Re: Thoughts on simple scanner approach for free page hinting

[David Hildenbrand]

On 08.04.19 22:51, David Hildenbrand wrote:
> On 08.04.19 22:10, Alexander Duyck wrote:
>> On Mon, Apr 8, 2019 at 11:40 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>
>>>>>>
>>>>>> In addition we will need some way to identify which pages have been
>>>>>> hinted on and which have not. The way I believe easiest to do this
>>>>>> would be to overload the PageType value so that we could essentially
>>>>>> have two values for "Buddy" pages. We would have our standard "Buddy"
>>>>>> pages, and "Buddy" pages that also have the "Offline" value set in the
>>>>>> PageType field. Tracking the Online vs Offline pages this way would
>>>>>> actually allow us to do this with almost no overhead as the mapcount
>>>>>> value is already being reset to clear the "Buddy" flag so adding a
>>>>>> "Offline" flag to this clearing should come at no additional cost.
>>>>>
>>>>> Just nothing here that this will require modifications to kdump
>>>>> (makedumpfile to be precise and the vmcore information exposed from the
>>>>> kernel), as kdump only checks for the the actual mapcount value to
>>>>> detect buddy and offline pages (to exclude them from dumps), they are
>>>>> not treated as flags.
>>>>>
>>>>> For now, any mapcount values are really only separate values, meaning
>>>>> not the separate bits are of interest, like flags would be. Reusing
>>>>> other flags would make our life a lot easier. E.g. PG_young or so. But
>>>>> clearing of these is then the problematic part.
>>>>>
>>>>> Of course we could use in the kernel two values, Buddy and BuddyOffline.
>>>>> But then we have to check for two different values whenever we want to
>>>>> identify a buddy page in the kernel.
>>>>
>>>> Actually this may not be working the way you think it is working.
>>>
>>> Trust me, I know how it works. That's why I was giving you the notice.
>>>
>>> Read the first paragraph again and ignore the others. I am only
>>> concerned about makedumpfile that has to be changed.
>>>
>>> PAGE_OFFLINE_MAPCOUNT_VALUE
>>> PAGE_BUDDY_MAPCOUNT_VALUE
>>>
>>> Once you find out how these values are used, you should understand what
>>> has to be changed and where.
>>
>> Ugh. Is there an official repo I am supposed to refer to for makedumpfile?
>>
>> As far as the changes needed I don't think this would necessitate
>> additional exports. We could probably just get away with having
>> makedumpfile generate a new value by simply doing an "&" of the two
>> values to determine what an offline buddy would be. If need be I can
>> submit a patch for that. I find it kind of annoying that the kernel is
>> handling identifying these bits one way, and makedumpfile is doing it
>> another way. It should have been setup to handle this all the same
>> way.
>>
>>>
>>>>>>
>>>>>> Lastly we would need to create a specialized function for allocating
>>>>>> the non-"Offline" pages, and to tweak __free_one_page to tail enqueue
>>>>>> "Offline" pages. I'm thinking the alloc function it would look
>>>>>> something like __rmqueue_smallest but without the "expand" and needing
>>>>>> to modify the !page check to also include a check to verify the page
>>>>>> is not "Offline". As far as the changes to __free_one_page it would be
>>>>>> a 2 line change to test for the PageType being offline, and if it is
>>>>>> to call add_to_free_area_tail instead of add_to_free_area.
>>>>>
>>>>> As already mentioned, there might be scenarios where the additional
>>>>> hinting thread might consume too much CPU cycles, especially if there is
>>>>> little guest activity any you mostly spend time scanning a handful of
>>>>> free pages and reporting them. I wonder if we can somehow limit the
>>>>> amount of wakeups/scans for a given period to mitigate this issue.
>>>>
>>>> That is why I was talking about breaking nr_free into nr_freed and
>>>> nr_bound. By doing that I can record the nr_free value to a
>>>> virtio-balloon specific location at the start of any walk and should
>>>> know exactly now many pages were freed between that call and the next
>>>> one. By ordering things such that we place the "Offline" pages on the
>>>> tail of the list it should make the search quite fast since we would
>>>> just be always allocating off of the head of the queue until we have
>>>> hinted everything int he queue. So when we hit the last call to alloc
>>>> the non-"Offline" pages and shut down our thread we can use the
>>>> nr_freed value that we recorded to know exactly how many pages have
>>>> been added that haven't been hinted.
>>>>
>>>>> One main issue I see with your approach is that we need quite a lot of
>>>>> core memory management changes. This is a problem. I wonder if we can
>>>>> factor out most parts into callbacks.
>>>>
>>>> I think that is something we can't get away from. However if we make
>>>> this generic enough there would likely be others beyond just the
>>>> virtualization drivers that could make use of the infrastructure. For
>>>> example being able to track the rate at which the free areas are
>>>> cycling in and out pages seems like something that would be useful
>>>> outside of just the virtualization areas.
>>>
>>> Might be, but might be the other extreme, people not wanting such
>>> special cases in core mm. I assume the latter until I see a very clear
>>> design where such stuff has been properly factored out.
>>
>> The only real pain point I am seeing right now is the assumptions
>> makedumpfile is currently making about how mapcount is being used to
>> indicate pagetype. If we patch it to fix it most of the other bits are
>> minor.
> 
> I'll be curious how splitting etc. will be handled. Especially if you
> want to set Offline for all affected sub pages.
> 

Answering that myself, I guess you are planning to change the buddy to
basically copy the offline value to sub-pages when splitting, also
attaching them to the tail of the list instead of the head.

-- 

Thanks,

David / dhildenb