[RFC PATCH 11/22] sched/fair: Prepare unthrottle_cfs_rq() to demote throttle status

[K Prateek Nayak]

If an entity which was blocked when running in kernel mode wakes up, a
fully throttled hierarchy needs to be demoted to a partially throttled
one.

Prepare unthrottle_cfs_rq() to demote the status where the caller
explicitly specifies via "demote_to_partial" indicator that it is
requesting the throttle status to be demoted.

Modify all current callers of unthrottle_cfs_rq()() setting
"demote_to_partial" as false since all the existing scenarios
completely unthrottles a cfs_rq.

Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 kernel/sched/core.c  |  2 +-
 kernel/sched/fair.c  | 39 +++++++++++++++++++++++++++++----------
 kernel/sched/sched.h |  2 +-
 3 files changed, 31 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0851cdad9242..a797517d3dcf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9488,7 +9488,7 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota,
 		cfs_rq->runtime_remaining = 0;
 
 		if (cfs_rq->throttled)
-			unthrottle_cfs_rq(cfs_rq);
+			unthrottle_cfs_rq(cfs_rq, false);
 	}
 
 	if (runtime_was_enabled && !runtime_enabled)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8e1df614e82f..091493bc8506 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6082,17 +6082,26 @@ static bool throttle_cfs_rq(struct cfs_rq *cfs_rq)
 	return false;
 }
 
-void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
+void unthrottle_cfs_rq(struct cfs_rq *cfs_rq, bool demote_to_partial)
 {
 	struct rq *rq = rq_of(cfs_rq);
 	struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
 	struct sched_entity *se;
 	long queued_delta, runnable_delta, idle_delta;
 	long rq_h_nr_queued = rq->cfs.h_nr_queued;
+	int throttled_state = cfs_rq->throttled;
 
 	se = cfs_rq->tg->se[cpu_of(rq)];
 
-	cfs_rq->throttled = CFS_UNTHROTTLED;
+	if (demote_to_partial) {
+		/*
+		 * A demotion to partially throttled state can only be
+		 * requested on a fully throttled hierarchy.
+		 */
+		SCHED_WARN_ON(!cfs_rq_h_throttled(cfs_rq));
+		cfs_rq->throttled = CFS_THROTTLED_PARTIAL;
+	} else
+		cfs_rq->throttled = CFS_UNTHROTTLED;
 
 	update_rq_clock(rq);
 
@@ -6101,9 +6110,16 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 		cfs_b->throttled_time += rq_clock(rq) - cfs_rq->throttled_clock;
 		cfs_rq->throttled_clock = 0;
 	}
-	list_del_rcu(&cfs_rq->throttled_list);
+
+	/* Partial throttle should retain itself in the throttled_list */
+	if (!demote_to_partial)
+		list_del_rcu(&cfs_rq->throttled_list);
 	raw_spin_unlock(&cfs_b->lock);
 
+	/* If cfs_rq was partially throttled, we have nothing to do */
+	if (throttled_state == CFS_THROTTLED_PARTIAL)
+		goto unthrottle_throttle;
+
 	/* update hierarchical throttle state */
 	walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
 
@@ -6176,8 +6192,11 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 unthrottle_throttle:
 	assert_list_leaf_cfs_rq(rq);
 
-	/* Determine whether we need to wake up potentially idle CPU: */
-	if (rq->curr == rq->idle && rq->cfs.nr_queued)
+	/*
+	 * Determine whether we need to wake up potentially idle CPU or
+	 * reevalutate our pick on the throttled hierarchy.
+	 */
+	if (cfs_rq->curr || (rq->curr == rq->idle && rq->cfs.nr_queued))
 		resched_curr(rq);
 }
 
@@ -6212,7 +6231,7 @@ static void __cfsb_csd_unthrottle(void *arg)
 		list_del_init(&cursor->throttled_csd_list);
 
 		if (cfs_rq_throttled(cursor))
-			unthrottle_cfs_rq(cursor);
+			unthrottle_cfs_rq(cursor, false);
 	}
 
 	rcu_read_unlock();
@@ -6227,7 +6246,7 @@ static inline void __unthrottle_cfs_rq_async(struct cfs_rq *cfs_rq)
 	bool first;
 
 	if (rq == this_rq()) {
-		unthrottle_cfs_rq(cfs_rq);
+		unthrottle_cfs_rq(cfs_rq, false);
 		return;
 	}
 
@@ -6243,7 +6262,7 @@ static inline void __unthrottle_cfs_rq_async(struct cfs_rq *cfs_rq)
 #else
 static inline void __unthrottle_cfs_rq_async(struct cfs_rq *cfs_rq)
 {
-	unthrottle_cfs_rq(cfs_rq);
+	unthrottle_cfs_rq(cfs_rq, false);
 }
 #endif
 
@@ -6329,7 +6348,7 @@ static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b)
 		list_del_init(&cfs_rq->throttled_csd_list);
 
 		if (cfs_rq_throttled(cfs_rq))
-			unthrottle_cfs_rq(cfs_rq);
+			unthrottle_cfs_rq(cfs_rq, false);
 
 		rq_unlock_irqrestore(rq, &rf);
 	}
@@ -6786,7 +6805,7 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
 		 * there's some valid quota amount
 		 */
 		cfs_rq->runtime_remaining = 1;
-		unthrottle_cfs_rq(cfs_rq);
+		unthrottle_cfs_rq(cfs_rq, false);
 	}
 	rcu_read_unlock();
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 22567d236f82..bd43271fa166 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -554,7 +554,7 @@ extern void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth
 
 extern void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b);
 extern void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
-extern void unthrottle_cfs_rq(struct cfs_rq *cfs_rq);
+extern void unthrottle_cfs_rq(struct cfs_rq *cfs_rq, bool demote_to_partial);
 extern bool cfs_task_bw_constrained(struct task_struct *p);
 
 extern void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
-- 
2.43.0