[Patch v4 04/22] sched/cache: Introduce helper functions to enforce LLC migration policy

Tim Chen
2026-04-01T21:46:48Z

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

Cache-aware scheduling aggregates threads onto their preferred LLC,
mainly through load balancing. When the preferred LLC becomes
saturated, more threads are still placed there, increasing latency.
A mechanism is needed to limit aggregation so that the preferred LLC
does not become overloaded.

Introduce helper functions can_migrate_llc() and
can_migrate_llc_task() to enforce the LLC migration policy:

1. Aggregate a task to its preferred LLC if both source and
destination LLCs are not too busy, or if doing so will not
leave the preferred LLC much more imbalanced than the
non-preferred one (>20% utilization difference, a little
higher than the default imbalance_pct(17%) of the LLC domain
as hysteresis). Later this threshold will be turned into tunable
debugfs.
2. Allow moving a task from overloaded preferred LLC to a non
preferred LLC if this will not cause the non preferred LLC
to become too imbalanced to cause a later migration back.
3. If both LLCs are too busy, let the generic load balance to
spread the tasks.

Further (hysteresis)action could be taken in the future to prevent tasks
from being migrated into and out of the preferred LLC frequently (back and
forth): the threshold for migrating a task out of its preferred LLC should
be higher than that for migrating it into the LLC.

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

Notes:
v3->v4:
Moving the capacity check after the utilization adjustment to make
it consistent with the decision matrix(Madadi Vineeth Reddy).

Raise the default threshold in fits_llc_capacity() from 50%
to 75% for single-core system. Because single-core system
does not need to consider the capacity of SMT. This helps
further improve the performance base on the test conducted
on single-core system.

kernel/sched/fair.c | 167 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 167 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f6692d9de017..6244443ecdc0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9790,6 +9790,38 @@ static inline int task_is_ineligible_on_dst_cpu(struct task_struct *p, int dest_
}

#ifdef CONFIG_SCHED_CACHE
+/*
+ * The margin used when comparing LLC utilization with CPU capacity.
+ * It determines the LLC load level where active LLC aggregation is
+ * done.
+ * Derived from fits_capacity().
+ *
+ * (default: ~50%, tunable via debugfs)
+ */
+static bool fits_llc_capacity(unsigned long util, unsigned long max)
+{
+ u32 aggr_pct = 50;
+
+ /*
+ * For single core systems, raise the aggregation
+ * threshold to accommodate more tasks.
+ */
+ if (cpu_smt_num_threads == 1)
+ aggr_pct = (aggr_pct * 3 / 2);
+
+ return util * 100 < max * aggr_pct;
+}
+
+/*
+ * The margin used when comparing utilization.
+ * is 'util1' noticeably greater than 'util2'
+ * Derived from capacity_greater().
+ * Bias is in perentage.
+ */
+/* Allows dst util to be bigger than src util by up to bias percent */
+#define util_greater(util1, util2) \
+ ((util1) * 100 > (util2) * 120)
+
static __maybe_unused bool get_llc_stats(int cpu, unsigned long *util,
unsigned long *cap)
{
@@ -9804,6 +9836,141 @@ static __maybe_unused bool get_llc_stats(int cpu, unsigned long *util,

return true;
}
+
+/*
+ * Decision matrix according to the LLC utilization. To
+ * decide whether we can do task aggregation across LLC.
+ *
+ * By default, 50% is the threshold for treating the LLC
+ * as busy. The reason for choosing 50% is to avoid saturation
+ * of SMT-2, and it is also a safe cutoff for other SMT-n
+ * platforms. SMT-1 has higher threshold because it is
+ * supposed to accommodate more tasks, see fits_llc_capacity().
+ *
+ * 20% is the utilization imbalance percentage to decide
+ * if the preferred LLC is busier than the non-preferred LLC.
+ * 20 is a little higher than the LLC domain's imbalance_pct
+ * 17. The hysteresis is used to avoid task bouncing between the
+ * preferred LLC and the non-preferred LLC, and it will
+ * be turned into tunable debugfs.
+ *
+ * 1. moving towards the preferred LLC, dst is the preferred
+ * LLC, src is not.
+ *
+ * src \ dst 30% 40% 50% 60%
+ * 30% Y Y Y N
+ * 40% Y Y Y Y
+ * 50% Y Y G G
+ * 60% Y Y G G
+ *
+ * 2. moving out of the preferred LLC, src is the preferred
+ * LLC, dst is not:
+ *
+ * src \ dst 30% 40% 50% 60%
+ * 30% N N N N
+ * 40% N N N N
+ * 50% N N G G
+ * 60% Y N G G
+ *
+ * src : src_util
+ * dst : dst_util
+ * Y : Yes, migrate
+ * N : No, do not migrate
+ * G : let the Generic load balance to even the load.
+ *
+ * The intention is that if both LLCs are quite busy, cache aware
+ * load balance should not be performed, and generic load balance
+ * should take effect. However, if one is busy and the other is not,
+ * the preferred LLC capacity(50%) and imbalance criteria(20%) should
+ * be considered to determine whether LLC aggregation should be
+ * performed to bias the load towards the preferred LLC.
+ */
+
+/* migration decision, 3 states are orthogonal. */
+enum llc_mig {
+ mig_forbid = 0, /* N: Don't migrate task, respect LLC preference */
+ mig_llc, /* Y: Do LLC preference based migration */
+ mig_unrestricted /* G: Don't restrict generic load balance migration */
+};
+
+/*
+ * Check if task can be moved from the source LLC to the
+ * destination LLC without breaking cache aware preferrence.
+ * src_cpu and dst_cpu are arbitrary CPUs within the source
+ * and destination LLCs, respectively.
+ */
+static enum llc_mig can_migrate_llc(int src_cpu, int dst_cpu,
+ unsigned long tsk_util,
+ bool to_pref)
+{
+ unsigned long src_util, dst_util, src_cap, dst_cap;
+
+ if (!get_llc_stats(src_cpu, &src_util, &src_cap) ||
+ !get_llc_stats(dst_cpu, &dst_util, &dst_cap))
+ return mig_unrestricted;
+
+ src_util = src_util < tsk_util ? 0 : src_util - tsk_util;
+ dst_util = dst_util + tsk_util;
+
+ if (!fits_llc_capacity(dst_util, dst_cap) &&
+ !fits_llc_capacity(src_util, src_cap))
+ return mig_unrestricted;
+
+ if (to_pref) {
+ /*
+ * Don't migrate if we will get preferred LLC too
+ * heavily loaded and if the dest is much busier
+ * than the src, in which case migration will
+ * increase the imbalance too much.
+ */
+ if (!fits_llc_capacity(dst_util, dst_cap) &&
+ util_greater(dst_util, src_util))
+ return mig_forbid;
+ } else {
+ /*
+ * Don't migrate if we will leave preferred LLC
+ * too idle, or if this migration leads to the
+ * non-preferred LLC falls within sysctl_aggr_imb percent
+ * of preferred LLC, leading to migration again
+ * back to preferred LLC.
+ */
+ if (fits_llc_capacity(src_util, src_cap) ||
+ !util_greater(src_util, dst_util))
+ return mig_forbid;
+ }
+ return mig_llc;
+}
+
+/*
+ * Check if task p can migrate from source LLC to
+ * destination LLC in terms of cache aware load balance.
+ */
+static __maybe_unused enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
+ struct task_struct *p)
+{
+ struct mm_struct *mm;
+ bool to_pref;
+ int cpu;
+
+ mm = p->mm;
+ if (!mm)
+ return mig_unrestricted;
+
+ cpu = mm->sc_stat.cpu;
+ if (cpu < 0 || cpus_share_cache(src_cpu, dst_cpu))
+ return mig_unrestricted;
+
+ if (cpus_share_cache(dst_cpu, cpu))
+ to_pref = true;
+ else if (cpus_share_cache(src_cpu, cpu))
+ to_pref = false;
+ else
+ return mig_unrestricted;
+
+ return can_migrate_llc(src_cpu, dst_cpu,
+ task_util(p), to_pref);
+}
+
#else
static inline bool get_llc_stats(int cpu, unsigned long *util,
unsigned long *cap)
--
2.32.0