On Mon, Jun 07, 2021 at 09:54:18PM +0200, David Hildenbrand wrote: > Hi, > > this series aims at improving in-kernel auto-online support. It tackles the > fundamental problems that: Hi David, the idea sounds good to me, and I like that this series takes away part of the responsability from the user to know where the memory should go. I think the kernel is a much better fit for that as it has all the required information to balance things. I also glanced over the series and besides some things here and there the whole approach looks sane. I plan to have a look into it in a few days, just have some high level questions for the time being: > 1) We can create zone imbalances when onlining all memory blindly to > ZONE_MOVABLE, in the worst case crashing the system. We have to know > upfront how much memory we are going to hotplug such that we can > safely enable auto-onlining of all hotplugged memory to ZONE_MOVABLE > via "online_movable". This is far from practical and only applicable in > limited setups -- like inside VMs under the RHV/oVirt hypervisor which > will never hotplug more than 3 times the boot memory (and the > limitation is only in place due to the Linux limitation). Could you give more insight about the problems created by zone imbalances (e.g: a lot of movable memory and little kernel memory). > 2) We see more setups that implement dynamic VM resizing, hot(un)plugging > memory to resize VM memory. In these setups, we might hotplug a lot of > memory, but it might happen in various small steps in both directions > (e.g., 2 GiB -> 8 GiB -> 4 GiB -> 16 GiB ...). virtio-mem is the > primary driver of this upstream right now, performing such dynamic > resizing NUMA-aware via multiple virtio-mem devices. > > Onlining all hotplugged memory to ZONE_NORMAL means we basically have > no hotunplug guarantees. Onlining all to ZONE_MOVABLE means we can > easily run into zone imbalances when growing a VM. We want a mixture, > and we want as much memory as reasonable/configured in ZONE_MOVABLE. > > 3) Memory devices consist of 1..X memory block devices, however, the > kernel doesn't really track the relationship. Consequently, also user > space has no idea. We want to make per-device decisions. As one > example, for memory hotunplug it doesn't make sense to use a mixture of > zones within a single DIMM: we want all MOVABLE if possible, otherwise > all !MOVABLE, because any !MOVABLE part will easily block the DIMM from > getting hotunplugged. As another example, virtio-mem operates on > individual units that span 1..X memory blocks. Similar to a DIMM, we > want a unit to either be all MOVABLE or !MOVABLE. Further, we want > as much memory of a virtio-mem device to be MOVABLE as possible. So, a virtio-mem unit could be seen as DIMM right? > 4) We want memory onlining to be done right from the kernel while adding > memory; for example, this is reqired for fast memory hotplug for > drivers that add individual memory blocks, like virito-mem. We want a > way to configure a policy in the kernel and avoid implementing advanced > policies in user space. "we want memory onlining to be done right from the kernel while adding memory" is not that always the case when a driver adds memory? User has no interaction with that right? > The auto-onlining support we have in the kernel is not sufficient. All we > have is a) online everything movable (online_movable) b) online everything > !movable (online_kernel) c) keep zones contiguous (online). This series > allows configuring c) to mean instead "online movable if possible according > to the coniguration, driven by a maximum MOVABLE:KERNEL ratio" -- a new > onlining policy. > > This series does 3 things: > > 1) Introduces the "auto-movable" online policy that initially operates on > individual memory blocks only. It uses a maximum MOVABLE:KERNEL ratio > to make a decision whether a memory block will be onlined to > ZONE_MOVABLE or not. However, in the basic form, hotplugged KERNEL > memory does not allow for more MOVABLE memory (details in the > patches). CMA memory is treated like MOVABLE memory. How a user would know which ratio is sane? Could we add some info in the Docu part that kinda sets some "basic" rules? > 2) Introduces static (e.g., DIMM) and dynamic (e.g., virtio-mem) memory > groups and uses group information to make decisions in the > "auto-movable" online policy accross memory blocks of a single memory > device (modeled as memory group). So, the distinction being that a DIMM cannot grow larger but we can add more memory to a virtio-mem unit? I feel I am missing some insight here. > 3) Maximizes ZONE_MOVABLE memory within dynamic memory groups, by > allowing ZONE_NORMAL memory within a dynamic memory group to allow for > more ZONE_MOVABLE memory within the same memory group. The target use > case is dynamic VM resizing using virtio-mem. Sorry, I got lost in this one. Care to explain a bit more? > The target usage will be: > > 1) Linux boots with "mhp_default_online_type=offline" > > 2) User space (e.g., systemd unit) configures memory onlining (according > to a config file and system properties), for example: > * Setting memory_hotplug.online_policy=auto-movable > * Setting memory_hotplug.auto_movable_ratio=301 > * Setting memory_hotplug.auto_movable_numa_aware=true I think we would need to document those in order to let the user know what it is best for them. e.g: when do we want to enable auto_movable_numa_aware etc. > For DIMMs, hotplugging 4 GiB DIMMs to a 4 GiB VM with a configured ratio of > 301% results in the following layout: > Memory block 1-15: DMA32 (early) > Memory block 32-47: Normal (early) > Memory block 48-79: Movable (DIMM 0) > Memory block 80-111: Movable (DIMM 1) > Memory block 112-143: Movable (DIMM 2) > Memory block 144-275: Normal (DIMM 3) > Memory block 176-207: Normal (DIMM 4) > ... all Normal > (-> hotplugged Normal memory does not allow for more Movable memory) Uhm, I am sorry for being dense here: On x86_64, 4GB = 32 sections (of 128MB each). Why the memblock span from #1 to #47? > For virtio-mem, using a simple, single virtio-mem device with a 4 GiB VM > will result in the following layout: > Memory block 1-15: DMA32 (early) > Memory block 32-47: Normal (early) > Memory block 48-143: Movable (virtio-mem, first 12 GiB) > Memory block 144: Normal (virtio-mem, next 128 MiB) > Memory block 145-147: Movable (virtio-mem, next 384 MiB) > Memory block 148: Normal (virtio-mem, next 128 MiB) > Memory block 149-151: Movable (virtio-mem, next 384 MiB) > ... Normal/Movable mixture as above > (-> hotplugged Normal memory allows for more Movable memory within > the same device) > > Which gives us maximum flexibility when dynamically growing/shrinking a > VM in smaller steps. When shrinking, virtio-mem will prioritize unplug of > MOVABLE memory with [1] sent last week, such that we won't accidentially > trigger zone imbalances in more complicated setups that involve multiple > virtio-mem devices. -- Oscar Salvador SUSE L3
