linux-kernel - [PATCH 17/19] sched/fair: Disable cache aware scheduling for processes with high thread counts

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Message-Id: <a098a60d9b4fc8ccea3392096f8bb0cf03af070b.1760206683.git.tim.c.chen@linux.intel.com>
Date: Sat, 11 Oct 2025 11:24:54 -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: Chen Yu <yu.c.chen@...el.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>,
	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>,
	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 17/19] sched/fair: Disable cache aware scheduling for processes with high thread counts

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

If the number of active threads within the process
exceeds the number of Cores(divided by SMTs number)
in the LLC, do not enable cache-aware scheduling.
This is because there is a risk of cache contention
within the preferred LLC when too many threads are
present.

Reported-by: K Prateek Nayak <kprateek.nayak@....com>
Signed-off-by: Chen Yu <yu.c.chen@...el.com>
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 kernel/sched/fair.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 79d109f8a09f..6b8eace79eee 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1240,6 +1240,18 @@ static inline int pref_llc_idx(struct task_struct *p)
 	return llc_idx(p->preferred_llc);
 }
 
+static bool exceed_llc_nr(struct mm_struct *mm, int cpu)
+{
+	int smt_nr = 1;
+
+#ifdef CONFIG_SCHED_SMT
+	if (sched_smt_active())
+		smt_nr = cpumask_weight(cpu_smt_mask(cpu));
+#endif
+
+	return ((mm->nr_running_avg * smt_nr) > per_cpu(sd_llc_size, cpu));
+}
+
 static void account_llc_enqueue(struct rq *rq, struct task_struct *p)
 {
 	int pref_llc;
@@ -1385,10 +1397,12 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
 
 	/*
 	 * If this task hasn't hit task_cache_work() for a while, or it
-	 * has only 1 thread, invalidate its preferred state.
+	 * has only 1 thread, or has too many active threads, invalidate
+	 * its preferred state.
 	 */
 	if (epoch - READ_ONCE(mm->mm_sched_epoch) > EPOCH_LLC_AFFINITY_TIMEOUT ||
-	    get_nr_threads(p) <= 1) {
+	    get_nr_threads(p) <= 1 ||
+	    exceed_llc_nr(mm, cpu_of(rq))) {
 		if (mm->mm_sched_cpu != -1)
 			mm->mm_sched_cpu = -1;
 	}
@@ -1467,6 +1481,11 @@ static void __no_profile task_cache_work(struct callback_head *work)
 	if (p->flags & PF_EXITING)
 		return;
 
+	if (get_nr_threads(p) <= 1) {
+		mm->mm_sched_cpu = -1;
+		return;
+	}
+
 	if (!zalloc_cpumask_var(&cpus, GFP_KERNEL))
 		return;
 
@@ -9826,6 +9845,10 @@ static enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
 	if (cpu < 0 || cpus_share_cache(src_cpu, dst_cpu))
 		return mig_unrestricted;
 
+	 /* skip cache aware load balance for single/too many threads */
+	if (get_nr_threads(p) <= 1 || exceed_llc_nr(mm, dst_cpu))
+		return mig_unrestricted;
+
 	if (cpus_share_cache(dst_cpu, cpu))
 		to_pref = true;
 	else if (cpus_share_cache(src_cpu, cpu))
-- 
2.32.0