linux-kernel - [PATCH v2 22/23] -- DO NOT APPLY!!! -- sched/cache/debug: Add ftrace to track the load balance statistics

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Message-Id: <445303c70d8d464c35c97f33d4be7b752e8db5ae.1764801860.git.tim.c.chen@linux.intel.com>
Date: Wed,  3 Dec 2025 15:07:41 -0800
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>,
	Vincent Guittot <vincent.guittot@...aro.org>
Cc: Chen Yu <yu.c.chen@...el.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>,
	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>,
	Vern Hao <haoxing990@...il.com>,
	Len Brown <len.brown@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aubrey Li <aubrey.li@...el.com>,
	Zhao Liu <zhao1.liu@...el.com>,
	Chen Yu <yu.chen.surf@...il.com>,
	Adam Li <adamli@...amperecomputing.com>,
	Aaron Lu <ziqianlu@...edance.com>,
	Tim Chen <tim.c.chen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 22/23] -- DO NOT APPLY!!! -- sched/cache/debug: Add ftrace to track the load balance statistics

From: Chen Yu <yu.c.chen@...el.com>

Debug patch only.

The user leverages this trace event (via bpftrace, etc)to monitor the cache
aware load balance activity - whether the tasks are moved to their preferred
LLC, or moved out of their preferred LLC.

Signed-off-by: Chen Yu <yu.c.chen@...el.com>
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 include/trace/events/sched.h | 31 +++++++++++++++++++++++++++++++
 kernel/sched/fair.c          | 10 ++++++++++
 2 files changed, 41 insertions(+)

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 7b2645b50e78..bd03f49f7e3c 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -10,6 +10,37 @@
 #include <linux/tracepoint.h>
 #include <linux/binfmts.h>
 
+TRACE_EVENT(sched_attach_task,
+
+	TP_PROTO(struct task_struct *t, int pref_cpu, int pref_llc,
+		 int attach_cpu, int attach_llc),
+
+	TP_ARGS(t, pref_cpu, pref_llc, attach_cpu, attach_llc),
+
+	TP_STRUCT__entry(
+			__array(	char,	comm,	TASK_COMM_LEN	)
+			__field(	pid_t,	pid			)
+			__field(	int,	pref_cpu		)
+			__field(	int,	pref_llc		)
+			__field(	int,	attach_cpu		)
+			__field(	int,	attach_llc		)
+	),
+
+	TP_fast_assign(
+		      memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
+		      __entry->pid	= t->pid;
+		      __entry->pref_cpu	= pref_cpu;
+		      __entry->pref_llc	= pref_llc;
+		      __entry->attach_cpu	= attach_cpu;
+		      __entry->attach_llc	= attach_llc;
+	),
+
+	TP_printk("comm=%s pid=%d pref_cpu=%d pref_llc=%d attach_cpu=%d attach_llc=%d",
+		  __entry->comm, __entry->pid,
+		  __entry->pref_cpu, __entry->pref_llc,
+		  __entry->attach_cpu, __entry->attach_llc)
+);
+
 /*
  * Tracepoint for calling kthread_stop, performed to end a kthread:
  */
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 742e455b093e..e47b4096f0a6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10487,6 +10487,16 @@ static void attach_task(struct rq *rq, struct task_struct *p)
 {
 	lockdep_assert_rq_held(rq);
 
+#ifdef CONFIG_SCHED_CACHE
+	if (p->mm) {
+		int pref_cpu = p->mm->mm_sched_cpu;
+
+		trace_sched_attach_task(p,
+					pref_cpu,
+					pref_cpu != -1 ? llc_id(pref_cpu) : -1,
+					cpu_of(rq), llc_id(cpu_of(rq)));
+	}
+#endif
 	WARN_ON_ONCE(task_rq(p) != rq);
 	activate_task(rq, p, ENQUEUE_NOCLOCK);
 	wakeup_preempt(rq, p, 0);
-- 
2.32.0