *find_busiest_queue(struct lb_env *env,
* When comparing with imbalance, use weighted_cpuload()
* which is not scaled with the cpu capacity.
*/
- if (capacity_factor && rq->nr_running == 1 && wl > env->imbalance)
+
+ if (rq->nr_running == 1 && wl > env->imbalance &&
+ ((capacity * env->sd->imbalance_pct) >= (rq->cpu_capacity_orig * 100)))
continue;

/*
--
1.9.1


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