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

Boris Ostrovsky
2015-03-04T14:57:53Z

On 03/04/2015 09:43 AM, Paul E. McKenney wrote:
> On Tue, Mar 03, 2015 at 02:31:51PM -0800, Paul E. McKenney wrote:
>> On Tue, Mar 03, 2015 at 05:06:50PM -0500, Boris Ostrovsky wrote:
>>> On 03/03/2015 04:26 PM, Paul E. McKenney wrote:
>>>> On Tue, Mar 03, 2015 at 03:13:07PM -0500, Boris Ostrovsky wrote:
>>>>> 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.
>>>> OK, so I have the following patch on top of my previous patch, which
>>>> I will merge if testing goes well. So if a CPU times out going offline,
>>>> the above three functions will not be called, the "didn't die" message
>>>> will be printed, and any future attempt to online that CPU will fail.
>>>> Is that the correct semantics?
>>> Yes.
>>>
>>> I am not sure whether not ever onlining the CPU is the best outcome
>>> but then I don't think trying to online it again with all interrupts
>>> and such still set up will work well. And it's an improvement over
>>> what we have now anyway (with current code we may clean up things
>>> for a non-dead cpu).
>> Another strategy is to key off of the return value of cpu_check_up_prepare().
>> If it returns -EBUSY, then the outgoing CPU finished up after the
>> surviving CPU timed out. The CPU trying to bring the new CPU online
>> could (in theory, anyway) do the xen_smp_intr_free(), xen_uninit_lock_cpu(),
>> and xen_teardown_timer() at that point.
> And the code for this, in xen_cpu_up(), might look something like the
> following:
>
> rc = cpu_check_up_prepare(cpu);
> if (rc && rc != -EBUSY)
> return rc;
> if (rc == EBUSY) {
> xen_smp_intr_free(cpu);
> xen_uninit_lock_cpu(cpu);
> xen_teardown_timer(cpu);
> }
>
> The idea is that we detect when the CPU eventually took itself offline,
> but only did so after the surviving CPU timed out. (Of course, it
> would probably be best to put those three statements into a small
> function that is called from both places.)
>
> I have no idea whether this approach would really work, especially given
> your earlier statement that CPU_DEAD happens early on. But in case it
> is helpful or sparks some better idea.

Let me test this, I think it may work.

In the meantime, it turned out that HVM guests are broken by this patch
(with our without changes that we've been discussing), because HVM CPUs
die with

static void xen_hvm_cpu_die(unsigned int cpu)
{
xen_cpu_die(cpu);
native_cpu_die(cpu);
}

Which means that cpu_wait_death() is called twice, and second call moves
the CPU to CPU_BROKEN.

The simple solution is to stop calling native_cpu_die() above but I'd
like to use common code in native_cpu_die(). I'll see if I can carve it
out without too much damage to x86.

Thanks.
-boris

>
> Thanx, Paul
>
>> But I must defer to you on this sort of thing.
>>
>> Thanx, Paul
>>
>>> Thanks.
>>> -boris
>>>
>>>
>>>> Thanx, Paul
>>>>
>>>> ------------------------------------------------------------------------
>>>>
>>>> diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
>>>> index e2c7389c58c5..f2a06ff0614d 100644
>>>> --- a/arch/x86/xen/smp.c
>>>> +++ b/arch/x86/xen/smp.c
>>>> @@ -508,12 +508,13 @@ static void xen_cpu_die(unsigned int cpu)
>>>> schedule_timeout(HZ/10);
>>>> }
>>>> - (void)cpu_wait_death(cpu, 5);
>>>> - /* FIXME: Are the below calls really safe in case of timeout? */
>>>> -
>>>> - xen_smp_intr_free(cpu);
>>>> - xen_uninit_lock_cpu(cpu);
>>>> - xen_teardown_timer(cpu);
>>>> + 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);
>>>> + }
>>>> }
>>>> static void xen_play_dead(void) /* used only with HOTPLUG_CPU */
>>>>