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Subject[RFC][PATCH 2/5] sched, fair: Add some serialization to the sched_domain load-balance walk
Since the sched_domain walk is completely unserialized (!SD_SERIALIZE)
it is possible that multiple cpus in the group get elected to do the
next level. Avoid this by adding some serialization.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/sched.h | 1 +
kernel/sched/core.c | 2 ++
kernel/sched/fair.c | 9 +++++++--
3 files changed, 10 insertions(+), 2 deletions(-)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -927,6 +927,7 @@ struct sched_group_power {
struct sched_group {
struct sched_group *next; /* Must be a circular list */
atomic_t ref;
+ int balance_cpu;

unsigned int group_weight;
struct sched_group_power *sgp;
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6057,6 +6057,7 @@ build_overlap_sched_groups(struct sched_

sg->sgp = *per_cpu_ptr(sdd->sgp, cpumask_first(sg_span));
atomic_inc(&sg->sgp->ref);
+ sg->balance_cpu = -1;

if (cpumask_test_cpu(cpu, sg_span))
groups = sg;
@@ -6132,6 +6133,7 @@ build_sched_groups(struct sched_domain *

cpumask_clear(sched_group_cpus(sg));
sg->sgp->power = 0;
+ sg->balance_cpu = -1;

for_each_cpu(j, span) {
if (get_group(j, sdd, NULL) != group)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3831,7 +3831,8 @@ static inline void update_sg_lb_stats(st
*/
if (local_group) {
if (idle != CPU_NEWLY_IDLE) {
- if (balance_cpu != this_cpu) {
+ if (balance_cpu != this_cpu ||
+ cmpxchg(&group->balance_cpu, -1, balance_cpu) != -1) {
*balance = 0;
return;
}
@@ -4933,7 +4934,7 @@ static void rebalance_domains(int cpu, e
int balance = 1;
struct rq *rq = cpu_rq(cpu);
unsigned long interval;
- struct sched_domain *sd;
+ struct sched_domain *sd, *last = NULL;
/* Earliest time when we have to do rebalance again */
unsigned long next_balance = jiffies + 60*HZ;
int update_next_balance = 0;
@@ -4943,6 +4944,7 @@ static void rebalance_domains(int cpu, e

rcu_read_lock();
for_each_domain(cpu, sd) {
+ last = sd;
if (!(sd->flags & SD_LOAD_BALANCE))
continue;

@@ -4987,6 +4989,9 @@ static void rebalance_domains(int cpu, e
if (!balance)
break;
}
+ for (sd = last; sd; sd = sd->child)
+ (void)cmpxchg(&sd->groups->balance_cpu, cpu, -1);
+
rcu_read_unlock();

/*



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