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SubjectRe: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask in sched domain
Hi, Aubrey

On Fri, 11 Sep 2020 at 23:48, Aubrey Li <aubrey.li@intel.com> wrote:
>
> Added idle cpumask to track idle cpus in sched domain. When a CPU
> enters idle, its corresponding bit in the idle cpumask will be set,
> and when the CPU exits idle, its bit will be cleared.
>
> When a task wakes up to select an idle cpu, scanning idle cpumask
> has low cost than scanning all the cpus in last level cache domain,
> especially when the system is heavily loaded.
>
> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
> ---
> include/linux/sched/topology.h | 13 +++++++++++++
> kernel/sched/fair.c | 4 +++-
> kernel/sched/topology.c | 2 +-
> 3 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index fb11091129b3..43a641d26154 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -65,8 +65,21 @@ struct sched_domain_shared {
> atomic_t ref;
> atomic_t nr_busy_cpus;
> int has_idle_cores;
> + /*
> + * Span of all idle CPUs in this domain.
> + *
> + * NOTE: this field is variable length. (Allocated dynamically
> + * by attaching extra space to the end of the structure,
> + * depending on how many CPUs the kernel has booted up with)
> + */
> + unsigned long idle_cpus_span[];
> };
>
> +static inline struct cpumask *sds_idle_cpus(struct sched_domain_shared *sds)
> +{
> + return to_cpumask(sds->idle_cpus_span);
> +}
> +
> struct sched_domain {
> /* These fields must be setup */
> struct sched_domain __rcu *parent; /* top domain must be null terminated */
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6b3b59cc51d6..3b6f8a3589be 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6136,7 +6136,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>
> time = cpu_clock(this);
>
> - cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> + cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);
Is the sds_idle_cpus() always empty if nohz=off?
Do we need to initialize the idle_cpus_span with sched_domain_span(sd)?

>
> for_each_cpu_wrap(cpu, cpus, target) {
> if (!--nr)
> @@ -10182,6 +10182,7 @@ static void set_cpu_sd_state_busy(int cpu)
> sd->nohz_idle = 0;
>
> atomic_inc(&sd->shared->nr_busy_cpus);
> + cpumask_clear_cpu(cpu, sds_idle_cpus(sd->shared));
> unlock:
> rcu_read_unlock();
> }
> @@ -10212,6 +10213,7 @@ static void set_cpu_sd_state_idle(int cpu)
> sd->nohz_idle = 1;
>
> atomic_dec(&sd->shared->nr_busy_cpus);
> + cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
This only works when entering/exiting tickless mode? :)
Why not update idle_cpus_span during tick_nohz_idle_enter()/exit()?

Thx.
Regards,
Jiang

> unlock:
> rcu_read_unlock();
> }
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 9079d865a935..92d0aeef86bf 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1769,7 +1769,7 @@ static int __sdt_alloc(const struct cpumask *cpu_map)
>
> *per_cpu_ptr(sdd->sd, j) = sd;
>
> - sds = kzalloc_node(sizeof(struct sched_domain_shared),
> + sds = kzalloc_node(sizeof(struct sched_domain_shared) + cpumask_size(),
> GFP_KERNEL, cpu_to_node(j));
> if (!sds)
> return -ENOMEM;
> --
> 2.25.1
>

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