Re: [PATCH v2 1/3] sched/fair: Call newidle_balance() from balance_callback on PREEMPT_RT

[Vincent Guittot <vincent.guittot@linaro.org>]

On Thu, 29 Apr 2021 at 01:28, Scott Wood <swood@redhat.com> wrote:
>
> This is required in order to be able to enable interrupts in the next
> patch.  This is limited to PREEMPT_RT to avoid adding potentially
> measurable overhead to the non-RT case (requiring a double switch when
> pulling a task onto a newly idle cpu).

IIUC, only the newidle_balance is a problem and not the idle load
balance that runs softirq. In this case, why not skipping
newidle_balance entirely in case of preempt_rt and kick an idle load
balance instead as you switch to idle thread context anyway


>
> update_misfit_status() is factored out for the PREEMPT_RT case, to ensure
> that the misfit status is kept consistent before dropping the lock.
>
> Signed-off-by: Scott Wood <swood@redhat.com>
> ---
> v2: Use a balance callback, and limit to PREEMPT_RT
>
>  kernel/sched/fair.c | 34 ++++++++++++++++++++++++++++++----
>  1 file changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..ff369c38a5b5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5660,6 +5660,9 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>
>  #ifdef CONFIG_SMP
>
> +static const bool newidle_balance_in_callback = IS_ENABLED(CONFIG_PREEMPT_RT);
> +static DEFINE_PER_CPU(struct callback_head, rebalance_head);
> +
>  /* Working cpumask for: load_balance, load_balance_newidle. */
>  DEFINE_PER_CPU(cpumask_var_t, load_balance_mask);
>  DEFINE_PER_CPU(cpumask_var_t, select_idle_mask);
> @@ -10549,7 +10552,7 @@ static inline void nohz_newidle_balance(struct rq *this_rq) { }
>   *     0 - failed, no new tasks
>   *   > 0 - success, new (fair) tasks present
>   */
> -static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> +static int do_newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  {
>         unsigned long next_balance = jiffies + HZ;
>         int this_cpu = this_rq->cpu;
> @@ -10557,7 +10560,9 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>         int pulled_task = 0;
>         u64 curr_cost = 0;
>
> -       update_misfit_status(NULL, this_rq);
> +       if (!newidle_balance_in_callback)
> +               update_misfit_status(NULL, this_rq);
> +
>         /*
>          * We must set idle_stamp _before_ calling idle_balance(), such that we
>          * measure the duration of idle_balance() as idle time.
> @@ -10576,7 +10581,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>          * further scheduler activity on it and we're being very careful to
>          * re-start the picking loop.
>          */
> -       rq_unpin_lock(this_rq, rf);
> +       if (!newidle_balance_in_callback)
> +               rq_unpin_lock(this_rq, rf);
>
>         if (this_rq->avg_idle < sysctl_sched_migration_cost ||
>             !READ_ONCE(this_rq->rd->overload)) {
> @@ -10655,11 +10661,31 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>         if (pulled_task)
>                 this_rq->idle_stamp = 0;
>
> -       rq_repin_lock(this_rq, rf);
> +       if (!newidle_balance_in_callback)
> +               rq_repin_lock(this_rq, rf);
>
>         return pulled_task;
>  }
>
> +static void newidle_balance_cb(struct rq *this_rq)
> +{
> +       update_rq_clock(this_rq);
> +       do_newidle_balance(this_rq, NULL);
> +}
> +
> +static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> +{
> +       if (newidle_balance_in_callback) {
> +               update_misfit_status(NULL, this_rq);
> +               queue_balance_callback(this_rq,
> +                                      &per_cpu(rebalance_head, this_rq->cpu),
> +                                      newidle_balance_cb);
> +               return 0;
> +       }
> +
> +       return do_newidle_balance(this_rq, rf);
> +}
> +
>  /*
>   * run_rebalance_domains is triggered when needed from the scheduler tick.
>   * Also triggered for nohz idle balancing (with nohz_balancing_kick set).
> --
> 2.27.0
>