linux-kernel - Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP

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Message-ID: <20200312043859-mutt-send-email-mst@kernel.org>
Date:   Thu, 12 Mar 2020 04:47:11 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Hui Zhu <teawater@...il.com>, jasowang@...hat.com,
        akpm@...ux-foundation.org, pagupta@...hat.com,
        mojha@...eaurora.org, namit@...are.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        Hui Zhu <teawaterz@...ux.alibaba.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Subject: Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER
 to handle THP spilt issue

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?

-- 
MST