linux-kernel - Re: [PATCH] sched/fair: document the slow path and fast path in select_task_rq

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Message-ID: <20211015092641.2266cf59@gandalf.local.home>
Date:   Fri, 15 Oct 2021 09:26:41 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Barry Song <21cnbao@...il.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com,
        yangyicong@...ilicon.com, Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH] sched/fair: document the slow path and fast path in
 select_task_rq_fair

On Fri, 15 Oct 2021 12:14:12 +0800
Barry Song <21cnbao@...il.com> wrote:

> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6951,6 +6951,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  			break;
>  		}
>  
> +		/*
> +		 * This is usually true only for WF_EXEC and WF_FORK, for WF_TTWU
> +		 * it is almost always false as sched_domain hasn't SD_BALANCE_WAKE
> +		 * in default

Comments are fine, but the above needs some grammar work.

		/*
		 * Usually only true for WF_EXEC and WF_FORK, as
		 * sched_domains usually do not have SD_BALANCE_WAKE set.
		 */

?

> +		 */
>  		if (tmp->flags & sd_flag)
>  			sd = tmp;
>  		else if (!want_affine)
> @@ -6958,7 +6963,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	}
>  
>  	if (unlikely(sd)) {
> -		/* Slow path */
> +		/*
> +		 * Slow path, usually only for WF_EXEC and WF_FORK; WF_TTWU almost
> +		 * always goes to fast path as sched_domain hasn't SD_BALANCE_WAKE
> +		 * in default
> +		 */

As the only way to have sd set, is from the above if statement, I believe
this comment is redundant.

-- Steve


>  		new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
>  	} else if (wake_flags & WF_TTWU) { /* XXX always ? */
>  		/* Fast path */