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SubjectRe: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue
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> On Mar 31, 2020, at 7:09 AM, David Hildenbrand <david@redhat.com> wrote:
>
> On 31.03.20 16:07, Michael S. Tsirkin wrote:
>> On Tue, Mar 31, 2020 at 04:03:18PM +0200, David Hildenbrand wrote:
>>> On 31.03.20 15:37, Michael S. Tsirkin wrote:
>>>> On Tue, Mar 31, 2020 at 03:32:05PM +0200, David Hildenbrand wrote:
>>>>> On 31.03.20 15:24, Michael S. Tsirkin wrote:
>>>>>> On Tue, Mar 31, 2020 at 12:35:24PM +0200, David Hildenbrand wrote:
>>>>>>> On 26.03.20 10:49, Michael S. Tsirkin wrote:
>>>>>>>> On Thu, Mar 26, 2020 at 08:54:04AM +0100, David Hildenbrand wrote:
>>>>>>>>>> Am 26.03.2020 um 08:21 schrieb Michael S. Tsirkin <mst@redhat.com>:
>>>>>>>>>>
>>>>>>>>>> On Thu, Mar 12, 2020 at 09:51:25AM +0100, David Hildenbrand wrote:
>>>>>>>>>>>> On 12.03.20 09:47, Michael S. Tsirkin wrote:
>>>>>>>>>>>> On Thu, Mar 12, 2020 at 09:37:32AM +0100, David Hildenbrand wrote:
>>>>>>>>>>>>> 2. You are essentially stealing THPs in the guest. So the fastest
>>>>>>>>>>>>> mapping (THP in guest and host) is gone. The guest won't be able to make
>>>>>>>>>>>>> use of THP where it previously was able to. I can imagine this implies a
>>>>>>>>>>>>> performance degradation for some workloads. This needs a proper
>>>>>>>>>>>>> performance evaluation.
>>>>>>>>>>>>
>>>>>>>>>>>> I think the problem is more with the alloc_pages API.
>>>>>>>>>>>> That gives you exactly the given order, and if there's
>>>>>>>>>>>> a larger chunk available, it will split it up.
>>>>>>>>>>>>
>>>>>>>>>>>> But for balloon - I suspect lots of other users,
>>>>>>>>>>>> we do not want to stress the system but if a large
>>>>>>>>>>>> chunk is available anyway, then we could handle
>>>>>>>>>>>> that more optimally by getting it all in one go.
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> So if we want to address this, IMHO this calls for a new API.
>>>>>>>>>>>> Along the lines of
>>>>>>>>>>>>
>>>>>>>>>>>> struct page *alloc_page_range(gfp_t gfp, unsigned int min_order,
>>>>>>>>>>>> unsigned int max_order, unsigned int *order)
>>>>>>>>>>>>
>>>>>>>>>>>> the idea would then be to return at a number of pages in the given
>>>>>>>>>>>> range.
>>>>>>>>>>>>
>>>>>>>>>>>> What do you think? Want to try implementing that?
>>>>>>>>>>>
>>>>>>>>>>> You can just start with the highest order and decrement the order until
>>>>>>>>>>> your allocation succeeds using alloc_pages(), which would be enough for
>>>>>>>>>>> a first version. At least I don't see the immediate need for a new
>>>>>>>>>>> kernel API.
>>>>>>>>>>
>>>>>>>>>> OK I remember now. The problem is with reclaim. Unless reclaim is
>>>>>>>>>> completely disabled, any of these calls can sleep. After it wakes up,
>>>>>>>>>> we would like to get the larger order that has become available
>>>>>>>>>> meanwhile.
>>>>>>>>>
>>>>>>>>> Yes, but that‘s a pure optimization IMHO.
>>>>>>>>> So I think we should do a trivial implementation first and then see what we gain from a new allocator API. Then we might also be able to justify it using real numbers.
>>>>>>>>
>>>>>>>> Well how do you propose implement the necessary semantics?
>>>>>>>> I think we are both agreed that alloc_page_range is more or
>>>>>>>> less what's necessary anyway - so how would you approximate it
>>>>>>>> on top of existing APIs?
>>>>>>> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
>>>>
>>>> .....
>>>>
>>>>
>>>>>>> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
>>>>>>> index 26de020aae7b..067810b32813 100644
>>>>>>> --- a/mm/balloon_compaction.c
>>>>>>> +++ b/mm/balloon_compaction.c
>>>>>>> @@ -112,23 +112,35 @@ size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
>>>>>>> EXPORT_SYMBOL_GPL(balloon_page_list_dequeue);
>>>>>>>
>>>>>>> /*
>>>>>>> - * balloon_page_alloc - allocates a new page for insertion into the balloon
>>>>>>> - * page list.
>>>>>>> + * balloon_pages_alloc - allocates a new page (of at most the given order)
>>>>>>> + * for insertion into the balloon page list.
>>>>>>> *
>>>>>>> * Driver must call this function to properly allocate a new balloon page.
>>>>>>> * Driver must call balloon_page_enqueue before definitively removing the page
>>>>>>> * from the guest system.
>>>>>>> *
>>>>>>> + * Will fall back to smaller orders if allocation fails. The order of the
>>>>>>> + * allocated page is stored in page->private.
>>>>>>> + *
>>>>>>> * Return: struct page for the allocated page or NULL on allocation failure.
>>>>>>> */
>>>>>>> -struct page *balloon_page_alloc(void)
>>>>>>> +struct page *balloon_pages_alloc(int order)
>>>>>>> {
>>>>>>> - struct page *page = alloc_page(balloon_mapping_gfp_mask() |
>>>>>>> - __GFP_NOMEMALLOC | __GFP_NORETRY |
>>>>>>> - __GFP_NOWARN);
>>>>>>> - return page;
>>>>>>> + struct page *page;
>>>>>>> +
>>>>>>> + while (order >= 0) {
>>>>>>> + page = alloc_pages(balloon_mapping_gfp_mask() |
>>>>>>> + __GFP_NOMEMALLOC | __GFP_NORETRY |
>>>>>>> + __GFP_NOWARN, order);
>>>>>>> + if (page) {
>>>>>>> + set_page_private(page, order);
>>>>>>> + return page;
>>>>>>> + }
>>>>>>> + order--;
>>>>>>> + }
>>>>>>> + return NULL;
>>>>>>> }
>>>>>>> -EXPORT_SYMBOL_GPL(balloon_page_alloc);
>>>>>>> +EXPORT_SYMBOL_GPL(balloon_pages_alloc);
>>>>>>>
>>>>>>> /*
>>>>>>> * balloon_page_enqueue - inserts a new page into the balloon page list.
>>>>>>
>>>>>>
>>>>>> I think this will try to invoke direct reclaim from the first iteration
>>>>>> to free up the max order.
>>>>>
>>>>> %__GFP_NORETRY: The VM implementation will try only very lightweight
>>>>> memory direct reclaim to get some memory under memory pressure (thus it
>>>>> can sleep). It will avoid disruptive actions like OOM killer.
>>>>>
>>>>> Certainly good enough for a first version I would say, no?
>>>>
>>>> Frankly how well that behaves would depend a lot on the workload.
>>>> Can regress just as well.
>>>>
>>>> For the 1st version I'd prefer something that is the least disruptive,
>>>> and that IMHO means we only trigger reclaim at all in the same configuration
>>>> as now - when we can't satisfy the lowest order allocation.
>>>
>>> Agreed.
>>>
>>>> Anything else would be a huge amount of testing with all kind of
>>>> workloads.
>>>
>>> So doing a "& ~__GFP_RECLAIM" in case order > 0? (as done in
>>> GFP_TRANSHUGE_LIGHT)
>>
>> That will improve the situation when reclaim is not needed, but leave
>> the problem in place for when it's needed: if reclaim does trigger, we
>> can get a huge free page and immediately break it up.
>>
>> So it's ok as a first step but it will make the second step harder as
>> we'll need to test with reclaim :).
>
> I expect the whole "steal huge pages from your guest" to be problematic,
> as I already mentioned to Alex. This needs a performance evaluation.
>
> This all smells like a lot of workload dependent fine-tuning. :)

AFAIK the hardware overheads of keeping huge-pages in the guest and backing
them with 4KB pages are non-negligible. Did you take those into account?

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