linux-kernel - Re: [PATCH] sched/core: simpler function for sched

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Date:   Mon, 5 Sep 2016 15:11:48 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     cheng chao <chengchao@...acom.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] sched/core: simpler function for sched_exec migration

On 09/05, cheng chao wrote:
>
> @@ -2958,7 +2958,7 @@ void sched_exec(void)
>  		struct migration_arg arg = { p, dest_cpu };
>  
>  		raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> -		stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
> +		stop_one_cpu_sync(task_cpu(p), migration_cpu_stop, &arg);
>  		return;
>  	}
>  unlock:
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 4a1ca5f..24f8637 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -130,6 +130,27 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
>  	return done.ret;
>  }
>  
> +/**
> + * the caller keeps task_on_rq_queued, so it's more suitable for
> + * sched_exec on the case when needs migration
> + */
> +void stop_one_cpu_sync(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
> +{
> +	struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = NULL };
> +
> +	if (!cpu_stop_queue_work(cpu, &work))
> +		return;
> +
> +#if defined(CONFIG_PREEMPT_NONE) || defined(CONFIG_PREEMPT_VOLUNTARY)
> +	/*
> +	 * CONFIG_PREEMPT doesn't need call schedule here, because
> +	 * preempt_enable already does the similar thing when call
> +	 * cpu_stop_queue_work
> +	 */
> +	schedule();
> +#endif
> +}

Honestly, I don't really understand the changelog, but this looks wrong.

stop_one_cpu_sync() assumes that cpu == smp_processor_id/task_cpu(current),
and thus the stopper thread should preempt us at least after schedule()
(if CONFIG_PREEMPT_NONE), so we do not need to synchronize.

But this is not necessarily true? This task can migrate to another CPU
before cpu_stop_queue_work() ?

Oleg.