[PATCH v2 3/3] sched: update blocked load when newly idle

[Vincent Guittot]

When NEWLY_IDLE load balance is not triggered, we might need to update the
blocked load anyway. We can kick an ilb so an idle CPU will take care of
updating blocked load or we can try to update them locally before entering
idle. In the latter case, we reuse part of the nohz_idle_balance.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
 kernel/sched/fair.c | 102 ++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 84 insertions(+), 18 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e203297..dcac728 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8829,6 +8829,9 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
 		*next_balance = next;
 }
 
+static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_idle_type idle);
+static void kick_ilb(unsigned int flags);
+
 /*
  * idle_balance is called by schedule() if this_cpu is about to become
  * idle. Attempts to pull tasks from other CPUs.
@@ -8863,12 +8866,26 @@ static int idle_balance(struct rq *this_rq, struct rq_flags *rf)
 
 	if (this_rq->avg_idle < sysctl_sched_migration_cost ||
 	    !this_rq->rd->overload) {
+		unsigned long has_blocked = READ_ONCE(nohz.has_blocked);
+		unsigned long next_blocked = READ_ONCE(nohz.next_blocked);
+
 		rcu_read_lock();
 		sd = rcu_dereference_check_sched_domain(this_rq->sd);
 		if (sd)
 			update_next_balance(sd, &next_balance);
 		rcu_read_unlock();
 
+		/*
+		 * Update blocked idle load if it has not been done for a
+		 * while. Try to do it locally before entering idle but kick a
+		 * ilb if it takes too much time and/or might delay next local
+		 * wake up
+		 */
+		if (has_blocked && time_after_eq(jiffies, next_blocked) &&
+				(this_rq->avg_idle < sysctl_sched_migration_cost ||
+				!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE)))
+			kick_ilb(NOHZ_STATS_KICK);
+
 		goto out;
 	}
 
@@ -9393,30 +9410,24 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 
 #ifdef CONFIG_NO_HZ_COMMON
 /*
- * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
- * rebalancing for all the cpus for whom scheduler ticks are stopped.
+ * Internal function that runs load balance for all idle cpus. The load balance
+ * can be a simple update of blocked load or a complete load balance with
+ * tasks movement depending of flags.
+ * For newly idle mode, we abort the loop if it takes too much time and return
+ * false to notify that the loop has not be completed and a ilb should be kick.
  */
-static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
+static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_idle_type idle)
 {
 	/* Earliest time when we have to do rebalance again */
 	unsigned long now = jiffies;
 	unsigned long next_balance = now + 60*HZ;
-	unsigned long next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD);
+	bool has_blocked_load = false;
 	int update_next_balance = 0;
 	int this_cpu = this_rq->cpu;
-	unsigned int flags;
 	int balance_cpu;
+	int ret = false;
 	struct rq *rq;
-
-	if (!(atomic_read(nohz_flags(this_cpu)) & NOHZ_KICK_MASK))
-		return false;
-
-	if (idle != CPU_IDLE) {
-		atomic_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
-		return false;
-	}
-
-	flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
+	u64 curr_cost = 0;
 
 	SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);
 
@@ -9431,6 +9442,10 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 	WRITE_ONCE(nohz.has_blocked, 0);
 
 	for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
+		u64 t0, domain_cost;
+
+		t0 = sched_clock_cpu(this_cpu);
+
 		if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
 			continue;
 
@@ -9444,6 +9459,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 			goto abort;
 		}
 
+		/*
+		 * If the update is done while CPU becomes idle, we abort
+		 * the update when its cost is higher than the average idle
+		 * time in orde to not delay a possible wake up.
+		 */
+		if (idle == CPU_NEWLY_IDLE && this_rq->avg_idle < curr_cost) {
+			has_blocked_load = true;
+			goto abort;
+		}
+
 		rq = cpu_rq(balance_cpu);
 
 		update_blocked_averages(rq->cpu);
@@ -9456,10 +9481,10 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 		if (time_after_eq(jiffies, rq->next_balance)) {
 			struct rq_flags rf;
 
-			rq_lock_irq(rq, &rf);
+			rq_lock_irqsave(rq, &rf);
 			update_rq_clock(rq);
 			cpu_load_update_idle(rq);
-			rq_unlock_irq(rq, &rf);
+			rq_unlock_irqrestore(rq, &rf);
 
 			if (flags & NOHZ_BALANCE_KICK)
 				rebalance_domains(rq, CPU_IDLE);
@@ -9469,15 +9494,27 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 			next_balance = rq->next_balance;
 			update_next_balance = 1;
 		}
+
+		domain_cost = sched_clock_cpu(this_cpu) - t0;
+		curr_cost += domain_cost;
+
+	}
+
+	/* Newly idle CPU doesn't need an update */
+	if (idle != CPU_NEWLY_IDLE) {
+		update_blocked_averages(this_cpu);
+		has_blocked_load |= this_rq->has_blocked_load;
 	}
 
-	update_blocked_averages(this_cpu);
 	if (flags & NOHZ_BALANCE_KICK)
 		rebalance_domains(this_rq, CPU_IDLE);
 
 	WRITE_ONCE(nohz.next_blocked,
 		now + msecs_to_jiffies(LOAD_AVG_PERIOD));
 
+	/* The full idle balance loop has been done */
+	ret = true;
+
 abort:
 	/* There is still blocked load, enable periodic update */
 	if (has_blocked_load)
@@ -9491,6 +9528,35 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 	if (likely(update_next_balance))
 		nohz.next_balance = next_balance;
 
+	return ret;
+}
+
+/*
+ * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
+ * rebalancing for all the cpus for whom scheduler ticks are stopped.
+ */
+static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
+{
+	int this_cpu = this_rq->cpu;
+	unsigned int flags;
+
+	if (!(atomic_read(nohz_flags(this_cpu)) & NOHZ_KICK_MASK))
+		return false;
+
+	if (idle != CPU_IDLE) {
+		atomic_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
+		return false;
+	}
+
+	/*
+	 * barrier, pairs with nohz_balance_enter_idle(), ensures ...
+	 */
+	flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
+	if (!(flags & NOHZ_KICK_MASK))
+		return false;
+
+	_nohz_idle_balance(this_rq, flags, idle);
+
 	return true;
 }
 #else
-- 
2.7.4