linux-kernel - Re: [PATCH v4] stop_machine: Avoid a sleep and wakeup in the stop_one

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Date:   Sun, 18 Sep 2016 10:07:30 +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 v4] stop_machine: Avoid a sleep and wakeup in the
 stop_one_cpu()

Hi Peter,

  What should I do next? Thanks.

Cheng

on 09/14/2016 10:01 AM, Cheng Chao wrote:
> In case @cpu == smp_proccessor_id(), we can avoid a sleep+wakeup
> by doing a preemption.
> 
> the caller such as sched_exec can benefit from this change.
> 
> Signed-off-by: Cheng Chao <cs.os.kernel@...il.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  kernel/sched/core.c   | 8 ++++++--
>  kernel/stop_machine.c | 5 +++++
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a0086a5..283b662 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1063,8 +1063,12 @@ static int migration_cpu_stop(void *data)
>  	 * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because
>  	 * we're holding p->pi_lock.
>  	 */
> -	if (task_rq(p) == rq && task_on_rq_queued(p))
> -		rq = __migrate_task(rq, p, arg->dest_cpu);
> +	if (task_rq(p) == rq) {
> +		if (task_on_rq_queued(p))
> +			rq = __migrate_task(rq, p, arg->dest_cpu);
> +		else
> +			p->wake_cpu = arg->dest_cpu;
> +	}
>  	raw_spin_unlock(&rq->lock);
>  	raw_spin_unlock(&p->pi_lock);
>  
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 4a1ca5f..1a24890 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -126,6 +126,11 @@ 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;
> +	/*
> +	 * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
> +	 * by doing a preemption.
> +	 */
> +	cond_resched();
>  	wait_for_completion(&done.completion);
>  	return done.ret;
>  }
>