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Commit aae6d3dd authored by Suresh Siddha's avatar Suresh Siddha Committed by Ingo Molnar
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sched: Use group weight, idle cpu metrics to fix imbalances during idle 



Currently we consider a sched domain to be well balanced when the imbalance
is less than the domain's imablance_pct. As the number of cores and threads
are increasing, current values of imbalance_pct (for example 25% for a
NUMA domain) are not enough to detect imbalances like:

a) On a WSM-EP system (two sockets, each having 6 cores and 12 logical threads),
24 cpu-hogging tasks get scheduled as 13 on one socket and 11 on another
socket. Leading to an idle HT cpu.

b) On a hypothetial 2 socket NHM-EX system (each socket having 8 cores and
16 logical threads), 16 cpu-hogging tasks can get scheduled as 9 on one
socket and 7 on another socket. Leaving one core in a socket idle
whereas in another socket we have a core having both its HT siblings busy.

While this issue can be fixed by decreasing the domain's imbalance_pct
(by making it a function of number of logical cpus in the domain), it
can potentially cause more task migrations across sched groups in an
overloaded case.

Fix this by using imbalance_pct only during newly_idle and busy
load balancing. And during idle load balancing, check if there
is an imbalance in number of idle cpu's across the busiest and this
sched_group or if the busiest group has more tasks than its weight that
the idle cpu in this_group can pull.

Reported-by: default avatarNikhil Rao <ncrao@google.com>
Signed-off-by: default avatarSuresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1284760952.2676.11.camel@sbsiddha-MOBL3.sc.intel.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent f6614b7b

include/linux/sched.h

+1 −0
Original line number Diff line number Diff line
@@ -862,6 +862,7 @@ struct sched_group { 
	 * single CPU.

	 */

	unsigned int cpu_power, cpu_power_orig;

	unsigned int group_weight;



	/*

	 * The CPUs this group covers.

kernel/sched.c

+2 −0
Original line number Diff line number Diff line
@@ -6960,6 +6960,8 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd) 
	if (cpu != group_first_cpu(sd->groups))

		return;



	sd->groups->group_weight = cpumask_weight(sched_group_cpus(sd->groups));



	child = sd->child;



	sd->groups->cpu_power = 0;

kernel/sched_fair.c

+31 −3
Original line number Diff line number Diff line
@@ -2035,13 +2035,16 @@ struct sd_lb_stats { 
	unsigned long this_load_per_task;

	unsigned long this_nr_running;

	unsigned long this_has_capacity;

	unsigned int  this_idle_cpus;



	/* Statistics of the busiest group */

	unsigned int  busiest_idle_cpus;

	unsigned long max_load;

	unsigned long busiest_load_per_task;

	unsigned long busiest_nr_running;

	unsigned long busiest_group_capacity;

	unsigned long busiest_has_capacity;

	unsigned int  busiest_group_weight;



	int group_imb; /* Is there imbalance in this sd */

#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
@@ -2063,6 +2066,8 @@ struct sg_lb_stats { 
	unsigned long sum_nr_running; /* Nr tasks running in the group */

	unsigned long sum_weighted_load; /* Weighted load of group's tasks */

	unsigned long group_capacity;

	unsigned long idle_cpus;

	unsigned long group_weight;

	int group_imb; /* Is there an imbalance in the group ? */

	int group_has_capacity; /* Is there extra capacity in the group? */

};
@@ -2431,7 +2436,8 @@ static inline void update_sg_lb_stats(struct sched_domain *sd, 
		sgs->group_load += load;

		sgs->sum_nr_running += rq->nr_running;

		sgs->sum_weighted_load += weighted_cpuload(i);



		if (idle_cpu(i))

			sgs->idle_cpus++;

	}



	/*
@@ -2469,6 +2475,7 @@ static inline void update_sg_lb_stats(struct sched_domain *sd, 
	sgs->group_capacity = DIV_ROUND_CLOSEST(group->cpu_power, SCHED_LOAD_SCALE);

	if (!sgs->group_capacity)

		sgs->group_capacity = fix_small_capacity(sd, group);

	sgs->group_weight = group->group_weight;



	if (sgs->group_capacity > sgs->sum_nr_running)

		sgs->group_has_capacity = 1;
@@ -2576,13 +2583,16 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu, 
			sds->this_nr_running = sgs.sum_nr_running;

			sds->this_load_per_task = sgs.sum_weighted_load;

			sds->this_has_capacity = sgs.group_has_capacity;

			sds->this_idle_cpus = sgs.idle_cpus;

		} else if (update_sd_pick_busiest(sd, sds, sg, &sgs, this_cpu)) {

			sds->max_load = sgs.avg_load;

			sds->busiest = sg;

			sds->busiest_nr_running = sgs.sum_nr_running;

			sds->busiest_idle_cpus = sgs.idle_cpus;

			sds->busiest_group_capacity = sgs.group_capacity;

			sds->busiest_load_per_task = sgs.sum_weighted_load;

			sds->busiest_has_capacity = sgs.group_has_capacity;

			sds->busiest_group_weight = sgs.group_weight;

			sds->group_imb = sgs.group_imb;

		}
@@ -2860,8 +2870,26 @@ find_busiest_group(struct sched_domain *sd, int this_cpu, 
	if (sds.this_load >= sds.avg_load)

		goto out_balanced;



	/*

	 * In the CPU_NEWLY_IDLE, use imbalance_pct to be conservative.

	 * And to check for busy balance use !idle_cpu instead of

	 * CPU_NOT_IDLE. This is because HT siblings will use CPU_NOT_IDLE

	 * even when they are idle.

	 */

	if (idle == CPU_NEWLY_IDLE || !idle_cpu(this_cpu)) {

		if (100 * sds.max_load <= sd->imbalance_pct * sds.this_load)

			goto out_balanced;

	} else {

		/*

		 * This cpu is idle. If the busiest group load doesn't

		 * have more tasks than the number of available cpu's and

		 * there is no imbalance between this and busiest group

		 * wrt to idle cpu's, it is balanced.

		 */

		if ((sds.this_idle_cpus  <= sds.busiest_idle_cpus + 1) &&

		    sds.busiest_nr_running <= sds.busiest_group_weight)

			goto out_balanced;

	}



force_balance:

	/* Looks like there is an imbalance. Compute it */