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

Boris Ostrovsky
2015-03-03T20:15:27Z

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