Commit 57abff06 authored by Vincent Guittot's avatar Vincent Guittot Committed by Ingo Molnar
Browse files

sched/fair: Rework find_idlest_group() 



The slow wake up path computes per sched_group statisics to select the
idlest group, which is quite similar to what load_balance() is doing
for selecting busiest group. Rework find_idlest_group() to classify the
sched_group and select the idlest one following the same steps as
load_balance().

Signed-off-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Morten.Rasmussen@arm.com
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: hdanton@sina.com
Cc: parth@linux.ibm.com
Cc: pauld@redhat.com
Cc: quentin.perret@arm.com
Cc: riel@surriel.com
Cc: srikar@linux.vnet.ibm.com
Cc: valentin.schneider@arm.com
Link: https://lkml.kernel.org/r/1571405198-27570-12-git-send-email-vincent.guittot@linaro.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent fc1273f4

kernel/sched/fair.c

+256 −128
Original line number Diff line number Diff line
@@ -5531,127 +5531,9 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, 
	return target;

}



static unsigned long cpu_util_without(int cpu, struct task_struct *p);



static unsigned long capacity_spare_without(int cpu, struct task_struct *p)

{

	return max_t(long, capacity_of(cpu) - cpu_util_without(cpu, p), 0);

}



/*

 * find_idlest_group finds and returns the least busy CPU group within the

 * domain.

 *

 * Assumes p is allowed on at least one CPU in sd.

 */

static struct sched_group *

find_idlest_group(struct sched_domain *sd, struct task_struct *p,

		  int this_cpu, int sd_flag)

{

	struct sched_group *idlest = NULL, *group = sd->groups;

	struct sched_group *most_spare_sg = NULL;

	unsigned long min_load = ULONG_MAX, this_load = ULONG_MAX;

	unsigned long most_spare = 0, this_spare = 0;

	int imbalance_scale = 100 + (sd->imbalance_pct-100)/2;

	unsigned long imbalance = scale_load_down(NICE_0_LOAD) *

				(sd->imbalance_pct-100) / 100;



	do {

		unsigned long load;

		unsigned long spare_cap, max_spare_cap;

		int local_group;

		int i;



		/* Skip over this group if it has no CPUs allowed */

		if (!cpumask_intersects(sched_group_span(group),

					p->cpus_ptr))

			continue;



		local_group = cpumask_test_cpu(this_cpu,

					       sched_group_span(group));



		/*

		 * Tally up the load of all CPUs in the group and find

		 * the group containing the CPU with most spare capacity.

		 */

		load = 0;

		max_spare_cap = 0;



		for_each_cpu(i, sched_group_span(group)) {

			load += cpu_load(cpu_rq(i));



			spare_cap = capacity_spare_without(i, p);



			if (spare_cap > max_spare_cap)

				max_spare_cap = spare_cap;

		}



		/* Adjust by relative CPU capacity of the group */

		load = (load * SCHED_CAPACITY_SCALE) /

					group->sgc->capacity;



		if (local_group) {

			this_load = load;

			this_spare = max_spare_cap;

		} else {

			if (load < min_load) {

				min_load = load;

				idlest = group;

			}



			if (most_spare < max_spare_cap) {

				most_spare = max_spare_cap;

				most_spare_sg = group;

			}

		}

	} while (group = group->next, group != sd->groups);



	/*

	 * The cross-over point between using spare capacity or least load

	 * is too conservative for high utilization tasks on partially

	 * utilized systems if we require spare_capacity > task_util(p),

	 * so we allow for some task stuffing by using

	 * spare_capacity > task_util(p)/2.

	 *

	 * Spare capacity can't be used for fork because the utilization has

	 * not been set yet, we must first select a rq to compute the initial

	 * utilization.

	 */

	if (sd_flag & SD_BALANCE_FORK)

		goto skip_spare;



	if (this_spare > task_util(p) / 2 &&

	    imbalance_scale*this_spare > 100*most_spare)

		return NULL;



	if (most_spare > task_util(p) / 2)

		return most_spare_sg;



skip_spare:

	if (!idlest)

		return NULL;



	/*

	 * When comparing groups across NUMA domains, it's possible for the

	 * local domain to be very lightly loaded relative to the remote

	 * domains but "imbalance" skews the comparison making remote CPUs

	 * look much more favourable. When considering cross-domain, add

	 * imbalance to the load on the remote node and consider staying

	 * local.

	 */

	if ((sd->flags & SD_NUMA) &&

	     min_load + imbalance >= this_load)

		return NULL;



	if (min_load >= this_load + imbalance)

		return NULL;



	if ((this_load < (min_load + imbalance)) &&

	    (100*this_load < imbalance_scale*min_load))

		return NULL;



	return idlest;

}

		  int this_cpu, int sd_flag);



/*

 * find_idlest_group_cpu - find the idlest CPU among the CPUs in the group.
@@ -5724,7 +5606,7 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p 
		return prev_cpu;



	/*

	 * We need task's util for capacity_spare_without, sync it up to

	 * We need task's util for cpu_util_without, sync it up to

	 * prev_cpu's last_update_time.

	 */

	if (!(sd_flag & SD_BALANCE_FORK))
@@ -7905,13 +7787,13 @@ static inline int sg_imbalanced(struct sched_group *group) 
 * any benefit for the load balance.

 */

static inline bool

group_has_capacity(struct lb_env *env, struct sg_lb_stats *sgs)

group_has_capacity(unsigned int imbalance_pct, struct sg_lb_stats *sgs)

{

	if (sgs->sum_nr_running < sgs->group_weight)

		return true;



	if ((sgs->group_capacity * 100) >

			(sgs->group_util * env->sd->imbalance_pct))

			(sgs->group_util * imbalance_pct))

		return true;



	return false;
@@ -7926,13 +7808,13 @@ group_has_capacity(struct lb_env *env, struct sg_lb_stats *sgs) 
 *  false.

 */

static inline bool

group_is_overloaded(struct lb_env *env, struct sg_lb_stats *sgs)

group_is_overloaded(unsigned int imbalance_pct, struct sg_lb_stats *sgs)

{

	if (sgs->sum_nr_running <= sgs->group_weight)

		return false;



	if ((sgs->group_capacity * 100) <

			(sgs->group_util * env->sd->imbalance_pct))

			(sgs->group_util * imbalance_pct))

		return true;



	return false;
@@ -7959,11 +7841,11 @@ group_smaller_max_cpu_capacity(struct sched_group *sg, struct sched_group *ref) 
}



static inline enum

group_type group_classify(struct lb_env *env,

group_type group_classify(unsigned int imbalance_pct,

			  struct sched_group *group,

			  struct sg_lb_stats *sgs)

{

	if (group_is_overloaded(env, sgs))

	if (group_is_overloaded(imbalance_pct, sgs))

		return group_overloaded;



	if (sg_imbalanced(group))
@@ -7975,7 +7857,7 @@ group_type group_classify(struct lb_env *env, 
	if (sgs->group_misfit_task_load)

		return group_misfit_task;



	if (!group_has_capacity(env, sgs))

	if (!group_has_capacity(imbalance_pct, sgs))

		return group_fully_busy;



	return group_has_spare;
@@ -8076,7 +7958,7 @@ static inline void update_sg_lb_stats(struct lb_env *env, 


	sgs->group_weight = group->group_weight;



	sgs->group_type = group_classify(env, group, sgs);

	sgs->group_type = group_classify(env->sd->imbalance_pct, group, sgs);



	/* Computing avg_load makes sense only when group is overloaded */

	if (sgs->group_type == group_overloaded)
@@ -8231,6 +8113,252 @@ static inline enum fbq_type fbq_classify_rq(struct rq *rq) 
}

#endif /* CONFIG_NUMA_BALANCING */





struct sg_lb_stats;



/*

 * update_sg_wakeup_stats - Update sched_group's statistics for wakeup.

 * @denv: The ched_domain level to look for idlest group.

 * @group: sched_group whose statistics are to be updated.

 * @sgs: variable to hold the statistics for this group.

 */

static inline void update_sg_wakeup_stats(struct sched_domain *sd,

					  struct sched_group *group,

					  struct sg_lb_stats *sgs,

					  struct task_struct *p)

{

	int i, nr_running;



	memset(sgs, 0, sizeof(*sgs));



	for_each_cpu(i, sched_group_span(group)) {

		struct rq *rq = cpu_rq(i);



		sgs->group_load += cpu_load(rq);

		sgs->group_util += cpu_util_without(i, p);

		sgs->sum_h_nr_running += rq->cfs.h_nr_running;



		nr_running = rq->nr_running;

		sgs->sum_nr_running += nr_running;



		/*

		 * No need to call idle_cpu() if nr_running is not 0

		 */

		if (!nr_running && idle_cpu(i))

			sgs->idle_cpus++;





	}



	/* Check if task fits in the group */

	if (sd->flags & SD_ASYM_CPUCAPACITY &&

	    !task_fits_capacity(p, group->sgc->max_capacity)) {

		sgs->group_misfit_task_load = 1;

	}



	sgs->group_capacity = group->sgc->capacity;



	sgs->group_type = group_classify(sd->imbalance_pct, group, sgs);



	/*

	 * Computing avg_load makes sense only when group is fully busy or

	 * overloaded

	 */

	if (sgs->group_type < group_fully_busy)

		sgs->avg_load = (sgs->group_load * SCHED_CAPACITY_SCALE) /

				sgs->group_capacity;

}



static bool update_pick_idlest(struct sched_group *idlest,

			       struct sg_lb_stats *idlest_sgs,

			       struct sched_group *group,

			       struct sg_lb_stats *sgs)

{

	if (sgs->group_type < idlest_sgs->group_type)

		return true;



	if (sgs->group_type > idlest_sgs->group_type)

		return false;



	/*

	 * The candidate and the current idlest group are the same type of

	 * group. Let check which one is the idlest according to the type.

	 */



	switch (sgs->group_type) {

	case group_overloaded:

	case group_fully_busy:

		/* Select the group with lowest avg_load. */

		if (idlest_sgs->avg_load <= sgs->avg_load)

			return false;

		break;



	case group_imbalanced:

	case group_asym_packing:

		/* Those types are not used in the slow wakeup path */

		return false;



	case group_misfit_task:

		/* Select group with the highest max capacity */

		if (idlest->sgc->max_capacity >= group->sgc->max_capacity)

			return false;

		break;



	case group_has_spare:

		/* Select group with most idle CPUs */

		if (idlest_sgs->idle_cpus >= sgs->idle_cpus)

			return false;

		break;

	}



	return true;

}



/*

 * find_idlest_group() finds and returns the least busy CPU group within the

 * domain.

 *

 * Assumes p is allowed on at least one CPU in sd.

 */

static struct sched_group *

find_idlest_group(struct sched_domain *sd, struct task_struct *p,

		  int this_cpu, int sd_flag)

{

	struct sched_group *idlest = NULL, *local = NULL, *group = sd->groups;

	struct sg_lb_stats local_sgs, tmp_sgs;

	struct sg_lb_stats *sgs;

	unsigned long imbalance;

	struct sg_lb_stats idlest_sgs = {

			.avg_load = UINT_MAX,

			.group_type = group_overloaded,

	};



	imbalance = scale_load_down(NICE_0_LOAD) *

				(sd->imbalance_pct-100) / 100;



	do {

		int local_group;



		/* Skip over this group if it has no CPUs allowed */

		if (!cpumask_intersects(sched_group_span(group),

					p->cpus_ptr))

			continue;



		local_group = cpumask_test_cpu(this_cpu,

					       sched_group_span(group));



		if (local_group) {

			sgs = &local_sgs;

			local = group;

		} else {

			sgs = &tmp_sgs;

		}



		update_sg_wakeup_stats(sd, group, sgs, p);



		if (!local_group && update_pick_idlest(idlest, &idlest_sgs, group, sgs)) {

			idlest = group;

			idlest_sgs = *sgs;

		}



	} while (group = group->next, group != sd->groups);





	/* There is no idlest group to push tasks to */

	if (!idlest)

		return NULL;



	/*

	 * If the local group is idler than the selected idlest group

	 * don't try and push the task.

	 */

	if (local_sgs.group_type < idlest_sgs.group_type)

		return NULL;



	/*

	 * If the local group is busier than the selected idlest group

	 * try and push the task.

	 */

	if (local_sgs.group_type > idlest_sgs.group_type)

		return idlest;



	switch (local_sgs.group_type) {

	case group_overloaded:

	case group_fully_busy:

		/*

		 * When comparing groups across NUMA domains, it's possible for

		 * the local domain to be very lightly loaded relative to the

		 * remote domains but "imbalance" skews the comparison making

		 * remote CPUs look much more favourable. When considering

		 * cross-domain, add imbalance to the load on the remote node

		 * and consider staying local.

		 */



		if ((sd->flags & SD_NUMA) &&

		    ((idlest_sgs.avg_load + imbalance) >= local_sgs.avg_load))

			return NULL;



		/*

		 * If the local group is less loaded than the selected

		 * idlest group don't try and push any tasks.

		 */

		if (idlest_sgs.avg_load >= (local_sgs.avg_load + imbalance))

			return NULL;



		if (100 * local_sgs.avg_load <= sd->imbalance_pct * idlest_sgs.avg_load)

			return NULL;

		break;



	case group_imbalanced:

	case group_asym_packing:

		/* Those type are not used in the slow wakeup path */

		return NULL;



	case group_misfit_task:

		/* Select group with the highest max capacity */

		if (local->sgc->max_capacity >= idlest->sgc->max_capacity)

			return NULL;

		break;



	case group_has_spare:

		if (sd->flags & SD_NUMA) {

#ifdef CONFIG_NUMA_BALANCING

			int idlest_cpu;

			/*

			 * If there is spare capacity at NUMA, try to select

			 * the preferred node

			 */

			if (cpu_to_node(this_cpu) == p->numa_preferred_nid)

				return NULL;



			idlest_cpu = cpumask_first(sched_group_span(idlest));

			if (cpu_to_node(idlest_cpu) == p->numa_preferred_nid)

				return idlest;

#endif

			/*

			 * Otherwise, keep the task on this node to stay close

			 * its wakeup source and improve locality. If there is

			 * a real need of migration, periodic load balance will

			 * take care of it.

			 */

			if (local_sgs.idle_cpus)

				return NULL;

		}



		/*

		 * Select group with highest number of idle CPUs. We could also

		 * compare the utilization which is more stable but it can end

		 * up that the group has less spare capacity but finally more

		 * idle CPUs which means more opportunity to run task.

		 */

		if (local_sgs.idle_cpus >= idlest_sgs.idle_cpus)

			return NULL;

		break;

	}



	return idlest;

}



/**

 * update_sd_lb_stats - Update sched_domain's statistics for load balancing.

 * @env: The load balancing environment.

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