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Date:   Thu, 26 Mar 2020 03:10:10 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     David Hildenbrand <david@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>
Subject: Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER
 to handle THP spilt issue
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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.

Well there's still a chance of splitting a big page if one
becomes available meanwhile. But OK.

> -- 
> Thanks,
> 
> David / dhildenb