linux-kernel - Re: [PATCH V2] sched: pull tasks when CPU is about to run SCHED

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Date:   Thu, 25 Feb 2021 16:47:01 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     chin <ultrachin@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        heddchen@...cent.com,
        xiaoggchen(陈小光) <xiaoggchen@...cent.com>
Subject: Re: [PATCH V2] sched: pull tasks when CPU is about to run SCHED_IDLE tasks

On Mon, 22 Feb 2021 at 08:35, <ultrachin@....com> wrote:
>
> From: Chen Xiaoguang <xiaoggchen@...cent.com>
>
> In order to use the computer efficiently we usually deploy online
> tasks and offline tasks in the same computer.
>
> The online tasks are more important than the offline tasks and are
> latency sensitive we should make sure the online tasks preempt the
> offline tasks as soon as possible while there are online tasks
> waiting to run.
>
> Online tasks using the SCHED_NORMAL policy and offline tasks using
> the SCHED_ILDE policy. This patch decrease the latency of online
> tasks by doing a load balance before a offline tasks to run.
>
> Signed-off-by: Chen Xiaoguang <xiaoggchen@...cent.com>
> Signed-off-by: Chen He <heddchen@...cent.com>

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

Thanks

> ---
> v1 -> v2:
>  - Add checking in balance_fair
>  - Remove task state checking in pick_next_task_fair
>  - Add comment about the change
> ---
>  kernel/sched/fair.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8a8bd7b..80b69a2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6833,7 +6833,13 @@ static void task_dead_fair(struct task_struct *p)
>  static int
>  balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
> -       if (rq->nr_running)
> +       /*
> +        * Return if SCHED_NORMAL tasks exist.
> +        * Else if only SCHED_IDLE tasks in rq then do a load balance trying
> +        * to pull SCHED_NORMAL tasks to run so as to reduce the latency of
> +        * SCHED_NORMAL task.
> +        */
> +       if (rq->nr_running && !sched_idle_rq(rq))
>                 return 1;
>
>         return newidle_balance(rq, rf) != 0;
> @@ -7013,6 +7019,14 @@ struct task_struct *
>         struct task_struct *p;
>         int new_tasks;
>
> +       /*
> +        * Before a CPU switches from running SCHED_NORMAL task to SCHED_IDLE
> +        * task, do a load balance trying to pull SCHED_NORMAL tasks to run
> +        * so as to reduce the latency of SCHED_NORMAL task.
> +        */
> +       if (sched_idle_rq(rq) && prev && prev->policy != SCHED_IDLE)
> +               goto idle;
> +
>  again:
>         if (!sched_fair_runnable(rq))
>                 goto idle;
> --
> 1.8.3.1
>