Re: [PATCH 1/1] hotplug cpu: migrate a task within its cpuset

[Oleg Nesterov <oleg@tv-sign.ru>]

Cliff Wickman wrote:
>
> In order to do this, move_task_off_dead_cpu() must make a call to
> cpuset_cpus_allowed(), which may block.
>
> move_task_off_dead_cpu() has been within a critical region when called
> from migrate_live_tasks().  So this patch also changes migrate_live_tasks()
> to enable interrupts before calling move_task_off_dead_cpu().
> Since the tasklist_lock is dropped, the list scan must be restarted from
> the top.
>
> [... snip ...]
>
> - * NOTE: interrupts should be disabled by the caller
> + * NOTE: interrupts are not disabled by the caller
>   */
>  static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
>  {
> @@ -5008,6 +5008,17 @@ restart:
>  	if (dest_cpu == NR_CPUS)
>  		dest_cpu = any_online_cpu(p->cpus_allowed);
>
> +	/* try to stay on the same cpuset */
> +	if (dest_cpu == NR_CPUS) {
> +		/*
> +		 * Call to cpuset_cpus_allowed may sleep, so we depend
> +		 * on move_task_off_dead_cpu() being called in a non-critical
> +		 * region.
> +		 */
> +		p->cpus_allowed = cpuset_cpus_allowed(p);
> +		dest_cpu = any_online_cpu(p->cpus_allowed);
> +	}

I know nothing about cpuset.c, a _very_ naive question.

Do we really need task_lock() (used by cpuset_cpus_allowed) here ?

If not, probably we can make this simpler. CPU_DEAD takes cpuset_lock(),
move_task_off_dead_cpu() uses guarantee_online_cpus() which doesn't sleep,
so we don't need other changes.

Possible?

If not, this patch should also change migrate_dead(), it still calls
move_task_off_dead_cpu() with irqs disabled, no?

Oleg.