linux-kernel - Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT

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Message-ID: <20160912110307.GA2417@redhat.com>
Date:   Mon, 12 Sep 2016 13:03:07 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Cheng Chao <cs.os.kernel@...il.com>
Cc:     mingo@...nel.org, peterz@...radead.org, tj@...nel.org,
        akpm@...ux-foundation.org, chris@...is-wilson.co.uk,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful
        works for CONFIG_PREEMPT_NONE

On 09/10, Cheng Chao wrote:
>
> @@ -126,6 +126,17 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
>  	cpu_stop_init_done(&done, 1);
>  	if (!cpu_stop_queue_work(cpu, &work))
>  		return -ENOENT;
> +
> +#if defined(CONFIG_PREEMPT_NONE)
> +	/*
> +	 * Makes the stopper thread run as soon as possible.
> +	 * And if the caller is TASK_RUNNING, keeps the caller TASK_RUNNING.
> +	 * It's special useful for some callers which are expected to be
> +	 * TASK_ON_RQ_QUEUED.
> +	 * sched_exec does benefit from this improvement.
> +	 */
> +	schedule();
> +#endif
>  	wait_for_completion(&done.completion);
>  	return done.ret;
>  }

Cheng, I already tried twice to suggest to conditionalize this schedule,
because it can only help if cpu == smp_processor_id, and you didn't reply.
I still think _cond_resched() makes more sense.

I won't really argue if you prefer it this way. But did you see my emails?

Oleg.