[PATCH v2 3/4] sched/fair: Extend for_each_sched_entity() with a back-link

[Peter Zijlstra <peterz@infradead.org>]

Leave a trail of bread crumbs, such that we can find our way back down the
hierarchy. No actual users yet, but split out because its a bit tricky.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/fair.c  |   14 +++++++++++---
 kernel/sched/sched.h |    1 +
 2 files changed, 12 insertions(+), 3 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -318,8 +318,13 @@ const struct sched_class fair_sched_clas
 #ifdef CONFIG_FAIR_GROUP_SCHED
 
 /* Walk up scheduling entities hierarchy */
-#define for_each_sched_entity(se, cfs_rq)				\
-	for (; (se) && ((cfs_rq) = cfs_rq_of(se)); (se) = (se)->parent)
+#define for_each_sched_entity(se, cfs_rq)					\
+	for (struct sched_entity *_BL = NULL;					\
+	     (se) && ((cfs_rq) = cfs_rq_of(se), (cfs_rq)->backlink = _BL, true);\
+	     (se) = (se)->parent, _BL = (se))
+
+#define for_each_sched_entity_bl(se, cfs_rq) \
+	for (; ((se) = (cfs_rq)->backlink); (cfs_rq) = group_cfs_rq(se))
 
 static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
 {
@@ -456,6 +461,9 @@ static int se_is_idle(struct sched_entit
 #define for_each_sched_entity(se, cfs_rq) \
 	for (; (se) && ((cfs_rq) = cfs_rq_of(se)); (se) = NULL)
 
+#define for_each_sched_entity_bl(se, cfs_rq) \
+	for (; ((se) = NULL);)
+
 static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
 {
 	return true;
@@ -15233,7 +15241,7 @@ static inline void task_tick_core(struct
 /*
  * se_fi_update - Update the cfs_rq->zero_vruntime_fi in a CFS hierarchy if needed.
  */
-static void se_fi_update(const struct sched_entity *se, unsigned int fi_seq,
+static void se_fi_update(struct sched_entity *se, unsigned int fi_seq,
 			 bool forceidle)
 {
 	struct cfs_rq *cfs_rq;
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -726,6 +726,7 @@ struct cfs_rq {
 	unsigned long		tg_runnable_avg_contrib;
 	long			propagate;
 	long			prop_runnable_sum;
+	struct sched_entity	*backlink;
 
 	/*
 	 *   h_load = weight * f(tg)