[PATCH] sched/fair: add comments for group_type and balancing at SD_NUMA level

[Vincent Guittot]

Add comments to describe each state of goup_type and to add some details
about the load balance at NUMA level.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
 kernel/sched/fair.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c93d534..268e441 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6986,11 +6986,34 @@ enum fbq_type { regular, remote, all };
  * group. see update_sd_pick_busiest().
  */
 enum group_type {
+	/*
+	 * The group has spare capacity that can be used to process more work.
+	 */
 	group_has_spare = 0,
+	/*
+	 * The group is fully used and the tasks don't compete for more CPU
+	 * cycles. Nevetheless, some tasks might wait before running.
+	 */
 	group_fully_busy,
+	/*
+	 * One task doesn't fit with CPU's capacity and must be migrated on a
+	 * more powerful CPU.
+	 */
 	group_misfit_task,
+	/*
+	 * One local CPU with higher capacity is available and task should be
+	 * migrated on it instead on current CPU.
+	 */
 	group_asym_packing,
+	/*
+	 * The tasks affinity prevents the scheduler to balance the load across
+	 * the system.
+	 */
 	group_imbalanced,
+	/*
+	 * The CPU is overloaded and can't provide expected CPU cycles to all
+	 * tasks.
+	 */
 	group_overloaded
 };
 
@@ -8608,7 +8631,11 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 
 	/*
 	 * Try to use spare capacity of local group without overloading it or
-	 * emptying busiest
+	 * emptying busiest.
+	 * XXX Spreading tasks across numa nodes is not always the best policy
+	 * and special cares should be taken for SD_NUMA domain level before
+	 * spreading the tasks. For now, load_balance() fully relies on
+	 * NUMA_BALANCING and fbq_classify_group/rq to overide the decision.
 	 */
 	if (local->group_type == group_has_spare) {
 		if (busiest->group_type > group_fully_busy) {
-- 
2.7.4