Re: [Xen-devel] [PATCH] x86: show remote CPU state upon fatal NMI

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/06/16 13:59, Jan Beulich wrote:
>>>> On 15.06.16 at 13:03, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 15/06/16 08:55, Jan Beulich wrote:
>>>>> @@ -570,6 +600,15 @@ void fatal_trap(const struct cpu_user_re
>>>>>              printk("Faulting linear address: %p\n", _p(cr2));
>>>>>              show_page_walk(cr2);
>>>>>          }
>>>>> +        else if ( trapnr == TRAP_nmi )
>>>>> +        {
>>>>> +            cpumask_andnot(&nmi_show_state_mask, &cpu_online_map,
>>>>> +                           cpumask_of(smp_processor_id()));
>>>>> +            set_nmi_callback(nmi_show_execution_state);
>>>>> +            smp_send_nmi_allbutself();
>>>> This would cause far less spinlock contention as
>>>>
>>>> for_each_cpu( cpu, nmi_show_state_mask )
>>>>     smp_send_nmi(cpu);
>>>>
>>>> I realise this involves introducing a new smp function, but it would
>>>> substantially reduce contention on the console lock.
>>> Again, I don't see why lock contention would matter here. And then
>>> I also don't see how sending the IPIs individually would make matters
>>> significantly better: The sending will surely finish much faster than
>>> the printing.
>> Contention is a problem because you have replaced the NMI callback, and
>> the watchdog is still running.  Especially if sync_console is in effect,
>> you are liable to incur a further timeout, queueing up more NMIs.
>>
>> Although now I think of it, that won't matter so long as the NMIs don't
>> nest.
>>
>> The one advantage of sending the NMIs in order is that the information
>> dump will happen in order, which is slightly more use than having them
>> in a random order on a large machine.
> How that? All the NMIs will still arrive at about the same time, so
> while some low numbered CPUs may indeed get their state printed
> in order, higher numbered ones may still make it into the lock region
> in any order. (And no, building upon ticket locks making randomness
> much less likely is neither an option, nor would it really help: Just
> think of a lower numbered CPU first having to come out of a deep
> C-state or running at a much lower P-state than a higher numbered
> one.)

Hmm true.  There isn't a acknowledgement of the start of the NMI
handler, and putting one in sounds like far more effort and fragility
than it is worth.

>
>>>> I would recommend moving this clause into nmi_watchdog_tick(), so that
>>>> it doesn't get involved for non-watchdog NMIs.  IOCK/SERR NMIs won't
>>>> have anything interesting to print from here.  I would also recommend
>>>> disabling the watchdog before IPI'ing.
>>> And indeed I would have wanted it there, but I can't see how it can
>>> reasonably be put there: fatal_trap() doesn't return, so we can't put
>>> it after. And we definitely want to get state of the local CPU out
>>> before we try to log state of any of the remote CPUs. So the only
>>> options I see would be to
>>> - somehow specially flag the regs structure, but that feels hackish
>>>   (among other aspects nmi_watchdog_tick() has that parameter
>>>   const qualified for the very reason that it isn't supposed to fiddle
>>>   with it),
>>> - introduce a global flag.
>> How about a boolean flag to fatal_trap()?  It doesn't have many callers,
>> and this kind of printing might also be useful for some MCEs.
> Ah, right, there indeed aren't that many. Can you qualify "some"
> a bit better, so that maybe I can have the patch pass true there
> right away?

Any MCE which is in-practice synchronous might benefit.  I wouldn't
worry about updating the MCE callers as part of this.

~Andrew

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