linux-kernel - Re: [PATCH 2/2] sched: Extract the task putting code from pick_next

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Message-ID: <20200420183232.16b83374@gandalf.local.home>
Date:   Mon, 20 Apr 2020 18:32:32 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 2/2] sched: Extract the task putting code from
 pick_next_task()

On Mon, 20 Apr 2020 00:31:52 +0800
Chen Yu <yu.c.chen@...el.com> wrote:

> Introduce a new function finish_prev_task() to do the balance
> when necessary, and then put previous task back to the run queue.
> This function is extracted from pick_next_task() to prepare for
> future usage by other type of task picking logic.
> 
> No functional change.
> 
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
>  kernel/sched/core.c | 39 +++++++++++++++++++++++----------------
>  1 file changed, 23 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3a61a3b8eaa9..bf59a5cf030c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3904,6 +3904,28 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
>  	schedstat_inc(this_rq()->sched_count);
>  }
>  
> +static void finish_prev_task(struct rq *rq, struct task_struct *prev,
> +			     struct rq_flags *rf)
> +{
> +	const struct sched_class *class;
> +#ifdef CONFIG_SMP
> +	/*
> +	 * We must do the balancing pass before put_next_task(), such

I know this is just a cut and paste move, but I'm thinking that this
comment is wrong. Shouldn't this be "put_prev_task()" as we have no
"put_next_task()" function.


> +	 * that when we release the rq->lock the task is in the same
> +	 * state as before we took rq->lock.
> +	 *
> +	 * We can terminate the balance pass as soon as we know there is
> +	 * a runnable task of @class priority or higher.
> +	 */
> +	for_class_range(class, prev->sched_class, &idle_sched_class) {
> +		if (class->balance(rq, prev, rf))
> +			break;
> +	}
> +#endif
> +
> +	put_prev_task(rq, prev);
> +}
> +
>  /*
>   * Pick up the highest-prio task:
>   */
> @@ -3937,22 +3959,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  	}
>  
>  restart:
> -#ifdef CONFIG_SMP
> -	/*
> -	 * We must do the balancing pass before put_next_task(), such
> -	 * that when we release the rq->lock the task is in the same
> -	 * state as before we took rq->lock.
> -	 *
> -	 * We can terminate the balance pass as soon as we know there is
> -	 * a runnable task of @class priority or higher.
> -	 */
> -	for_class_range(class, prev->sched_class, &idle_sched_class) {
> -		if (class->balance(rq, prev, rf))
> -			break;
> -	}
> -#endif
> -
> -	put_prev_task(rq, prev);
> +	finish_prev_task(rq, prev, rf);

I'm not sure I like the name of this function. Perhaps
"balance_and_put_prev_task()"? Something more in kind to what the function
does.

-- Steve

>  
>  	for_each_class(class) {
>  		p = class->pick_next_task(rq);