Re: [RFC PATCH 5/6] sched/fair: trigger the update of blocked load on newly idle cpu

[Valentin Schneider <valentin.schneider@arm.com>]

On 05/02/21 12:48, Vincent Guittot wrote:
> Instead of waking up a random and already idle CPU, we can take advantage
> of this_cpu being about to enter idle to run the ILB and update the
> blocked load.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  include/linux/sched/nohz.h |  2 ++
>  kernel/sched/fair.c        | 11 ++++++++---
>  kernel/sched/idle.c        |  6 ++++++
>  3 files changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/sched/nohz.h b/include/linux/sched/nohz.h
> index 6d67e9a5af6b..74cdc4e87310 100644
> --- a/include/linux/sched/nohz.h
> +++ b/include/linux/sched/nohz.h
> @@ -9,8 +9,10 @@
>  #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
>  extern void nohz_balance_enter_idle(int cpu);
>  extern int get_nohz_timer_target(void);
> +extern void nohz_run_idle_balance(int cpu);
>  #else
>  static inline void nohz_balance_enter_idle(int cpu) { }
> +static inline void nohz_run_idle_balance(int cpu) { }
>  #endif
>
>  #ifdef CONFIG_NO_HZ_COMMON
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 935594cd5430..3d2ab28d5736 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10461,6 +10461,11 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>       return true;
>  }
>
> +void nohz_run_idle_balance(int cpu)
> +{
> +	nohz_idle_balance(cpu_rq(cpu), CPU_IDLE);
> +}
> +
>  static void nohz_newidle_balance(struct rq *this_rq)
>  {
>       int this_cpu = this_rq->cpu;
> @@ -10482,10 +10487,10 @@ static void nohz_newidle_balance(struct rq *this_rq)
>               return;
>
>       /*
> -	 * Blocked load of idle CPUs need to be updated.
> -	 * Kick an ILB to update statistics.
> +	 * Set the need to trigger ILB in order to update blocked load
> +	 * before entering idle state.
>        */
> -	kick_ilb(NOHZ_STATS_KICK);
> +	this_rq->nohz_idle_balance = NOHZ_STATS_KICK;
>  }
>
>  #else /* !CONFIG_NO_HZ_COMMON */
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 305727ea0677..52a4e9ce2f9b 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -261,6 +261,12 @@ static void cpuidle_idle_call(void)
>  static void do_idle(void)
>  {
>       int cpu = smp_processor_id();
> +
> +	/*
> +	 * Check if we need to update some blocked load
> +	 */
> +	nohz_run_idle_balance(cpu);
> +

What do we gain from doing this here vs having a stats update in
newidle_balance()?

The current approach is to have a combined load_balance() + blocked load
update during newidle, and I get that this can take too long. But then,
we could still have what you're adding to do_idle() in the tail of
newidle_balance() itself, no? i.e.

  newidle_balance()
    ...
    for_each_domain(this_cpu, sd) {
       ...
       pulled_task = load_balance(...);
       ...
    }
    ...
    if (!pulled_task && !this_rq->nr_running) {
      this_rq->nohz_idle_balance = NOHZ_STATS_KICK;
      _nohz_idle_balance();
    }

or somesuch.

>       /*
>        * If the arch has a polling bit, we maintain an invariant:
>        *
> --
> 2.17.1