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

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Date:   Sat, 10 Sep 2016 17:51:02 +0800
From:   Cheng Chao <cs.os.kernel@...il.com>
To:     peterz@...radead.org
Cc:     mingo@...nel.org, oleg@...hat.com, 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

hi Peter, I guess you can receive the mail from me now,
I have changed the mailbox to gmail.

Oleg has already done much work for this patch, I am really obliged.
please review this patch, thanks.

on 09/10/2016 04:52 PM, Cheng Chao wrote:
> For CONFIG_PREEMPT_NONE=y, when sched_exec() needs migration, sched_exec()
> calls stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg).
> 
> If the migration_cpu_stop() can not migrate,why do we call stop_one_cpu()?
> It just makes the task TASK_UNINTERRUPTIBLE, wakes up the stopper thread,
> executes migration_cpu_stop(), and the stopper thread wakes up the task.
> 
> But in fact, all above works are almost useless(wasteful),the reason is
> migration_cpu_stop() can not migrate. why? migration_cpu_stop() needs the
> task is TASK_ON_RQ_QUEUED before it calls __migrate_task().
> 
> This patch keeps the task TASK_RUNNING instead of TASK_UNINTERRUPTIBLE,
> so the migration_cpu_stop() can do useful works.
> 
> Signed-off-by: Cheng Chao <cs.os.kernel@...il.com>
> ---
>  kernel/stop_machine.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 4a1ca5f..41aea5e 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -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;
>  }
>