Subject: Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux

From: Purcareata Bogdan <[email-masked]>
Date: Fri, 3 Apr 2015 11:07:29 +0300

On 03.04.2015 02:11, Scott Wood wrote:
> On Fri, 2015-03-27 at 19:07 +0200, Purcareata Bogdan wrote:
>> On 27.02.2015 03:05, Scott Wood wrote:
>>> On Thu, 2015-02-26 at 14:31 +0100, Sebastian Andrzej Siewior wrote:
>>>> On 02/26/2015 02:02 PM, Paolo Bonzini wrote:
>>>>>
>>>>>
>>>>> On 24/02/2015 00:27, Scott Wood wrote:
>>>>>> This isn't a host PIC driver.  It's guest PIC emulation, some of which
>>>>>> is indeed not suitable for a rawlock (in particular, openpic_update_irq
>>>>>> which loops on the number of vcpus, with a loop body that calls
>>>>>> IRQ_check() which loops over all pending IRQs).
>>>>>
>>>>> The question is what behavior is wanted of code that isn't quite
>>>>> RT-ready.  What is preferred, bugs or bad latency?
>>>>>
>>>>> If the answer is bad latency (which can be avoided simply by not running
>>>>> KVM on a RT kernel in production), patch 1 can be applied.  If the
>>>> can be applied *but* makes no difference if applied or not.
>>>>
>>>>> answer is bugs, patch 1 is not upstream material.
>>>>>
>>>>> I myself prefer to have bad latency; if something takes a spinlock in
>>>>> atomic context, that spinlock should be raw.  If it hurts (latency),
>>>>> don't do it (use the affected code).
>>>>
>>>> The problem, that is fixed by this s/spin_lock/raw_spin_lock/, exists
>>>> only in -RT. There is no change upstream. In general we fix such things
>>>> in -RT first and forward the patches upstream if possible. This convert
>>>> thingy would be possible.
>>>> Bug fixing comes before latency no matter if RT or not. Converting
>>>> every lock into a rawlock is not always the answer.
>>>> Last thing I read from Scott is that he is not entirely sure if this is
>>>> the right approach or not and patch #1 was not acked-by him either.
>>>>
>>>> So for now I wait for Scott's feedback and maybe a backtrace :)
>>>
>>> Obviously leaving it in a buggy state is not what we want -- but I lean
>>> towards a short term "fix" of putting "depends on !PREEMPT_RT" on the
>>> in-kernel MPIC emulation (which is itself just an optimization -- you
>>> can still use KVM without it).  This way people don't enable it with RT
>>> without being aware of the issue, and there's more of an incentive to
>>> fix it properly.
>>>
>>> I'll let Bogdan supply the backtrace.
>>
>> So about the backtrace. Wasn't really sure how to "catch" this, so what
>> I did was to start a 24 VCPUs guest on a 24 CPU board, and in the guest
>> run 24 netperf flows with an external back to back board of the same
>> kind. I assumed this would provide the sufficient VCPUs and external
>> interrupt to expose an alleged culprit.
>>
>> With regards to measuring the latency, I thought of using ftrace,
>> specifically the preemptirqsoff latency histogram. Unfortunately, I
>> wasn't able to capture any major differences between running a guest
>> with in-kernel MPIC emulation (with the openpic raw_spinlock_conversion
>> applied) vs. no in-kernel MPIC emulation. Function profiling
>> (trace_stat) shows that in the second case there's a far greater time
>> spent in kvm_handle_exit (100x), but overall, the maximum latencies for
>> preemptirqsoff don't look that much different.
>>
>> Here are the max numbers (preemptirqsoff) for the 24 CPUs, on the host
>> RT Linux, sorted in descending order, expressed in microseconds:
>>
>> In-kernel MPIC		QEMU MPIC
>> 3975			5105
>
> What are you measuring?  Latency in the host, or in the guest?

This is in the host kernel. It's the maximum continuous period of time 
when both interrupts and preemption were disabled on the host kernel 
(basically making it unresponsive). This has been tracked while the 
guest was running with high prio, with 24 VCPUs, and in the guest there 
were 24 netperf flows - so a lot of VCPUs and a lot of external 
interrupts - for about 15 minutes.

Bogdan P.