linux-kernel - [PATCH 7/7] Propagate negative bias

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Date: Mon, 24 Jun 2024 11:23:56 +0100
From: Hongyan Xia <hongyan.xia2@....com>
To: Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>
Cc: Qais Yousef <qyousef@...alina.io>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Lukasz Luba <lukasz.luba@....com>,
	Christian Loehle <christian.loehle@....com>,
	Pierre Gondois <pierre.gondois@....com>,
	Youssef Esmat <youssefesmat@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 7/7] Propagate negative bias

Negative bias is interesting, because dequeuing such a task will
actually increase utilization.

Solve by applying PELT decay to negative biases as well. This in fact
can be implemented easily with some math tricks.

Signed-off-by: Hongyan Xia <hongyan.xia2@....com>
---
 kernel/sched/fair.c  | 40 ++++++++++++++++++++++++++++++++++++++++
 kernel/sched/sched.h |  4 ++++
 2 files changed, 44 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3bb077df52ae..d09af6abf464 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4878,6 +4878,45 @@ static inline unsigned long root_cfs_util_uclamp(struct rq *rq)
 
 	return max(ret, 0L);
 }
+
+/*
+ * Negative biases are tricky. If we remove them right away then dequeuing a
+ * uclamp_max task has the interesting effect that dequeuing results in a higher
+ * rq utilization. Solve this by applying PELT decay to the bias itself.
+ *
+ * Keeping track of a PELT-decayed negative bias is extra overhead. However, we
+ * observe this interesting math property, where y is the decay factor and p is
+ * the number of periods elapsed:
+ *
+ *	util_new = util_old * y^p - neg_bias * y^p
+ *		 = (util_old - neg_bias) * y^p
+ *
+ * Therefore, we simply subtract the negative bias from util_avg the moment we
+ * dequeue, then the PELT signal itself is the total of util_avg and the decayed
+ * negative bias, and we no longer need to track the decayed bias separately.
+ */
+static void propagate_negative_bias(struct task_struct *p)
+{
+	if (task_util_bias(p) < 0 && !task_on_rq_migrating(p)) {
+		unsigned long neg_bias = -task_util_bias(p);
+		struct sched_entity *se = &p->se;
+		struct cfs_rq *cfs_rq;
+
+		p->se.avg.util_avg_bias = 0;
+
+		for_each_sched_entity(se) {
+			u32 divider, neg_sum;
+
+			cfs_rq = cfs_rq_of(se);
+			divider = get_pelt_divider(&cfs_rq->avg);
+			neg_sum = neg_bias * divider;
+			sub_positive(&se->avg.util_avg, neg_bias);
+			sub_positive(&se->avg.util_sum, neg_sum);
+			sub_positive(&cfs_rq->avg.util_avg, neg_bias);
+			sub_positive(&cfs_rq->avg.util_sum, neg_sum);
+		}
+	}
+}
 #else
 static inline long task_util_bias(struct task_struct *p)
 {
@@ -6869,6 +6908,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	/* At this point se is NULL and we are at root level*/
 	sub_nr_running(rq, 1);
 	util_bias_dequeue(rq, p);
+	propagate_negative_bias(p);
 
 	/* balance early to pull high priority tasks */
 	if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 59e5ea421a4c..9d14ef9c717e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3140,6 +3140,10 @@ static inline void util_bias_dequeue(struct rq *rq, struct task_struct *p)
 {
 }
 
+static inline void propagate_negative_bias(struct task_struct *p)
+{
+}
+
 #endif /* !CONFIG_UCLAMP_TASK */
 
 #ifdef CONFIG_HAVE_SCHED_AVG_IRQ
-- 
2.34.1