[RFC PATCH v1] sched/fair: limit load balance redo times at the same sched_domain level

[Aubrey Li <aubrey.li@intel.com>]

A long-tail load balance cost is observed on the newly idle path,
this is caused by a race window between the first nr_running check
of the busiest runqueue and its nr_running recheck in detach_tasks.

Before the busiest runqueue is locked, the tasks on the busiest
runqueue could be pulled by other CPUs and nr_running of the busiest
runqueu becomes 1, this causes detach_tasks breaks with LBF_ALL_PINNED
flag set, and triggers load_balance redo at the same sched_domain level.

In order to find the new busiest sched_group and CPU, load balance will
recompute and update the various load statistics, which eventually leads
to the long-tail load balance cost.

This patch introduces a variable(sched_nr_lb_redo) to limit load balance
redo times, combined with sysctl_sched_nr_migrate, the max load balance
cost is reduced from 100+ us to 70+ us, measured on a 4s x86 system with
192 logical CPUs.

Cc: Andi Kleen <ak@linux.intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Aubrey Li <aubrey.li@linux.intel.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 ae7ceba..b59f371 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7407,6 +7407,8 @@ struct lb_env {
 	unsigned int		loop;
 	unsigned int		loop_break;
 	unsigned int		loop_max;
+	unsigned int		redo_cnt;
+	unsigned int		redo_max;
 
 	enum fbq_type		fbq_type;
 	enum migration_type	migration_type;
@@ -9525,6 +9527,7 @@ static int should_we_balance(struct lb_env *env)
 	return group_balance_cpu(sg) == env->dst_cpu;
 }
 
+static const unsigned int sched_nr_lb_redo = 1;
 /*
  * Check this_cpu to ensure it is balanced within domain. Attempt to move
  * tasks if there is an imbalance.
@@ -9547,6 +9550,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		.dst_grpmask    = sched_group_span(sd->groups),
 		.idle		= idle,
 		.loop_break	= sched_nr_migrate_break,
+		.redo_max	= sched_nr_lb_redo,
 		.cpus		= cpus,
 		.fbq_type	= all,
 		.tasks		= LIST_HEAD_INIT(env.tasks),
@@ -9682,7 +9686,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 			 * destination group that is receiving any migrated
 			 * load.
 			 */
-			if (!cpumask_subset(cpus, env.dst_grpmask)) {
+			if (!cpumask_subset(cpus, env.dst_grpmask) &&
+					++env.redo_cnt < env.redo_max) {
 				env.loop = 0;
 				env.loop_break = sched_nr_migrate_break;
 				goto redo;
-- 
2.7.4