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Subject: Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to
 handle THP spilt issue
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Hui Zhu <teawater@gmail.com>, jasowang@redhat.com,
        akpm@linux-foundation.org, pagupta@redhat.com,
        mojha@codeaurora.org, namit@vmware.com,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, qemu-devel@nongnu.org,
        Hui Zhu <teawaterz@linux.alibaba.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>
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From:   David Hildenbrand <david@redhat.com>
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Date:   Tue, 31 Mar 2020 16:03:18 +0200
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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=
>:
>>>>>>>
>>>>>>> =EF=BB=BFOn Thu, Mar 12, 2020 at 09:51:25AM +0100, David Hildenbr=
and wrote:
>>>>>>>>> On 12.03.20 09:47, Michael S. Tsirkin wrote:
>>>>>>>>> On Thu, Mar 12, 2020 at 09:37:32AM +0100, David Hildenbrand wro=
te:
>>>>>>>>>> 2. You are essentially stealing THPs in the guest. So the fast=
est
>>>>>>>>>> mapping (THP in guest and host) is gone. The guest won't be ab=
le 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 prope=
r
>>>>>>>>>> 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_or=
der,
>>>>>>>>>                    unsigned int max_order, unsigned int *order)
>>>>>>>>>
>>>>>>>>> the idea would then be to return at a number of pages in the gi=
ven
>>>>>>>>> range.
>>>>>>>>>
>>>>>>>>> What do you think? Want to try implementing that?
>>>>>>>>
>>>>>>>> You can just start with the highest order and decrement the orde=
r until
>>>>>>>> your allocation succeeds using alloc_pages(), which would be eno=
ugh for
>>>>>>>> a first version. At least I don't see the immediate need for a n=
ew
>>>>>>>> kernel API.
>>>>>>>
>>>>>>> OK I remember now.  The problem is with reclaim. Unless reclaim i=
s
>>>>>>> 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=E2=80=98s a pure optimization IMHO.
>>>>>> So I think we should do a trivial implementation first and then se=
e what we gain from a new allocator API. Then we might also be able to ju=
stify 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/ball=
oon_compaction.h
>=20
> .....
>=20
>=20
>>>> 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 balloo=
n_dev_info *b_dev_info,
>>>>  EXPORT_SYMBOL_GPL(balloon_page_list_dequeue);
>>>> =20
>>>>  /*
>>>> - * 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 balloo=
n page.
>>>>   * Driver must call balloon_page_enqueue before definitively removi=
ng 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 =3D alloc_page(balloon_mapping_gfp_mask() |
>>>> -				       __GFP_NOMEMALLOC | __GFP_NORETRY |
>>>> -				       __GFP_NOWARN);
>>>> -	return page;
>>>> +	struct page *page;
>>>> +
>>>> +	while (order >=3D 0) {
>>>> +		page =3D 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);
>>>> =20
>>>>  /*
>>>>   * balloon_page_enqueue - inserts a new page into the balloon page =
list.
>>>
>>>
>>> I think this will try to invoke direct reclaim from the first iterati=
on
>>> 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 i=
t
>> can sleep). It will avoid disruptive actions like OOM killer.
>>
>> Certainly good enough for a first version I would say, no?
>=20
> Frankly how well that behaves would depend a lot on the workload.
> Can regress just as well.
>=20
> 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 configur=
ation
> as now - when we can't satisfy the lowest order allocation.

Agreed.

>=20
> Anything else would be a huge amount of testing with all kind of
> workloads.
>=20

So doing a "& ~__GFP_RECLAIM" in case order > 0? (as done in
GFP_TRANSHUGE_LIGHT)

--=20
Thanks,

David / dhildenb