[Patch v4 06/22] sched/cache: Assign preferred LLC ID to processes

Tim Chen
2026-04-01T21:46:51Z

With cache-aware scheduling enabled, each task is assigned a
preferred LLC ID. This allows quick identification of the LLC domain
where the task prefers to run, similar to numa_preferred_nid in
NUMA balancing.

Co-developed-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---

Notes:
v3->v4:
Use WRITE_ONCE()/READ_ONCE() on p->preferred_llc
(Madadi Vineeth Reddy)

include/linux/sched.h | 1 +
init/init_task.c | 3 +++
kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++++++++++++
3 files changed, 47 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index bd33f5b9096b..526108acc483 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1408,6 +1408,7 @@ struct task_struct {

#ifdef CONFIG_SCHED_CACHE
struct callback_head cache_work;
+ int preferred_llc;
#endif

struct rseq_data rseq;
diff --git a/init/init_task.c b/init/init_task.c
index 5c838757fc10..9f964898d40e 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -214,6 +214,9 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = {
.numa_group = NULL,
.numa_faults = NULL,
#endif
+#ifdef CONFIG_SCHED_CACHE
+ .preferred_llc = -1,
+#endif
#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
.kasan_depth = 1,
#endif
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6244443ecdc0..1eda689e0136 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1366,11 +1366,43 @@ static unsigned long fraction_mm_sched(struct rq *rq,
return div64_u64(NICE_0_LOAD * pcpu_sched->runtime, rq->cpu_runtime + 1);
}

+static int get_pref_llc(struct task_struct *p, struct mm_struct *mm)
+{
+ int mm_sched_llc = -1;
+
+ if (!mm)
+ return -1;
+
+ if (mm->sc_stat.cpu != -1) {
+ mm_sched_llc = llc_id(mm->sc_stat.cpu);
+
+#ifdef CONFIG_NUMA_BALANCING
+ /*
+ * Don't assign preferred LLC if it
+ * conflicts with NUMA balancing.
+ * This can happen when sched_setnuma() gets
+ * called, however it is not much of an issue
+ * because we expect account_mm_sched() to get
+ * called fairly regularly -- at a higher rate
+ * than sched_setnuma() at least -- and thus the
+ * conflict only exists for a short period of time.
+ */
+ if (static_branch_likely(&sched_numa_balancing) &&
+ p->numa_preferred_nid >= 0 &&
+ cpu_to_node(mm->sc_stat.cpu) != p->numa_preferred_nid)
+ mm_sched_llc = -1;
+#endif
+ }
+
+ return mm_sched_llc;
+}
+
static inline
void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
{
struct sched_cache_time *pcpu_sched;
struct mm_struct *mm = p->mm;
+ int mm_sched_llc = -1;
unsigned long epoch;

if (!sched_cache_enabled())
@@ -1404,6 +1436,11 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
if (mm->sc_stat.cpu != -1)
mm->sc_stat.cpu = -1;
}
+
+ mm_sched_llc = get_pref_llc(p, mm);
+
+ if (READ_ONCE(p->preferred_llc) != mm_sched_llc)
+ WRITE_ONCE(p->preferred_llc, mm_sched_llc);
}

static void task_tick_cache(struct rq *rq, struct task_struct *p)
@@ -1577,6 +1614,12 @@ void init_sched_mm(struct task_struct *p) { }

static void task_tick_cache(struct rq *rq, struct task_struct *p) { }

+static inline int get_pref_llc(struct task_struct *p,
+ struct mm_struct *mm)
+{
+ return -1;
+}
+
#endif /* CONFIG_SCHED_CACHE */

/*
--
2.32.0