Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue

[David Hildenbrand <david@redhat.com>]

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)

-- 
Thanks,

David / dhildenb