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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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Message-ID: <118bc13b-76b2-f5a1-6aca-65bd10a22f6c@redhat.com>
Date:   Tue, 31 Mar 2020 16:29:02 +0200
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On 31.03.20 16:18, Michael S. Tsirkin wrote:
> On Tue, Mar 31, 2020 at 04:09:59PM +0200, David Hildenbrand wrote:
>=20
> ...
>=20
>>>>>>>>>>>>> So if we want to address this, IMHO this calls for a new AP=
I.
>>>>>>>>>>>>> Along the lines of
>>>>>>>>>>>>>
>>>>>>>>>>>>>    struct page *alloc_page_range(gfp_t gfp, unsigned int mi=
n_order,
>>>>>>>>>>>>>                    unsigned int max_order, unsigned int *or=
der)
>>>>>>>>>>>>>
>>>>>>>>>>>>> the idea would then be to return at a number of pages in th=
e given
>>>>>>>>>>>>> range.
>>>>>>>>>>>>>
>>>>>>>>>>>>> What do you think? Want to try implementing that?
>=20
> ..
>=20
>> I expect the whole "steal huge pages from your guest" to be problemati=
c,
>> as I already mentioned to Alex. This needs a performance evaluation.
>>
>> This all smells like a lot of workload dependent fine-tuning. :)
>=20
>=20
> So that's why I proposed the API above.
>=20
> The idea is that *if we are allocating a huge page anyway*,
> rather than break it up let's send it whole to the device.
> If we have smaller pages, return smaller pages.
>=20

Sorry, I still fail to see why you cannot do that with my version of
balloon_pages_alloc(). But maybe I haven't understood the magic you
expect to happen in alloc_page_range() :)

It's just going via a different inflate queue once we have that page, as
I stated in front of my draft patch "but with an
optimized reporting interface".

> That seems like it would always be an improvement, whatever the
> workload.
>=20

Don't think so. Assume there are plenty of 4k pages lying around. It
might actually be *bad* for guest performance if you take a huge page
instead of all the leftover 4k pages that cannot be merged. Only at the
point where you would want to break a bigger page up and report it in
pieces, where it would definitely make no difference.

I guess Hui Zhu now has something to look into/work on :)

--=20
Thanks,

David / dhildenb