[RFC PATCH v3 06/10] sched/fair: Prepare switched_from & switched_to for per-task throttling

[Valentin Schneider <vschneid@redhat.com>]

Later commits will change CFS bandwidth control throttling from a
per-cfs_rq basis to a per-task basis. This means special care needs to be
taken around any transition a task can have into and out of a cfs_rq.

To ease reviewing, the transitions are patched with dummy-helpers that are
implemented later on.

Add helpers to switched_from_fair() and switched_to_fair() to cover class
changes. If switching from CFS, a task may need to be unthrottled. If
switching to CFS, a task may need to be throttled.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 kernel/sched/fair.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 095357bd17f0e..acac0829c71f3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5694,6 +5694,10 @@ static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
 	return cfs_bandwidth_used() && cfs_rq->throttle_count;
 }
 
+static inline bool task_needs_throttling(struct task_struct *p) { return false; }
+static inline void task_throttle_setup(struct task_struct *p) { }
+static inline void task_throttle_cancel(struct task_struct *p) { }
+
 /*
  * Ensure that neither of the group entities corresponding to src_cpu or
  * dest_cpu are members of a throttled hierarchy when performing group
@@ -6622,6 +6626,10 @@ static inline int throttled_lb_pair(struct task_group *tg,
 	return 0;
 }
 
+static inline bool task_needs_throttling(struct task_struct *p) { return false; }
+static inline void task_throttle_setup(struct task_struct *p) { }
+static inline void task_throttle_cancel(struct task_struct *p) { }
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
 void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth *parent) {}
 static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}
@@ -12847,11 +12855,15 @@ static void attach_task_cfs_rq(struct task_struct *p)
 static void switched_from_fair(struct rq *rq, struct task_struct *p)
 {
 	detach_task_cfs_rq(p);
+	if (cfs_bandwidth_used())
+		task_throttle_cancel(p);
 }
 
 static void switched_to_fair(struct rq *rq, struct task_struct *p)
 {
 	attach_task_cfs_rq(p);
+	if (cfs_bandwidth_used() && task_needs_throttling(p))
+		task_throttle_setup(p);
 
 	set_task_max_allowed_capacity(p);
 
-- 
2.43.0