Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE

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From	Oleg Nesterov <oleg@redhat.com>
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Subject	Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE
Date	Mon, 12 Sep 2016 15:10:01 +0200
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On 09/12, Peter Zijlstra wrote:
>
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -121,6 +121,11 @@ 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();

Yes, this is what I tried to suggest too.

But this leads to the question which I wanted to ask many times.

Why cond_resched() is not NOP if CONFIG_PREEMPT=y ?

Perhaps we have some users like, just for example,

	preempt_enable_no_resched();
	cond_resched();

which actually want the should_resched() check even if CONFIG_PREEMPT,
but most callers do not?

Oleg.

Thread

[PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE Cheng Chao <cs.os.kernel@gmail.com> - 2016-09-10 11:00 +0200
  Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Cheng Chao <cs.os.kernel@gmail.com> - 2016-09-10 12:00 +0200
    Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Peter Zijlstra <peterz@infradead.org> - 2016-09-10 18:40 +0200
  Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful         works for CONFIG_PREEMPT_NONE Oleg Nesterov <oleg@redhat.com> - 2016-09-12 13:10 +0200
    Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Cheng Chao <cs.os.kernel@gmail.com> - 2016-09-13 04:50 +0200
  Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Peter Zijlstra <peterz@infradead.org> - 2016-09-12 13:40 +0200
    Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Peter Zijlstra <peterz@infradead.org> - 2016-09-12 13:50 +0200
      Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful         works for CONFIG_PREEMPT_NONE Oleg Nesterov <oleg@redhat.com> - 2016-09-12 15:10 +0200
        Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Peter Zijlstra <peterz@infradead.org> - 2016-09-12 17:10 +0200
          Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful         works for CONFIG_PREEMPT_NONE Oleg Nesterov <oleg@redhat.com> - 2016-09-13 18:20 +0200
            Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Peter Zijlstra <peterz@infradead.org> - 2016-09-13 18:40 +0200
              Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Cheng Chao <cs.os.kernel@gmail.com> - 2016-09-14 04:10 +0200
                Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Peter Zijlstra <peterz@infradead.org> - 2016-09-14 10:00 +0200
              Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful         works for CONFIG_PREEMPT_NONE Oleg Nesterov <oleg@redhat.com> - 2016-09-14 17:50 +0200
              [tip:sched/core] sched/core: Optimize __schedule() tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2016-09-22 16:10 +0200
      Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Cheng Chao <cs.os.kernel@gmail.com> - 2016-09-13 06:10 +0200
        Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful  works for CONFIG_PREEMPT_NONE Peter Zijlstra <peterz@infradead.org> - 2016-09-13 10:50 +0200
  [PATCH v4] stop_machine: Avoid a sleep and wakeup in the stop_one_cpu() Cheng Chao <cs.os.kernel@gmail.com> - 2016-09-14 04:10 +0200
    Re: [PATCH v4] stop_machine: Avoid a sleep and wakeup in the         stop_one_cpu() Oleg Nesterov <oleg@redhat.com> - 2016-09-14 18:00 +0200
    Re: [PATCH v4] stop_machine: Avoid a sleep and wakeup in the  stop_one_cpu() Cheng Chao <cs.os.kernel@gmail.com> - 2016-09-18 04:10 +0200
    [tip:sched/core] stop_machine: Avoid a sleep and wakeup in  stop_one_cpu() tip-bot for Cheng Chao <tipbot@zytor.com> - 2016-09-22 16:10 +0200

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