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>
Link: http://lkml.kernel.org/n/tip-vqh9ai6s0ewmeakjz80w4qz6@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4a559bf..3cbfb55 100644 (file)
--- 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;

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0533a68..6001e5c 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6060,6 +6060,7 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)
 
                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;
@@ -6135,6 +6136,7 @@ build_sched_groups(struct sched_domain *sd, int cpu)
 
                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)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 968ffee..cf86f74 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3828,7 +3828,8 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
         */
        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;
                        }
@@ -4929,7 +4930,7 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
        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;
@@ -4939,6 +4940,7 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
 
        rcu_read_lock();
        for_each_domain(cpu, sd) {
+               last = sd;
                if (!(sd->flags & SD_LOAD_BALANCE))
                        continue;
 
@@ -4983,6 +4985,9 @@ out:
                if (!balance)
                        break;
        }
+       for (sd = last; sd; sd = sd->child)
+               (void)cmpxchg(&sd->groups->balance_cpu, cpu, -1);
+
        rcu_read_unlock();
 
        /*