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

[Michael S. Tsirkin]

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.


> -- 
> Thanks,
> 
> David / dhildenb