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

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On 10/08, Oleg Nesterov wrote:
>
> 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()".

Forgot to mention... And note that both these cpu_active() are obviously
racy, CPU_DOWN_PREPARE can make it inactive right after the check.

> > 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?

So I think this series makes sense anyway and hopefully should fix the
problem with or without your change.

Oleg.

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[RFC][PATCH] sched: Start stopper early Peter Zijlstra <peterz@infradead.org> - 2015-10-07 10:50 +0200
  Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-07 14:40 +0200
    Re: [RFC][PATCH] sched: Start stopper early Peter Zijlstra <peterz@infradead.org> - 2015-10-07 14:40 +0200
      Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-07 15:30 +0200
        Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-07 15:30 +0200
        Re: Re: [RFC][PATCH] sched: Start stopper early kbuild test robot <lkp@intel.com> - 2015-10-07 15:40 +0200
  [PATCH 0/3] (Was: [RFC][PATCH] sched: Start stopper early) Oleg Nesterov <oleg@redhat.com> - 2015-10-08 17:00 +0200
    [PATCH 1/3] stop_machine: ensure that a queued callback will be  called before cpu_stop_park() Oleg Nesterov <oleg@redhat.com> - 2015-10-08 17:00 +0200
      Re: [PATCH 1/3] stop_machine: ensure that a queued callback will be  called before cpu_stop_park() Peter Zijlstra <peterz@infradead.org> - 2015-10-14 17:40 +0200
        Re: [PATCH 1/3] stop_machine: ensure that a queued callback will  be called before cpu_stop_park() Oleg Nesterov <oleg@redhat.com> - 2015-10-14 21:10 +0200
          Re: [PATCH 1/3] stop_machine: ensure that a queued callback will be  called before cpu_stop_park() Peter Zijlstra <peterz@infradead.org> - 2015-10-14 22:40 +0200
            Re: [PATCH 1/3] stop_machine: ensure that a queued callback will  be called before cpu_stop_park() Oleg Nesterov <oleg@redhat.com> - 2015-10-15 19:10 +0200
              Re: [PATCH 1/3] stop_machine: ensure that a queued callback will be  called before cpu_stop_park() Peter Zijlstra <peterz@infradead.org> - 2015-10-16 13:00 +0200
    [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-08 17:00 +0200
      Re: [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Peter Zijlstra <peterz@infradead.org> - 2015-10-08 17:10 +0200
        Re: [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-08 18:10 +0200
          Re: [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-08 18:20 +0200
      [PATCH v2 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-08 19:10 +0200
        Re: [PATCH v2 3/3] stop_machine: change cpu_stop_queue_two_works()  to rely on stopper->enabled Peter Zijlstra <peterz@infradead.org> - 2015-10-09 18:40 +0200
          Re: [PATCH v2 3/3] stop_machine: change cpu_stop_queue_two_works()  to rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:50 +0200
    [PATCH 2/3] stop_machine: introduce __cpu_stop_queue_work() and  cpu_stop_queue_two_works() Oleg Nesterov <oleg@redhat.com> - 2015-10-08 17:00 +0200
  Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-08 20:10 +0200
    Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-08 21:00 +0200
  [PATCH 0/3] make stopper threads more "selfparking" Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:10 +0200
    [PATCH 2/3] stop_machine: kill cpu_stop_threads->setup() and  cpu_stop_unpark() Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:10 +0200
    [PATCH 3/3] sched: start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:10 +0200
      Re: [PATCH 3/3] sched: start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-09 19:00 +0200
    [PATCH 1/3] stop_machine: kill smp_hotplug_thread->pre_unpark,  introduce stop_machine_unpark() Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:10 +0200
  Re: [RFC][PATCH] sched: Start stopper early Heiko Carstens <heiko.carstens@de.ibm.com> - 2015-10-16 10:30 +0200
    Re: [RFC][PATCH] sched: Start stopper early Peter Zijlstra <peterz@infradead.org> - 2015-10-16 12:00 +0200
      Re: [RFC][PATCH] sched: Start stopper early Heiko Carstens <heiko.carstens@de.ibm.com> - 2015-10-16 14:10 +0200

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