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

[Oleg Nesterov]

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.