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 <bhar...@linux.vnet.ibm.com> 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 <bhar...@linux.vnet.ibm.com> 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 <bhar...@linux.vnet.ibm.com> > > > > > > > > > > > > Reviewed-by: David Gibson <da...@gibson.dropbear.id.au> > > > > > > > > > > > > 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
signature.asc
Description: PGP signature