Re: WTH is going on with memory hotplug sysf interface (was: Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks)

[Michal Hocko]

On Mon 13-03-17 11:31:10, Igor Mammedov wrote:
> On Fri, 10 Mar 2017 14:58:07 +0100
[...]
> > [    0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x00000000-0x0009ffff]
> > [    0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x00100000-0x3fffffff]
> > [    0.000000] ACPI: SRAT: Node 1 PXM 1 [mem 0x40000000-0x7fffffff]
> > [    0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x100000000-0x27fffffff] hotplug
> > [    0.000000] NUMA: Node 0 [mem 0x00000000-0x0009ffff] + [mem 0x00100000-0x3fffffff] -> [mem 0x00000000-0x3fffffff]
> > [    0.000000] NODE_DATA(0) allocated [mem 0x3fffc000-0x3fffffff]
> > [    0.000000] NODE_DATA(1) allocated [mem 0x7ffdc000-0x7ffdffff]
> > [    0.000000] Zone ranges:
> > [    0.000000]   DMA      [mem 0x0000000000001000-0x0000000000ffffff]
> > [    0.000000]   DMA32    [mem 0x0000000001000000-0x000000007ffdffff]
> > [    0.000000]   Normal   empty
> > [    0.000000] Movable zone start for each node
> > [    0.000000] Early memory node ranges
> > [    0.000000]   node   0: [mem 0x0000000000001000-0x000000000009efff]
> > [    0.000000]   node   0: [mem 0x0000000000100000-0x000000003fffffff]
> > [    0.000000]   node   1: [mem 0x0000000040000000-0x000000007ffdffff]
> > 
> > so there is neither any normal zone nor movable one at the boot time.
> it could be if hotpluggable memory were present at boot time in E802 table
> (if I remember right when running on hyperv there is movable zone at boot time),
> 
> but in qemu hotpluggable memory isn't put into E820,
> so zone is allocated later when memory is enumerated
> by ACPI subsystem and onlined.
> It causes less issues wrt movable zone and works for
> different versions of linux/windows as well.
> 
> That's where in kernel auto-onlining could be also useful,
> since user would be able to start-up with with small
> non removable memory plus several removable DIMMs
> and have all the memory onlined/available by the time
> initrd is loaded. (missing piece here is onling
> removable memory as movable by default).

Why we should even care to online that memory that early rather than
making it available via e820?
 
> > Then I hotplugged 1G slot
> > (qemu) object_add memory-backend-ram,id=mem1,size=1G
> > (qemu) device_add pc-dimm,id=dimm1,memdev=mem1
> You can also specify node a pc-dimm goes to with 'node' property
> if it should go to other then node 0.
> 
> device_add pc-dimm,id=dimm1,memdev=mem1,node=1

thanks for the tip

> > unfortunatelly the memory didn't show up automatically and I got
> > [  116.375781] acpi PNP0C80:00: Enumeration failure
> it should work,
> do you have CONFIG_ACPI_HOTPLUG_MEMORY enabled?

No I didn't. Thanks, good to know!
-- 
Michal Hocko
SUSE Labs