linux-kernel - [PATCH 1/1] sched/fair: Update blocked averages on tick

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Message-Id: <20241011123222.1282936-2-pierre.gondois@arm.com>
Date: Fri, 11 Oct 2024 14:32:22 +0200
From: Pierre Gondois <pierre.gondois@....com>
To: linux-kernel@...r.kernel.org
Cc: Hongyan Xia <hongyan.xia2@....com>,
	Chritian Loehle <christian.loehle@....com>,
	Pierre Gondois <pierre.gondois@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>
Subject: [PATCH 1/1] sched/fair: Update blocked averages on tick

The Energy Aware Scheduler (EAS) relies on CPU/tasks utilization
signals. On an idle CPU, the blocked load is updated during
load balancing.

sd->balance_interval increases with the number of CPUs in the domain.
On an Arm DynamIQ system, sched domains containing CPUs with the same
capacity do not exist. On a Pixel6 with 4 little, 2 mid, 2 big CPUs:
- sd->min_interval = 8
- sd->min_interval = 16

The balance interval is doubled if the system is balanced, meaning
that a balanced system will likely update blocked load every 16ms.

The find_energy_efficient_cpu() function might thus relies on outdated
util signals to place tasks, leading to bad energy placement.

Update blocked load on sched tick if:
- the rq is idle
- the load balancer will not be triggered.

Signed-off-by: Pierre Gondois <pierre.gondois@....com>
---
 kernel/sched/fair.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 225b31aaee55..2f03bd10ac7a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9841,15 +9841,12 @@ static unsigned long task_h_load(struct task_struct *p)
 }
 #endif
 
-static void sched_balance_update_blocked_averages(int cpu)
+static void update_blocked_averages(struct rq *rq)
 {
 	bool decayed = false, done = true;
-	struct rq *rq = cpu_rq(cpu);
-	struct rq_flags rf;
 
-	rq_lock_irqsave(rq, &rf);
-	update_blocked_load_tick(rq);
 	update_rq_clock(rq);
+	update_blocked_load_tick(rq);
 
 	decayed |= __update_blocked_others(rq, &done);
 	decayed |= __update_blocked_fair(rq, &done);
@@ -9857,6 +9854,18 @@ static void sched_balance_update_blocked_averages(int cpu)
 	update_blocked_load_status(rq, !done);
 	if (decayed)
 		cpufreq_update_util(rq, 0);
+}
+
+static void sched_balance_update_blocked_averages(int cpu)
+{
+	struct rq *rq = cpu_rq(cpu);
+	struct cfs_rq *cfs_rq;
+	struct rq_flags rf;
+
+	cfs_rq = &rq->cfs;
+
+	rq_lock_irqsave(rq, &rf);
+	update_blocked_averages(rq);
 	rq_unlock_irqrestore(rq, &rf);
 }
 
@@ -12877,6 +12886,8 @@ void sched_balance_trigger(struct rq *rq)
 
 	if (time_after_eq(jiffies, rq->next_balance))
 		raise_softirq(SCHED_SOFTIRQ);
+	else if (idle_cpu(rq->cpu))
+		update_blocked_averages(rq);
 
 	nohz_balancer_kick(rq);
 }
-- 
2.25.1