linux-kernel - [RFC patch v3 05/20] sched: Add hysteresis to switch a task's preferred LLC

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Message-Id: <7371f30196b317c0c5a0ae3fa463ec76a4dc69ef.1750268218.git.tim.c.chen@linux.intel.com>
Date: Wed, 18 Jun 2025 11:27:53 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	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>,
	Tim Chen <tim.c.chen@...el.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Libo Chen <libo.chen@...cle.com>,
	Abel Wu <wuyun.abel@...edance.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>,
	Len Brown <len.brown@...el.com>,
	linux-kernel@...r.kernel.org,
	Chen Yu <yu.c.chen@...el.com>
Subject: [RFC patch v3 05/20] sched: Add hysteresis to switch a task's preferred LLC

Switching a process's preferred LLC generates lots of task
migrations across LLCs. To avoid frequent switches
of home LLC, implement the following policy:

1. Require a 2x occ change threshold to switch preferred LLC
2. Don't discard preferred LLC for a task

Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 kernel/sched/fair.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6a2678f9d44a..7fb2322c5d9e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1175,6 +1175,14 @@ static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
 #define EPOCH_PERIOD	(HZ/100)	/* 10 ms */
 #define EPOCH_OLD	5		/* 50 ms */
 
+static int llc_id(int cpu)
+{
+	if (cpu < 0)
+		return -1;
+
+	return per_cpu(sd_llc_id, cpu);
+}
+
 void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu *_pcpu_sched)
 {
 	unsigned long epoch;
@@ -1299,6 +1307,7 @@ static void task_cache_work(struct callback_head *work)
 	struct task_struct *p = current;
 	struct mm_struct *mm = p->mm;
 	unsigned long m_a_occ = 0;
+	unsigned long last_m_a_occ = 0;
 	int cpu, m_a_cpu = -1;
 	cpumask_var_t cpus;
 
@@ -1337,11 +1346,13 @@ static void task_cache_work(struct callback_head *work)
 					     per_cpu(sd_llc_id, i), occ, m_occ, m_cpu, nr);
 			}
 
-			a_occ /= nr;
+			// a_occ /= nr;
 			if (a_occ > m_a_occ) {
 				m_a_occ = a_occ;
 				m_a_cpu = m_cpu;
 			}
+			if (llc_id(cpu) == llc_id(mm->mm_sched_cpu))
+				last_m_a_occ = a_occ;
 
 			trace_printk("(%d) a_occ: %ld m_a_occ: %ld\n",
 				     per_cpu(sd_llc_id, cpu), a_occ, m_a_occ);
@@ -1355,13 +1366,10 @@ static void task_cache_work(struct callback_head *work)
 		}
 	}
 
-	/*
-	 * If the max average cache occupancy is 'small' we don't care.
-	 */
-	if (m_a_occ < (NICE_0_LOAD >> EPOCH_OLD))
-		m_a_cpu = -1;
-
-	mm->mm_sched_cpu = m_a_cpu;
+	if (m_a_occ > (2 * last_m_a_occ)) {
+		/* avoid the bouncing of mm_sched_cpu */
+		mm->mm_sched_cpu = m_a_cpu;
+	}
 
 	free_cpumask_var(cpus);
 }
-- 
2.32.0