[libvirt] [PATCH v2 0/2] qemu: Honor memory mode='strict'

[Michal Privoznik]

On 4/12/19 12:11 PM, Daniel Henrique Barboza wrote:
> 
> 
> On 4/12/19 6:10 AM, Michal Privoznik wrote:
>> On 4/11/19 7:29 PM, Daniel Henrique Barboza wrote:
>>>
>>>
>>> On 4/11/19 11:56 AM, Michal Privoznik wrote:
>>>> On 4/11/19 4:23 PM, Daniel Henrique Barboza wrote:
>>>>> Hi,
>>>>>
>>>>> I've tested these patches again, twice, in similar setups like I 
>>>>> tested
>>>>> the first version (first in a Power8, then in a Power9 server).
>>>>>
>>>>> Same results, though. Libvirt will not avoid the launch of a 
>>>>> pseries guest,
>>>>> with numanode=strict, even if the numa node does not have available
>>>>> RAM. If I stress test the memory of the guest to force the allocation,
>>>>> QEMU exits with an error as soon as the memory of the host numa node
>>>>> is exhausted.
>>>>
>>>> Yes, this is expected. I mean, by default qemu doesn't allocate 
>>>> memory for the guest fully. You'd have to force it:
>>>>
>>>> <memoryBacking>
>>>>   <allocation mode='immediate'/>
>>>> </memoryBacking>
>>>>
>>>
>>> Tried with this extra setting, still no good. Domain still boots, 
>>> even if
>>> there is not enough memory to load up all its ram in the NUMA node
>>> I am setting. For reference, this is the top of the guest XML:
>>>
>>>
>>>    <name>vm1</name>
>>>    <uuid>f48e9e35-8406-4784-875f-5185cb4d47d7</uuid>
>>>    <memory unit='KiB'>314572800</memory>
>>>    <currentMemory unit='KiB'>314572800</currentMemory>
>>>    <memoryBacking>
>>>      <allocation mode='immediate'/>
>>>    </memoryBacking>
>>>    <vcpu placement='static'>16</vcpu>
>>>    <numatune>
>>>      <memory mode='strict' nodeset='0'/>
>>>    </numatune>
>>>    <os>
>>>      <type arch='ppc64' machine='pseries'>hvm</type>
>>>      <boot dev='hd'/>
>>>    </os>
>>>    <clock offset='utc'/>
>>>
>>> While doing this test, I recalled that some of my IBM peers recently
>>> mentioned that they were unable to do a pre-allocation of the RAM
>>> of a pseries guest using Libvirt, but they were able to do it using QEMU
>>> directly (using -realtime mlock=on). In fact, I just tried it out 
>>> with command
>>> line QEMU and the guest allocated all the memory at boot.
>>
>> Ah, so looks like -mem-prealloc doesn't work at Power? Can you please 
>> check:
>>
>> 1) that -mem-prealloc is on the qemu command line
> 
> Yes. This is the cmd line generated:
> 
> /usr/bin/qemu-system-ppc64 \
> -name guest=vm1,debug-threads=on \
> -S \
> -object 
> secret,id=masterKey0,format=raw,file=/home/user/usr/var/lib/libvirt/qemu/domain-2-vm1/master-key.aes 
> \
> -machine pseries-2.11,accel=kvm,usb=off,dump-guest-core=off \
> -bios /home/user/boot_rom.bin \
> -m 307200 \
> -mem-prealloc \
> -realtime mlock=off \

This looks correct.

> -smp 16,sockets=16,cores=1,threads=1 \
> -uuid f48e9e35-8406-4784-875f-5185cb4d47d7 \
> -display none \
> -no-user-config \
> -nodefaults \
> -chardev 
> socket,id=charmonitor,path=/home/user/usr/var/lib/libvirt/qemu/domain-2-vm1/monitor.sock,server,nowait 
> \
> -mon chardev=charmonitor,id=monitor,mode=control \
> -rtc base=utc \
> -no-shutdown \
> -boot strict=on \
> -device spapr-pci-host-bridge,index=1,id=pci.1 \
> -device qemu-xhci,id=usb,bus=pci.0,addr=0x3 \
> -drive 
> file=/home/user/nv2-vm1.qcow2,format=qcow2,if=none,id=drive-virtio-disk0 \
> -device 
> virtio-blk-pci,scsi=off,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 
> \
> -chardev pty,id=charserial0 \
> -device spapr-vty,chardev=charserial0,id=serial0,reg=0x30000000 \
> -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x1 \
> -sandbox 
> on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
> -msg timestamp=on
> 
> 
> 
> 
>> 2) how much memory qemu allocates right after it started the guest? I 
>> mean, before you start some mem stress test which causes it to 
>> allocate the memory fully.
> 
> It starts with 300Gb. It depletes its assigned NUMA node (that has 256Gb),
> then it takes ~70Gb from another NUMA node to complete the 300Gb.

Huh, than -mem-prealloc is working but something else is not. What 
strikes me is that once guest starts using the memory then host kernel 
kills the guest. So host kernel knows about the limits we've set but 
doesn't enforce them when allocating the memory.

> 
> 
>>
>>>
>>> This means that the pseries guest is able to do mem pre-alloc. I'd 
>>> say that
>>> there might be something missing somewhere (XML, host setup, libvirt
>>> config ...) or perhaps even a bug that is preventing Libvirt from doing
>>> this pre-alloc. This explains why I can't verify this patch series. 
>>> I'll see if
>>> I dig it further to understand why when I have the time.
>>
>> Yeah, I don't know Power well enough to help you. Sorry.
> 
> 
> No problem. One question: Libvirt is supposed to let the VM do the full
> allocation of its RAM using -mem-prealloc and with -realtime mlock=off,
> is that correct?

-mem-prealloc should be enough. -realtime mlock is ther to lock the 
allocated memory so that it doesn't get swapped out. You can enable 
memory locking via:

   <memoryBacking>
     <locked/>
   </memoryBacking>

Michal