[RFC 1/3] sched: set loop_max after rq lock is taken

[RFC 1/3] sched: set loop_max after rq lock is taken

[Uladzislau Rezki]

From: Uladzislau 2 Rezki <uladzislau2.rezki@sonymobile.com>

While doing a load balance there is a race in setting
loop_max variable since nr_running can be changed causing
incorect iteration loops.

As a result we may skip some candidates or check the same
tasks again.

Signed-off-by: Uladzislau 2 Rezki <uladzislau2.rezki@sonymobile.com>
---
 kernel/sched/fair.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c242944..c5d9351 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7744,12 +7744,17 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		 * correctly treated as an imbalance.
 		 */
 		env.flags |= LBF_ALL_PINNED;
-		env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
 
 more_balance:
 		raw_spin_lock_irqsave(&busiest->lock, flags);
 
 		/*
+		 * Set loop_max when rq's lock is taken to prevent a race.
+		 */
+		env.loop_max = min(sysctl_sched_nr_migrate,
+						busiest->nr_running);
+
+		/*
 		 * cur_ld_moved - load moved in current iteration
 		 * ld_moved     - cumulative load moved across iterations
 		 */
-- 
2.1.4

[RFC 2/3] sched: set number of iterations to h_nr_running

[Uladzislau Rezki]

From: Uladzislau 2 Rezki <uladzislau2.rezki@sonymobile.com>

It is possible that busiest run queue has multiple RT tasks,
whereas no CFS tasks, that is why it is reasonable to use
h_nr_running instead, because a load balance only applies
for CFS related tasks.

Signed-off-by: Uladzislau 2 Rezki <uladzislau2.rezki@sonymobile.com>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c5d9351..9ebcab1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7752,7 +7752,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		 * Set loop_max when rq's lock is taken to prevent a race.
 		 */
 		env.loop_max = min(sysctl_sched_nr_migrate,
-						busiest->nr_running);
+						busiest->cfs.h_nr_running);
 
 		/*
 		 * cur_ld_moved - load moved in current iteration
-- 
2.1.4

[RFC 3/3] sched: ignore task_h_load for CPU_NEWLY_IDLE

[Uladzislau Rezki]

From: Uladzislau 2 Rezki <uladzislau2.rezki@sonymobile.com>

A load balancer calculates imbalance factor for particular shed
domain and tries to steal up the prescribed amount of weighted load.
However, a small imbalance factor would sometimes prevent us from
stealing any tasks at all. When a CPU is newly idle, it should
steal first task which passes a migration criteria.

Signed-off-by: Uladzislau 2 Rezki <uladzislau2.rezki@sonymobile.com>
---
 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9ebcab1..00b63cc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6534,8 +6534,9 @@ static int detach_tasks(struct lb_env *env)
 		if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed)
 			goto next;
 
-		if ((load / 2) > env->imbalance)
-			goto next;
+		if (env->idle != CPU_NEWLY_IDLE)
+			if ((load / 2) > env->imbalance)
+				goto next;
 
 		detach_task(p, env);
 		list_add(&p->se.group_node, &env->tasks);
-- 
2.1.4