Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support

[David Gibson]

On Tue, Apr 26, 2016 at 04:03:37PM -0500, Michael Roth wrote:
> Quoting Igor Mammedov (2016-04-26 02:52:36)
> > On Tue, 26 Apr 2016 10:39:23 +0530
> > Bharata B Rao <address@hidden> wrote:
> > 
> > > On Mon, Apr 25, 2016 at 11:20:50AM +0200, Igor Mammedov wrote:
> > > > On Wed, 16 Mar 2016 10:11:54 +0530
> > > > Bharata B Rao <address@hidden> wrote:
> > > >   
> > > > > On Wed, Mar 16, 2016 at 12:36:05PM +1100, David Gibson wrote:  
> > > > > > On Tue, Mar 15, 2016 at 10:08:56AM +0530, Bharata B Rao wrote:    
> > > > > > > Add support to hot remove pc-dimm memory devices.
> > > > > > > 
> > > > > > > Signed-off-by: Bharata B Rao <address@hidden>    
> > > > > > 
> > > > > > Reviewed-by: David Gibson <address@hidden>
> > > > > > 
> > > > > > Looks correct, but again, needs to wait on the PAPR change.  
> > > > [...]  
> > > > > 
> > > > > While we are here, I would also like to get some opinion on the real
> > > > > need for memory unplug. Is there anything that memory unplug gives us
> > > > > which memory ballooning (shrinking mem via ballooning) can't give ?  
> > > > Sure ballooning can complement memory hotplug but turning it on would
> > > > effectively reduce hotplug to balloning as it would enable overcommit
> > > > capability instead of hard partitioning pc-dimms provides. So one
> > > > could just use ballooning only and not bother with hotplug at all.
> > > > 
> > > > On the other hand memory hotplug/unplug (at least on x86) tries
> > > > to model real hardware, thus removing need in paravirt ballooning
> > > > solution in favor of native guest support.  
> > > 
> > > Thanks for your views.
> > > 
> > > > 
> > > > PS:
> > > > Guest wise, currently hot-unplug is not well supported in linux,
> > > > i.e. it's not guarantied that guest will honor unplug request
> > > > as it may pin dimm by using it as a non migratable memory. So
> > > > there is something to work on guest side to make unplug more
> > > > reliable/guarantied.  
> > > 
> > > In the above scenario where the guest doesn't allow removal of certain
> > > parts of DIMM memory, what is the expected behaviour as far as QEMU
> > > DIMM device is concerned ? I seem to be running into this situation
> > > very often with PowerPC mem unplug where I am left with a DIMM device
> > > that has only some memory blocks released. In this situation, I would like
> > > to block further unplug requests on the same device, but QEMU seems
> > > to allow more such unplug requests to come in via the monitor. So
> > > qdev won't help me here ? Should I detect such condition from the
> > > machine unplug() handler and take required action ?
> > I think offlining is a guests task along with recovering from
> > inability to offline (i.e. offline all + eject or restore original state).
> > QUEM does it's job by notifying guest what dimm it wants to remove
> > and removes it when guest asks it (at least in x86 world).
> 
> In the case of pseries, the DIMM abstraction isn't really exposed to
> the guest, but rather the memory blocks we use to make the backing
> memdev memory available to the guest. During unplug, the guest
> completely releases these blocks back to QEMU, and if it can only
> release a subset of what's requested it does not attempt to recover.
> We can potentially change that behavior on the guest side, since
> partially-freed DIMMs aren't currently useful on the host-side...
> 
> But, in the case of pseries, I wonder if it makes sense to maybe go
> ahead and MADV_DONTNEED the ranges backing these released blocks so the
> host can at least partially reclaim the memory from a partially
> unplugged DIMM?

Urgh.. I can see the benefit, but I'm a bit uneasy about making the
DIMM semantics different in this way on Power.

I'm shoehorning the PAPR DR memory mechanism into the qemu DIMM model
was a good idea after all.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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