Re: [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to rely on stopper->enabled

[Oleg Nesterov <oleg@redhat.com>]

On 10/08, Peter Zijlstra wrote:
>
> On Thu, Oct 08, 2015 at 04:51:36PM +0200, Oleg Nesterov wrote:
> > @@ -261,12 +276,8 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
> >  	set_state(&msdata, MULTI_STOP_PREPARE);
> >
> >  	/*
> > +	 * We do not want to migrate to inactive CPU. FIXME: move this
> > +	 * into the caller.
> >  	 */
> >  	if (!cpu_active(cpu1) || !cpu_active(cpu2)) {
> >  		preempt_enable();
>
> So we cannot move that into the caller..

Why?

> because this function sleeps
> with wait_for_completion().
>
> Or rather, it would force the caller to use get_online_cpus(), which we
> worked really hard to avoid.

Aaah wait. Sorry for confusion!

I meant "move this into the callback, migrate_swap_stop()".

> Also, I think we still want the patch I proposed which ensures the
> stopper thread is active 'early', because the load balancer pretty much

Perhaps. Although I do not really understand why it is important.
I mean, either way we unpark it at CPU_ONLINE stage, just
sched_cpu_active() has a higher priority.

But this is off-topic in a sense that the main point of this patch
is that stop_two_cpus() no longer needs to abuse cpu_active() checks
to avoid the race with cpu_up/down, we can simply rely on ->enabled.

And again, we need to take both locks to remove "lglock stop_cpus_lock".

So I think your change can be applied after this series too. Or I missed
something?

Oleg.