Re: use of shrinker in virtio balloon free page hinting

[David Hildenbrand <david@redhat.com>]

On 17.07.19 16:34, Michael S. Tsirkin wrote:
> On Wed, Jul 17, 2019 at 04:10:47PM +0200, David Hildenbrand wrote:
>> On 17.07.19 13:20, Michael S. Tsirkin wrote:
>>> Wei, others,
>>>
>>> ATM virtio_balloon_shrinker_scan will only get registered
>>> when deflate on oom feature bit is set.
>>>
>>> Not sure whether that's intentional.  Assuming it is:
>>>
>>> virtio_balloon_shrinker_scan will try to locate and free
>>> pages that are processed by host.
>>> The above seems broken in several ways:
>>> - count ignores the free page list completely
>>> - if free pages are being reported, pages freed
>>>   by shrinker will just get re-allocated again
>>
>> Trying to answer your questions (not sure if I fully understood what you
>> mean)
>>
>> virtio_balloon_shrinker_scan() will not be called due to inflation
>> requests (balloon_page_alloc()). It will be called whenever the system
>> is OOM, e.g., when starting a new application.
>>
>> I assume you were expecting the shrinker getting called due to
>> balloon_page_alloc(). however, that is not the case as we pass
>> "__GFP_NORETRY".
> 
> Right but it's possible we exhaust all memory, then
> someone else asks for a single page and that invokes
> the shrinker.

Yes, I think that can happen.

> 
>>
>> To test, something like:
>>
>> 1. Start a VM with
>>
>> -device virtio-balloon-pci,deflate-on-oom=true
>>
>> 2. Inflate the balloon, e.g.,
>>
>> QMP: balloon 1024
>> QMP: info balloon
>> -> 1024
>>
>> See how "MemTotal" in /proc/meminfo in the guest won't change
>>
>> 3. Run a workload that exhausts memory in the guest (OOM).
>>
>> See how the balloon was automatically deflated
>>
>> QMP: info balloon
>> -> Something bigger than 1024
>>
>>
>> Not sure if it is broken, last time I played with it, it worked, but
>> that was ~1-2 years ago.
>>
>> -- 
>>
>> Thanks,
>>
>> David / dhildenb
> 
> Sorry I was unclear.  The question was about
> VIRTIO_BALLOON_F_FREE_PAGE_HINT specifically.

Ah, I see. Never used both things together.

-- 

Thanks,

David / dhildenb