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Subject[109/115] sched: Use group weight, idle cpu metrics to fix imbalances during idle
2.6.32-longterm review patch.  If anyone has any objections, please let us know.

------------------

Commit: aae6d3ddd8b90f5b2c8d79a2b914d1706d124193 upstream

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: Nikhil Rao <ncrao@google.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1284760952.2676.11.camel@sbsiddha-MOBL3.sc.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
include/linux/sched.h | 1 +
kernel/sched.c | 36 +++++++++++++++++++++++++++++++++---
2 files changed, 34 insertions(+), 3 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -894,6 +894,7 @@ struct sched_group {
* single CPU.
*/
unsigned int cpu_power;
+ unsigned int group_weight;

/*
* The CPUs this group covers.
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3534,13 +3534,16 @@ 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)
@@ -3562,6 +3565,8 @@ 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? */
};
@@ -3905,7 +3910,8 @@ static inline void update_sg_lb_stats(st
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++;
}

/*
@@ -3939,6 +3945,7 @@ static inline void update_sg_lb_stats(st
sgs->group_imb = 1;

sgs->group_capacity = DIV_ROUND_CLOSEST(group->cpu_power, SCHED_LOAD_SCALE);
+ sgs->group_weight = group->group_weight;

if (sgs->group_capacity > sgs->sum_nr_running)
sgs->group_has_capacity = 1;
@@ -4004,13 +4011,16 @@ static inline void update_sd_lb_stats(st
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 (sgs.avg_load > sds->max_load &&
(sgs.sum_nr_running > sgs.group_capacity ||
sgs.group_imb)) {
sds->max_load = sgs.avg_load;
sds->busiest = group;
sds->busiest_nr_running = sgs.sum_nr_running;
+ sds->busiest_idle_cpus = sgs.idle_cpus;
sds->busiest_group_capacity = sgs.group_capacity;
+ sds->busiest_group_weight = sgs.group_weight;
sds->busiest_load_per_task = sgs.sum_weighted_load;
sds->busiest_has_capacity = sgs.group_has_capacity;
sds->group_imb = sgs.group_imb;
@@ -4235,8 +4245,26 @@ find_busiest_group(struct sched_domain *
if (sds.this_load >= sds.avg_load)
goto out_balanced;

- if (100 * sds.max_load <= sd->imbalance_pct * sds.this_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 */
@@ -8751,6 +8779,8 @@ static void init_sched_groups_power(int
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;



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