lkml.org 
[lkml]   [2026]   [May]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 5/5] sched/fair: Add SIS_UTIL support to select_idle_capacity()
On Tue, 28 Apr 2026 at 16:44, Andrea Righi <arighi@nvidia.com> wrote:
>
> From: K Prateek Nayak <kprateek.nayak@amd.com>
>
> Add to select_idle_capacity() the same SIS_UTIL-controlled idle-scan
> mechanism, already used by select_idle_cpu(): when sched_feat(SIS_UTIL)
> is enabled and the LLC domain has sched_domain_shared data, derive the
> per-attempt scan limit from sd->shared->nr_idle_scan.
>
> That bounds the walk on large LLCs and allows an early return once the
> scan limit is reached, if we already picked a sufficiently strong
> idle-core candidate (best_fits == ASYM_IDLE_CORE_UCLAMP_MISFIT).
>
> Co-developed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
> kernel/sched/fair.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a1f4d70f6b3d9..1cde3a9b1e0f5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8018,6 +8018,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> int fits, best_fits = ASYM_IDLE_COMPLETE_MISFIT;
> int cpu, best_cpu = -1;
> struct cpumask *cpus;
> + int nr = INT_MAX;
>
> cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
> cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> @@ -8026,10 +8027,28 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> util_min = uclamp_eff_value(p, UCLAMP_MIN);
> util_max = uclamp_eff_value(p, UCLAMP_MAX);
>
> + if (sched_feat(SIS_UTIL) && sd->shared) {
> + /*
> + * Same nr_idle_scan hint as select_idle_cpu(), nr only limits
> + * the scan when not preferring an idle core.
> + */
> + nr = READ_ONCE(sd->shared->nr_idle_scan) + 1;
> + /* overloaded domain is unlikely to have idle cpu/core */
> + if (nr == 1)
> + return -1;
> + }
> +
> for_each_cpu_wrap(cpu, cpus, target) {
> bool preferred_core = !prefers_idle_core || is_core_idle(cpu);
> unsigned long cpu_cap = capacity_of(cpu);
>
> + /*
> + * Good-enough early exit (mirrors select_idle_cpu() logic).
> + */
> + if (!prefers_idle_core &&
> + --nr <= 0 && best_fits == ASYM_IDLE_CORE_UCLAMP_MISFIT)

With SMT, !prefers_idle_core implies that there is no idle core; Is
best_fits == ASYM_IDLE_CORE_UCLAMP_MISFIT really expected in such case
?

With !SMT, !prefers_idle_core is always true and we will bail out
early as expected


> + return best_cpu;
> +
> if (!choose_idle_cpu(cpu, p))
> continue;
>
> --
> 2.54.0
>

\
 
 \ /
  Last update: 2026-05-06 15:03    [W:0.189 / U:4.057 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and my Meterkast|Read the blog