Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

[David Hildenbrand]

On 18.02.19 20:16, Michael S. Tsirkin wrote:
> On Mon, Feb 18, 2019 at 07:29:44PM +0100, David Hildenbrand wrote:
>>>
>>>>>
>>>>> But really what business has something that is supposedly
>>>>> an optimization blocking a VCPU? We are just freeing up
>>>>> lots of memory why is it a good idea to slow that
>>>>> process down?
>>>>
>>>> I first want to know that it is a problem before we declare it a
>>>> problem. I provided an example (s390x) where it does not seem to be a
>>>> problem. One hypercall ~every 512 frees. As simple as it can get.
>>>>
>>>> No trying to deny that it could be a problem on x86, but then I assume
>>>> it is only a problem in specific setups.
>>>
>>> But which setups? How are we going to identify them?
>>
>> I guess is simple (I should be carefuly with this word ;) ): As long as
>> you don't isolate + pin your CPUs in the hypervisor, you can expect any
>> kind of sudden hickups. We're in a virtualized world. Real time is one
>> example.
>>
>> Using kernel threads like Nitesh does right now? It can be scheduled
>> anytime by the hypervisor on the exact same cpu. Unless you isolate +
>> pin in the hypervor. So the same problem applies.
> 
> Right but we know how to handle this. Many deployments already use tools
> to detect host threads kicking VCPUs out.
> Getting VCPU blocked by a kfree call would be something new.
> 

Yes, and for s390x we already have some kfree's taking longer than
others. We have to identify when it is not okay.

> 
>>> So I'm fine with a simple implementation but the interface needs to
>>> allow the hypervisor to process hints in parallel while guest is
>>> running.  We can then fix any issues on hypervisor without breaking
>>> guests.
>>
>> Yes, I am fine with defining an interface that theoretically let's us
>> change the implementation in the guest later.
>> I consider this even a
>> prerequisite. IMHO the interface shouldn't be different, it will be
>> exactly the same.
>>
>> It is just "who" calls the batch freeing and waits for it. And as I
>> outlined here, doing it without additional threads at least avoids us
>> for now having to think about dynamic data structures and that we can
>> sometimes not report "because the thread is still busy reporting or
>> wasn't scheduled yet".
> 
> Sorry I wasn't clear. I think we need ability to change the
> implementation in the *host* later. IOW don't rely on
> host being synchronous.
> 
> 
I actually misread it :) . In any way, there has to be a mechanism to
synchronize.

If we are going via a bare hypercall (like s390x, like what Alexander
proposes), it is going to be a synchronous interface either way. Just a
bare hypercall, there will not really be any blocking on the guest side.

Via virtio, I guess it is waiting for a response to a requests, right?

-- 

Thanks,

David / dhildenb