linux-kernel - [PATCH 08/19] sched/fair: Introduce per runqueue task LLC preference counter

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Message-Id: <a002ffc53c06bfa0ef0700631b0cb5413bdbf06c.1760206683.git.tim.c.chen@linux.intel.com>
Date: Sat, 11 Oct 2025 11:24:45 -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>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	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>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Jianyong Wu <jianyong.wu@...look.com>,
	Yangyu Chen <cyy@...self.name>,
	Tingyin Duan <tingyin.duan@...il.com>,
	Vern Hao <vernhao@...cent.com>,
	Len Brown <len.brown@...el.com>,
	Aubrey Li <aubrey.li@...el.com>,
	Zhao Liu <zhao1.liu@...el.com>,
	Chen Yu <yu.chen.surf@...il.com>,
	Chen Yu <yu.c.chen@...el.com>,
	Libo Chen <libo.chen@...cle.com>,
	Adam Li <adamli@...amperecomputing.com>,
	Tim Chen <tim.c.chen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 08/19] sched/fair: Introduce per runqueue task LLC preference counter

Each runqueue is assigned a static array where each element tracks
the number of tasks preferring a given LLC, indexed from 0 to
NR_LLCS.

For example, rq->nr_pref_llc[3] = 2 signifies that there are 2 tasks on
this runqueue which prefer to run within LLC3 (indexed from 0 to NR_LLCS

The load balancer can use this information to identify busy runqueues
and migrate tasks to their preferred LLC domains.

Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 kernel/sched/fair.c  | 35 +++++++++++++++++++++++++++++++++++
 kernel/sched/sched.h |  1 +
 2 files changed, 36 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fd315937c0cf..b7a68fe7601b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1235,22 +1235,51 @@ static inline int llc_idx(int cpu)
 	return per_cpu(sd_llc_idx, cpu);
 }
 
+static inline int pref_llc_idx(struct task_struct *p)
+{
+	return llc_idx(p->preferred_llc);
+}
+
 static void account_llc_enqueue(struct rq *rq, struct task_struct *p)
 {
+	int pref_llc;
+
 	if (!sched_cache_enabled())
 		return;
 
 	rq->nr_llc_running += (p->preferred_llc != -1);
 	rq->nr_pref_llc_running += (p->preferred_llc == task_llc(p));
+
+	if (p->preferred_llc < 0)
+		return;
+
+	pref_llc = pref_llc_idx(p);
+	if (pref_llc < 0)
+		return;
+
+	++rq->nr_pref_llc[pref_llc];
 }
 
 static void account_llc_dequeue(struct rq *rq, struct task_struct *p)
 {
+	int pref_llc;
+
 	if (!sched_cache_enabled())
 		return;
 
 	rq->nr_llc_running -= (p->preferred_llc != -1);
 	rq->nr_pref_llc_running -= (p->preferred_llc == task_llc(p));
+
+	if (p->preferred_llc < 0)
+		return;
+
+	pref_llc = pref_llc_idx(p);
+	if (pref_llc < 0)
+		return;
+
+	/* avoid negative counter */
+	if (rq->nr_pref_llc[pref_llc] > 0)
+		--rq->nr_pref_llc[pref_llc];
 }
 
 void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu *_pcpu_sched)
@@ -1524,10 +1553,16 @@ void init_sched_mm(struct task_struct *p)
 
 void reset_llc_stats(struct rq *rq)
 {
+	int i = 0;
+
 	if (!sched_cache_enabled())
 		return;
 
 	rq->nr_llc_running = 0;
+
+	for (i = 0; i < max_llcs; ++i)
+		rq->nr_pref_llc[i] = 0;
+
 	rq->nr_pref_llc_running = 0;
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3ab64067acc6..b801d32d5fba 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1101,6 +1101,7 @@ struct rq {
 #ifdef CONFIG_SCHED_CACHE
 	unsigned int		nr_pref_llc_running;
 	unsigned int		nr_llc_running;
+	unsigned int		nr_pref_llc[NR_LLCS];
 #endif
 #ifdef CONFIG_NO_HZ_COMMON
 	unsigned long		last_blocked_load_update_tick;
-- 
2.32.0