[PATCH] sched/topology: Avoid spurious asymmetry from CPU capacity noise

[Andrea Righi]

On some platforms, the firmware may expose per-CPU performance
differences (e.g., via ACPI CPPC highest_perf) even when the system is
effectively symmetric. These small variations, typically due to silicon
binning, are reflected in arch_scale_cpu_capacity() and end up being
interpreted as real capacity asymmetry.

As a result, the scheduler incorrectly enables SD_ASYM_CPUCAPACITY,
triggering asymmetry-specific behaviors, even though all CPUs have
comparable performance.

Prevent this by treating CPU capacities within 20% of the maximum value
as equivalent when building the asymmetry topology. This filters out
firmware noise, while preserving correct behavior on real heterogeneous
systems, where capacity differences are significantly larger.

Reported-by: Felix Abecassis <fabecassis@xxxxxxxxxx>
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/topology.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 061f8c85f5552..fe71ea9f3bda7 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1432,9 +1432,8 @@ static void free_asym_cap_entry(struct rcu_head *head)
 	kfree(entry);
 }
 
-static inline void asym_cpu_capacity_update_data(int cpu)
+static inline void asym_cpu_capacity_update_data(int cpu, unsigned long capacity)
 {
-	unsigned long capacity = arch_scale_cpu_capacity(cpu);
 	struct asym_cap_data *insert_entry = NULL;
 	struct asym_cap_data *entry;
 
@@ -1471,13 +1470,27 @@ static inline void asym_cpu_capacity_update_data(int cpu)
 static void asym_cpu_capacity_scan(void)
 {
 	struct asym_cap_data *entry, *next;
+	unsigned long max_cap = 0;
+	unsigned long capacity;
 	int cpu;
 
 	list_for_each_entry(entry, &asym_cap_list, link)
 		cpumask_clear(cpu_capacity_span(entry));
 
 	for_each_cpu_and(cpu, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN))
-		asym_cpu_capacity_update_data(cpu);
+		max_cap = max(max_cap, arch_scale_cpu_capacity(cpu));
+
+	/*
+	 * Treat small capacity differences (< 20% max capacity) as noise,
+	 * to prevent enabling SD_ASYM_CPUCAPACITY when it's not really
+	 * needed.
+	 */
+	for_each_cpu_and(cpu, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN)) {
+		capacity = arch_scale_cpu_capacity(cpu);
+		if (capacity * 5 >= max_cap * 4)
+			capacity = max_cap;
+		asym_cpu_capacity_update_data(cpu, capacity);
+	}
 
 	list_for_each_entry_safe(entry, next, &asym_cap_list, link) {
 		if (cpumask_empty(cpu_capacity_span(entry))) {
-- 
2.53.0