Re: [PATCH 1/2] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 03.04.26 07:31, Andrea Righi wrote:
> On systems with asymmetric CPU capacity (e.g., ACPI/CPPC reporting
> different per-core frequencies), the wakeup path uses
> select_idle_capacity() and prioritizes idle CPUs with higher capacity
> for better task placement.
> 
> However, when those CPUs belong to SMT cores, their effective capacity
> can be much lower than the nominal capacity when the sibling thread is
> busy: SMT siblings compete for shared resources, so a "high capacity"
> CPU that is idle but whose sibling is busy does not deliver its full
> capacity. This effective capacity reduction cannot be modeled by the
> static capacity value alone.
> 
> When SMT is active, teach asym-capacity idle selection to treat a
> logical CPU as a weaker target if its physical core is only partially
> idle: select_idle_capacity() no longer returns on the first idle CPU
> whose static capacity fits the task when that CPU still has a busy
> sibling, it keeps scanning for an idle CPU on a fully-idle core and only
> if none qualify does it fall back to partially-idle cores, using shifted
> fit scores so fully-idle cores win ties; asym_fits_cpu() applies the
> same fully-idle core requirement when asym capacity and SMT are both
> active.
> 
> This improves task placement, since partially-idle SMT siblings deliver
> less than their nominal capacity. Favoring fully idle cores, when
> available, can significantly enhance both throughput and wakeup latency
> on systems with both SMT and CPU asymmetry.
> 
> No functional changes on systems with only asymmetric CPUs or only SMT.
> 
> Cc: K Prateek Nayak <kprateek.nayak@amd.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Christian Loehle <christian.loehle@arm.com>
> Cc: Koba Ko <kobak@nvidia.com>
> Reported-by: Felix Abecassis <fabecassis@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/fair.c | 36 ++++++++++++++++++++++++++++++++----
>  1 file changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bf948db905ed1..7f09191014d18 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7774,6 +7774,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  static int
>  select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  {
> +	bool prefers_idle_core = sched_smt_active() && test_idle_cores(target);

Somehow I miss a:

    if (prefers_idle_core)
        set_idle_cores(target, false)

The one in select_idle_sibling() -> select_idle_cpu() isn't executed
anymore in with ASYM_CPUCAPACITY.


Another thing is that sic() iterates over CPUs sd_asym_cpucapacity
whereas the idle core thing lives in sd_llc/sd_llc_shared. Both sd's are
probably th same on your system.


>  	unsigned long task_util, util_min, util_max, best_cap = 0;
>  	int fits, best_fits = 0;
>  	int cpu, best_cpu = -1;
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