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

[Vincent Guittot]

On Tue, 24 Mar 2026 at 01:55, Andrea Righi <arighi@xxxxxxxxxx> wrote:
>
> 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

20% is a bit high, my snapdragon rb5 has a mid CPU with a capacity of
871 but we still want to keep them different

Why would 5% not be enough?



> 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
>