Re: [Xen-devel] [PATCH tip/core/rcu 02/20] x86: Use common outgoing-CPU-notification code

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/03/2015 02:42 PM, Paul E. McKenney wrote:
> On Tue, Mar 03, 2015 at 02:17:24PM -0500, Boris Ostrovsky wrote:
> > On 03/03/2015 12:42 PM, Paul E. McKenney wrote:
> > >   }
> > > @@ -511,7 +508,8 @@ static void xen_cpu_die(unsigned int cpu)
> > >                 schedule_timeout(HZ/10);
> > >         }
> > > -       cpu_die_common(cpu);
> > > +       (void)cpu_wait_death(cpu, 5);
> > > +       /* FIXME: Are the below calls really safe in case of timeout? */
> >
> > Not for HVM guests (PV guests will only reach this point after
> > target cpu has been marked as down by the hypervisor).
> >
> > We need at least to have a message similar to what native_cpu_die()
> > prints on cpu_wait_death() failure. And I think we should not call
> > the two routines below (three, actually --- there is also
> > xen_teardown_timer() below, which is not part of the diff).
> >
> > -boris
> >
> >
> > >         xen_smp_intr_free(cpu);
> > >         xen_uninit_lock_cpu(cpu);
> So something like this, then?
>
>         if (cpu_wait_death(cpu, 5)) {
>                 xen_smp_intr_free(cpu);
>                 xen_uninit_lock_cpu(cpu);
>                 xen_teardown_timer(cpu);
>         }
        else
                pr_err("CPU %u didn't die...\n", cpu);


> Easy change for me to make if so!
>
> Or do I need some other check for HVM-vs.-PV guests, and, if so, what
> would that check be?  And also if so, is it OK to online a PV guest's
> CPU that timed out during its previous offline?

I believe PV VCPUs will always be CPU_DEAD by the time we get here since 
we are (indirectly) waiting for this in the loop at the beginning of 
xen_cpu_die():
'while (xen_pv_domain() && HYPERVISOR_vcpu_op(VCPUOP_is_up, cpu, NULL))' 
will exit only after 'HYPERVISOR_vcpu_op(VCPUOP_down, 
smp_processor_id()' in xen_play_dead(). Which happens after 
play_dead_common() has marked the cpu as CPU_DEAD.
So no test is needed.

Thanks.
-boris



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