Messages in this thread |  | | From | Vincent Guittot <> | | Date | Wed, 6 May 2026 12:29:10 +0200 | | Subject | Re: [PATCH 3/5] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection |
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On Tue, 28 Apr 2026 at 16:44, Andrea Righi <arighi@nvidia.com> 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. > > Introduce SMT awareness in the asym-capacity idle selection policy: when > SMT is active, always prefer fully-idle SMT cores over partially-idle > ones. > > Prioritizing fully-idle SMT cores yields better task placement because > the effective capacity of partially-idle SMT cores is reduced; always > preferring them when available leads to more accurate capacity usage on > task wakeup. > > On an SMT system with asymmetric CPU capacities, SMT-aware idle > selection has been shown to improve throughput by around 15-18% for > CPU-bound workloads, running an amount of tasks equal to the amount of > SMT cores. > > 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> > Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com> > Reported-by: Felix Abecassis <fabecassis@nvidia.com> > Signed-off-by: Andrea Righi <arighi@nvidia.com> > --- > kernel/sched/fair.c | 70 +++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 65 insertions(+), 5 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index bbdf537f61154..6a7e4943804b5 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7989,6 +7989,22 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool > return idle_cpu; > } > > +/* > + * Idle-capacity scan ranks transformed util_fits_cpu() outcomes; lower values > + * are more preferred (see select_idle_capacity()). > + */ > +enum asym_fits_state { > + /* In descending order of preference */ > + ASYM_IDLE_CORE_UCLAMP_MISFIT = -4, > + ASYM_IDLE_CORE_COMPLETE_MISFIT, > + ASYM_IDLE_THREAD_FITS, > + ASYM_IDLE_THREAD_UCLAMP_MISFIT, > + ASYM_IDLE_COMPLETE_MISFIT, > + > + /* util_fits_cpu() bias for an idle core. */ > + ASYM_IDLE_CORE_BIAS = -3, > +}; > + > /* > * Scan the asym_capacity domain for idle CPUs; pick the first idle one on which > * the task fits. If no CPU is big enough, but there are idle ones, try to > @@ -7997,8 +8013,9 @@ 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); > unsigned long task_util, util_min, util_max, best_cap = 0; > - int fits, best_fits = 0; > + int fits, best_fits = ASYM_IDLE_COMPLETE_MISFIT; > int cpu, best_cpu = -1; > struct cpumask *cpus; > > @@ -8010,6 +8027,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target) > util_max = uclamp_eff_value(p, UCLAMP_MAX); > > for_each_cpu_wrap(cpu, cpus, target) { > + bool preferred_core = !prefers_idle_core || is_core_idle(cpu); > unsigned long cpu_cap = capacity_of(cpu); > > if (!choose_idle_cpu(cpu, p)) > @@ -8018,7 +8036,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target) > fits = util_fits_cpu(task_util, util_min, util_max, cpu); > > /* This CPU fits with all requirements */ > - if (fits > 0) > + if (fits > 0 && preferred_core) > return cpu; > /* > * Only the min performance hint (i.e. uclamp_min) doesn't fit. > @@ -8026,9 +8044,33 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target) > */ > else if (fits < 0) > cpu_cap = get_actual_cpu_capacity(cpu); > + /* > + * fits > 0 implies we are not on a preferred core > + * but the util fits CPU capacity. Set fits to ASYM_IDLE_THREAD_FITS > + * so the effective range becomes > + * [ASYM_IDLE_THREAD_FITS, ASYM_IDLE_COMPLETE_MISFIT], where: > + * ASYM_IDLE_COMPLETE_MISFIT - does not fit > + * ASYM_IDLE_THREAD_UCLAMP_MISFIT - fits with the exception of UCLAMP_MIN > + * ASYM_IDLE_THREAD_FITS - fits with the exception of preferred_core > + */ > + else if (fits > 0) > + fits = ASYM_IDLE_THREAD_FITS; > + > + /* > + * If we are on a preferred core, translate the range of fits > + * of [ASYM_IDLE_THREAD_UCLAMP_MISFIT, ASYM_IDLE_COMPLETE_MISFIT] to > + * [ASYM_IDLE_CORE_UCLAMP_MISFIT, ASYM_IDLE_CORE_COMPLETE_MISFIT]. > + * This ensures that an idle core is always given priority over > + * (partially) busy core. > + * > + * A fully fitting idle core would have returned early and hence > + * fits > 0 for preferred_core need not be dealt with. > + */ > + if (preferred_core) > + fits += ASYM_IDLE_CORE_BIAS;
It might be good to add a comment stating that if the system doesn't have SMT, prefers_idle_core and preferred_core are always true.
This is okay because CPU == Core in this case but the value differs from the default 0 or -1 of util_fits_cpu
> > /* > - * First, select CPU which fits better (-1 being better than 0). > + * First, select CPU which fits better (lower is more preferred). > * Then, select the one with best capacity at same level. > */ > if ((fits < best_fits) || > @@ -8039,6 +8081,19 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target) > } > } > > + /* > + * A value in the [ASYM_IDLE_CORE_UCLAMP_MISFIT, ASYM_IDLE_CORE_BIAS]
s/ASYM_IDLE_CORE_BIAS/ASYM_IDLE_CORE_COMPLETE_MISFIT/
ASYM_IDLE_CORE_BIAS is an offset to move an idle core that doesn't fully fit in the preferred range [ASYM_IDLE_CORE_UCLAMP_MISFIT, ASYM_IDLE_CORE_COMPLETE_MISFIT]
Keeping in mind that ASYM_IDLE_CORE_BIAS == -3 == ASYM_IDLE_CORE_BIAS
> + * range means the chosen CPU is in a fully idle SMT core. Values above > + * ASYM_IDLE_CORE_BIAS mean we never ranked such a CPU best.
s/ASYM_IDLE_CORE_BIAS/ASYM_IDLE_CORE_COMPLETE_MISFIT/
> + * > + * The asym-capacity wakeup path returns from select_idle_sibling() > + * after this function and never runs select_idle_cpu(), so the usual > + * select_idle_cpu() tail that clears idle cores must live here when the > + * idle-core preference did not win. > + */ > + if (prefers_idle_core && best_fits > ASYM_IDLE_CORE_BIAS)
s/ASYM_IDLE_CORE_BIAS/ASYM_IDLE_CORE_COMPLETE_MISFIT/
> + set_idle_cores(target, false); > + > return best_cpu; > } > > @@ -8047,12 +8102,17 @@ static inline bool asym_fits_cpu(unsigned long util, > unsigned long util_max, > int cpu) > { > - if (sched_asym_cpucap_active()) > + if (sched_asym_cpucap_active()) { > /* > * Return true only if the cpu fully fits the task requirements > * which include the utilization and the performance hints. > + * > + * When SMT is active, also require that the core has no busy > + * siblings. > */ > - return (util_fits_cpu(util, util_min, util_max, cpu) > 0); > + return (!sched_smt_active() || is_core_idle(cpu)) && > + (util_fits_cpu(util, util_min, util_max, cpu) > 0); > + } > > return true; > } > -- > 2.54.0 >
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