On Thu 02-03-17 18:03:15, Igor Mammedov wrote: > On Thu, 2 Mar 2017 15:28:16 +0100 > Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > On Thu 02-03-17 14:53:48, Igor Mammedov wrote: > > [...] > > > When trying to support memory unplug on guest side in RHEL7, > > > experience shows otherwise. Simplistic udev rule which onlines > > > added block doesn't work in case one wants to online it as movable. > > > > > > Hotplugged blocks in current kernel should be onlined in reverse > > > order to online blocks as movable depending on adjacent blocks zone. > > > > Could you be more specific please? Setting online_movable from the udev > > rule should just work regardless of the ordering or the state of other > > memblocks. If that doesn't work I would call it a bug. > It's rather an implementation constrain than a bug > for details and workaround patch see > [1] https://bugzilla.redhat.com/show_bug.cgi?id=1314306#c7 "You are not authorized to access bug #1314306" could you paste the reasoning here please? > patch attached there is limited by another memory hotplug > issue, which is NORMAL/MOVABLE zone balance, if kernel runs > on configuration where the most of memory is hot-removable > kernel might experience lack of memory in zone NORMAL. yes and that is an inherent problem of movable memory. > > > Which means simple udev rule isn't usable since it gets event from > > > the first to the last hotplugged block order. So now we would have > > > to write a daemon that would > > > - watch for all blocks in hotplugged memory appear (how would it know) > > > - online them in right order (order might also be different depending > > > on kernel version) > > > -- it becomes even more complicated in NUMA case when there are > > > multiple zones and kernel would have to provide user-space > > > with information about zone maps > > > > > > In short current experience shows that userspace approach > > > - doesn't solve issues that Vitaly has been fixing (i.e. onlining > > > fast and/or under memory pressure) when udev (or something else > > > might be killed) > > > > yeah and that is why the patch does the onlining from the kernel. > onlining in this patch is limited to hyperv and patch breaks > auto-online on x86 kvm/vmware/baremetal as they reuse the same > hotplug path. Those can use the udev or do you see any reason why they couldn't? > > > > Can you imagine any situation when somebody actually might want to have > > > > this knob enabled? From what I understand it doesn't seem to be the > > > > case. > > > For x86: > > > * this config option is enabled by default in recent Fedora, > > > > How do you want to support usecases which really want to online memory > > as movable? Do you expect those users to disable the option because > > unless I am missing something the in kernel auto onlining only supporst > > regular onlining. > > current auto onlining config option does what it's been designed for, > i.e. it onlines hotplugged memory. > It's possible for non average Fedora user to override default > (commit 86dd995d6) if she/he needs non default behavior > (i.e. user knows how to online manually and/or can write > a daemon that would handle all of nuances of kernel in use). > > For the rest when Fedora is used in cloud and user increases memory > via management interface of whatever cloud she/he uses, it just works. > > So it's choice of distribution to pick its own default that makes > majority of user-base happy and this patch removes it without taking > that in consideration. You still can have a udev rule to achive the same thing for non-ballooning based hotplug. > How to online memory is different issue not related to this patch, > current default onlining as ZONE_NORMAL works well for scaling > up VMs. > > Memory unplug is rather new and it doesn't work reliably so far, > moving onlining to user-space won't really help. Further work > is need to be done so that it would work reliably. The main problem I have with this is that this is a limited usecase driven configuration knob which doesn't work properly for other usecases (namely movable online once your distribution choses to set the config option to auto online). There is a userspace solution for this so this shouldn't have been merged in the first place! It sneaked a proper review process (linux-api wasn't CC to get a broader attenttion) which is really sad. So unless this causes a major regression which would be hard to fix I will submit the patch for inclusion. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
