Re: [RFC PATCH v8] sched/fair: select idle cpu from idle cpumask for task wakeup

[Vincent Guittot]

On Thu, 10 Dec 2020 at 02:44, Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx> wrote:
>
> Add idle cpumask to track idle cpus in sched domain. Every time
> a CPU enters idle, the CPU is set in idle cpumask to be a wakeup
> target. And if the CPU is not in idle, the CPU is cleared in idle
> cpumask during scheduler tick to ratelimit idle cpumask update.
>
> When a task wakes up to select an idle cpu, scanning idle cpumask
> has lower cost than scanning all the cpus in last level cache domain,
> especially when the system is heavily loaded.
>
> Benchmarks including hackbench, schbench, uperf, sysbench mysql and
> kbuild have been tested on a x86 4 socket system with 24 cores per
> socket and 2 hyperthreads per core, total 192 CPUs, no regression
> found.
>
> v7->v8:
> - refine update_idle_cpumask, no functionality change
> - fix a suspicious RCU usage warning with CONFIG_PROVE_RCU=y
>
> v6->v7:
> - place the whole idle cpumask mechanism under CONFIG_SMP
>
> v5->v6:
> - decouple idle cpumask update from stop_tick signal, set idle CPU
>   in idle cpumask every time the CPU enters idle
>
> v4->v5:
> - add update_idle_cpumask for s2idle case
> - keep the same ordering of tick_nohz_idle_stop_tick() and update_
>   idle_cpumask() everywhere
>
> v3->v4:
> - change setting idle cpumask from every idle entry to tickless idle
>   if cpu driver is available
> - move clearing idle cpumask to scheduler_tick to decouple nohz mode
>
> v2->v3:
> - change setting idle cpumask to every idle entry, otherwise schbench
>   has a regression of 99th percentile latency
> - change clearing idle cpumask to nohz_balancer_kick(), so updating
>   idle cpumask is ratelimited in the idle exiting path
> - set SCHED_IDLE cpu in idle cpumask to allow it as a wakeup target
>
> v1->v2:
> - idle cpumask is updated in the nohz routines, by initializing idle
>   cpumask with sched_domain_span(sd), nohz=off case remains the original
>   behavior
>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Qais Yousef <qais.yousef@xxxxxxx>
> Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> Cc: Jiang Biao <benbjiang@xxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>

This version looks good to me. I don't see regressions of v5 anymore
and see some improvements on heavy cases

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
>  include/linux/sched/topology.h | 13 ++++++++++
>  kernel/sched/core.c            |  2 ++
>  kernel/sched/fair.c            | 45 +++++++++++++++++++++++++++++++++-
>  kernel/sched/idle.c            |  5 ++++
>  kernel/sched/sched.h           |  4 +++
>  kernel/sched/topology.c        |  3 ++-
>  6 files changed, 70 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 820511289857..b47b85163607 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/core.c b/kernel/sched/core.c
> index c4da7e17b906..b136e2440ea4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4011,6 +4011,7 @@ void scheduler_tick(void)
>
>  #ifdef CONFIG_SMP
>         rq->idle_balance = idle_cpu(cpu);
> +       update_idle_cpumask(cpu, rq->idle_balance);
>         trigger_load_balance(rq);
>  #endif
>  }
> @@ -7186,6 +7187,7 @@ void __init sched_init(void)
>                 rq->idle_stamp = 0;
>                 rq->avg_idle = 2*sysctl_sched_migration_cost;
>                 rq->max_idle_balance_cost = sysctl_sched_migration_cost;
> +               rq->last_idle_state = 1;
>
>                 INIT_LIST_HEAD(&rq->cfs_tasks);
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c0c4d9ad7da8..25f36ecfee54 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6146,7 +6146,12 @@ 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);
> +       /*
> +        * sched_domain_shared is set only at shared cache level,
> +        * this works only because select_idle_cpu is called with
> +        * sd_llc.
> +        */
> +       cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);
>
>         for_each_cpu_wrap(cpu, cpus, target) {
>                 if (!--nr)
> @@ -6806,6 +6811,44 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>
>         return newidle_balance(rq, rf) != 0;
>  }
> +
> +/*
> + * Update cpu idle state and record this information
> + * in sd_llc_shared->idle_cpus_span.
> + *
> + * This function is called with interrupts disabled.
> + */
> +void update_idle_cpumask(int cpu, bool idle)
> +{
> +       struct sched_domain *sd;
> +       struct rq *rq = cpu_rq(cpu);
> +       int idle_state;
> +
> +       /*
> +        * Also set SCHED_IDLE cpu in idle cpumask to
> +        * allow SCHED_IDLE cpu as a wakeup target.
> +        */
> +       idle_state = idle || sched_idle_cpu(cpu);
> +       /*
> +        * No need to update idle cpumask if the state
> +        * does not change.
> +        */
> +       if (rq->last_idle_state == idle_state)
> +               return;
> +       /*
> +        * Called with irq disabled, rcu protection is not needed.
> +        */
> +       sd = per_cpu(sd_llc, cpu);
> +       if (unlikely(!sd))
> +               return;
> +
> +       if (idle_state)
> +               cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
> +       else
> +               cpumask_clear_cpu(cpu, sds_idle_cpus(sd->shared));
> +
> +       rq->last_idle_state = idle_state;
> +}
>  #endif /* CONFIG_SMP */
>
>  static unsigned long wakeup_gran(struct sched_entity *se)
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index f324dc36fc43..2c517d6a061a 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -257,6 +257,11 @@ static void do_idle(void)
>                         cpuhp_report_idle_dead();
>                         arch_cpu_idle_dead();
>                 }
> +               /*
> +                * The CPU is about to go idle, set it in idle cpumask
> +                * to be a wake up target.
> +                */
> +               update_idle_cpumask(cpu, true);
>
>                 arch_cpu_idle_enter();
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 8d1ca65db3b0..4041d5a10de5 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -976,6 +976,7 @@ struct rq {
>
>         unsigned char           nohz_idle_balance;
>         unsigned char           idle_balance;
> +       unsigned char           last_idle_state;
>
>         unsigned long           misfit_task_load;
>
> @@ -1516,6 +1517,8 @@ static inline unsigned int group_first_cpu(struct sched_group *group)
>
>  extern int group_balance_cpu(struct sched_group *sg);
>
> +void update_idle_cpumask(int cpu, bool idle);
> +
>  #if defined(CONFIG_SCHED_DEBUG) && defined(CONFIG_SYSCTL)
>  void register_sched_domain_sysctl(void);
>  void dirty_sched_domain_sysctl(int cpu);
> @@ -1536,6 +1539,7 @@ extern void flush_smp_call_function_from_idle(void);
>
>  #else /* !CONFIG_SMP: */
>  static inline void flush_smp_call_function_from_idle(void) { }
> +static inline void update_idle_cpumask(int cpu, bool idle) { }
>  #endif
>
>  #include "stats.h"
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 1bd7e3af904f..541bd3a7de4d 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1407,6 +1407,7 @@ sd_init(struct sched_domain_topology_level *tl,
>                 sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
>                 atomic_inc(&sd->shared->ref);
>                 atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
> +               cpumask_copy(sds_idle_cpus(sd->shared), sched_domain_span(sd));
>         }
>
>         sd->private = sdd;
> @@ -1769,7 +1770,7 @@ 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
>