[libvirt][RFC PATCH] add a new 'default' option for attribute mode in numatune

[Zhong, Luyao]

On 8/19/2020 11:24 PM, Martin Kletzander wrote:
> On Tue, Aug 18, 2020 at 07:49:30AM +0000, Zang, Rui wrote:
>>
>>
>>> -----Original Message-----
>>> From: Martin Kletzander <mkletzan at redhat.com>
>>> Sent: Monday, August 17, 2020 4:58 PM
>>> To: Zhong, Luyao <luyao.zhong at intel.com>
>>> Cc: libvir-list at redhat.com; Zang, Rui <rui.zang at intel.com>; Michal 
>>> Privoznik
>>> <mprivozn at redhat.com>
>>> Subject: Re: [libvirt][RFC PATCH] add a new 'default' option for 
>>> attribute mode
>>> in numatune
>>>
>>> On Tue, Aug 11, 2020 at 04:39:42PM +0800, Zhong, Luyao wrote:
>>> >
>>> >
>>> >On 8/7/2020 4:24 PM, Martin Kletzander wrote:
>>> >> On Fri, Aug 07, 2020 at 01:27:59PM +0800, Zhong, Luyao wrote:
>>> >>>
>>> >>>
>>> >>> On 8/3/2020 7:00 PM, Martin Kletzander wrote:
>>> >>>> On Mon, Aug 03, 2020 at 05:31:56PM +0800, Luyao Zhong wrote:
>>> >>>>> Hi Libvirt experts,
>>> >>>>>
>>> >>>>> I would like enhence the numatune snippet configuration. Given a
>>> >>>>> example snippet:
>>> >>>>>
>>> >>>>> <domain>
>>> >>>>>  ...
>>> >>>>>  <numatune>
>>> >>>>>    <memory mode="strict" nodeset="1-4,^3"/>  ÂÂ
>>> >>>>> <memnode cellid="0" mode="strict" nodeset="1"/>    <memnode
>>> >>>>> cellid="2" mode="preferred" nodeset="2"/>  </numatune>  ...
>>> >>>>> </domain>
>>> >>>>>
>>> >>>>> Currently, attribute mode is either 'interleave', 'strict', or
>>> >>>>> 'preferred', I propose to add a new 'default'  option. I give
>>> >>>>> the reason as following.
>>> >>>>>
>>> >>>>> Presume we are using cgroups v1, Libvirt sets cpuset.mems for all
>>> >>>>> vcpu threads according to 'nodeset' in memory element. And
>>> >>>>> translate the memnode element to qemu config options (--object
>>> >>>>> memory-backend-ram) for per numa cell, which invoking mbind()
>>> >>>>> system call at the end.[1]
>>> >>>>>
>>> >>>>> But what if we want using default memory policy and request each
>>> >>>>> guest numa cell pinned to different host memory nodes? We can't
>>> >>>>> use mbind via qemu config options, because (I quoto here) "For
>>> >>>>> MPOL_DEFAULT, the nodemask and maxnode arguments must be specify
>>> >>>>> the empty set of nodes." [2]
>>> >>>>>
>>> >>>>> So my solution is introducing a new 'default' option for attribute
>>> >>>>> mode. e.g.
>>> >>>>>
>>> >>>>> <domain>
>>> >>>>>  ...
>>> >>>>>  <numatune>
>>> >>>>>    <memory mode="default" nodeset="1-2"/>    <memnode
>>> >>>>> cellid="0" mode="default" nodeset="1"/>    <memnode
>>> >>>>> cellid="1" mode="default" nodeset="2"/>  </numatune>  ...
>>> >>>>> </domain>
>>> >>>>>
>>> >>>>> If the mode is 'default', libvirt should avoid generating qemu
>>> >>>>> command line '--object memory-backend-ram', and invokes cgroups to
>>> >>>>> set cpuset.mems for per guest numa combining with numa topology
>>> >>>>> config. Presume the numa topology is :
>>> >>>>>
>>> >>>>> <cpu>
>>> >>>>>  ...
>>> >>>>>  <numa>
>>> >>>>>    <cell id='0' cpus='0-3' memory='512000' unit='KiB' /> ÂÂ
>>> >>>>>   <cell id='1' cpus='4-7' memory='512000' unit='KiB' /> ÂÂ
>>> >>>>> </numa>  ...
>>> >>>>> </cpu>
>>> >>>>>
>>> >>>>> Then libvirt should set cpuset.mems to '1' for vcpus 0-3, and '2'
>>> >>>>> for vcpus 4-7.
>>> >>>>>
>>> >>>>>
>>> >>>>> Is this reasonable and feasible? Welcome any comments.
>>> >>>>>
>>> >>>>
>>> >>>> There are couple of problems here.  The memory is not (always)
>>> >>>> allocated by the vCPU threads.  I also remember it to not be
>>> >>>> allocated by the process, but in KVM in a way that was not affected
>>> >>>> by the cgroup settings.
>>> >>>
>>> >>> Thanks for your reply. Maybe I don't get what you mean, could you
>>> >>> give me more context? But what I proposed will have no effect on
>>> >>> other memory allocation.
>>> >>>
>>> >>
>>> >> Check how cgroups work.  We can set the memory nodes that a process
>>> >> will allocate from.  However to set the node for the process
>>> >> (thread) QEMU needs to be started with the vCPU threads already
>>> >> spawned (albeit stopped).  And for that QEMU already allocates some
>>> >> memory.  Moreover if extra memory was allocated after we set the
>>> >> cpuset.mems it is not guaranteed that it will be allocated by the
>>> >> vCPU in that NUMA cell, it might be done in the emulator instead or
>>> >> the KVM module in the kernel in which case it might not be accounted
>>> >> for the process actually causing the allocation (as we've already
>>> >> seen with Linux).  In all these cases cgroups will not do what you
>>> >> want them to do.  The last case might be fixed, the first ones are
>>> >> by default not going to work.
>>> >>
>>> >>>> That might be
>>> >>>> fixed now,
>>> >>>> however.
>>> >>>>
>>> >>>> But basically what we have against is all the reasons why we
>>> >>>> started using QEMU's command line arguments for all that.
>>> >>>>
>>> >>> I'm not proposing use QEMU's command line arguments, on contrary I
>>> >>> want using cgroups setting to support a new config/requirement. I
>>> >>> give a solution about if we require default memory policy and memory
>>> >>> numa pinning.
>>> >>>
>>> >>
>>> >> And I'm suggesting you look at the commit log to see why we *had* to
>>> >> add these command line arguments, even though I think I managed to
>>> >> describe most of them above already (except for one that _might_
>>> >> already be fixed in the kernel).  I understand the git log is huge
>>> >> and the code around NUMA memory allocation was changing a lot, so I
>>> >> hope my explanation will be enough.
>>> >>
>>> >Thank you for detailed explanation, I think I get it now. We can't
>>> >guarantee memory allocation matching requirement since there is a time
>>> >slot before setting cpuset.mems.
>>> >
>>>
>>> That's one of the things, although this one could be avoided (by 
>>> setting a global
>>> cgroup before exec()).
>>>
>>> >>> Thanks,
>>> >>> Luyao
>>> >>>> Sorry, but I think it will more likely break rather than fix stuff.
>>> >>>> Maybe this
>>> >>>> could be dealt with by a switch in `qemu.conf` with a huge warning
>>> >>>> above it.
>>> >>>>
>>> >>> I'm not trying to fix something, I propose how to support a new
>>> >>> requirement just like I stated above.
>>> >>>
>>> >>
>>> >> I guess we should take a couple of steps back, I don't get what you
>>> >> are trying to achieve.  Maybe if you describe your use case it will
>>> >> be easier to reach a conclusion.
>>> >>
>>> >Yeah, I do have a usecase I didn't mention before. It's a feature in
>>> >kernel but not merged yet, we call it memory tiering.
>>> >(https://lwn.net/Articles/802544/)
>>> >
>>> >If memory tiering is enabled on host, DRAM is top tier memory, and
>>> >PMEM(persistent memory) is second tier memory, PMEM is shown as numa
>>> >node without cpu. For short, pages can be migrated between DRAM and
>>> >PMEM based on DRAM pressure and how cold/hot they are.
>>> >
>>> >We could configure multiple memory migrating path. For example, node 0:
>>> >DRAM, node 1: DRAM, node 2: PMEM, node 3: PMEM we can make 0+2 to a
>>> >group, and 1+3 to a group. In each group, page is allowed to migrated
>>> >down(demotion) and up(promotion).
>>> >
>>> >If **we want our VMs utilizing memory tiering and with NUMA topology**,
>>> >we need handle the guest memory mapping to host memory, that means we
>>> >need bind each guest numa node to a memory nodes group(DRAM node +
>>> PMEM
>>> >node) on host. For example, guest node 0 -> host node 0+2.
>>> >
>>> >However, only cgroups setting can make the memory tiering work, if we
>>> >use mbind() system call, demoted pages will never go back to DRAM.
>>> >That's why I propose to add 'default' option and bypass mbind in QEMU.
>>> >
>>> >I hope I make myself understandable. I'll appreciate if you could give
>>> >some suggestion.
>>> >
>>>
>>> This comes around every couple of months/years and bites us in the 
>>> back no
>>> matter what way we go (every time there is someone who wants it the 
>>> other
>>> way).
>>> That's why I think there could be a way for the user to specify 
>>> whether they will
>>> likely move the memory or not and based on that we would specify `host-
>>> nodes` and `policy` to qemu or not.  I think I even suggested this 
>>> before (or
>>> probably delegated it to someone else for a suggestion so that there 
>>> is more
>>> discussion), but nobody really replied.
>>>
>>> So what we need, I think, is a way for someone to set a per-domain 
>>> information
>>> whether we should bind the memory to nodes in a changeable fashion or 
>>> not.
>>> I'd like to have it in as well.  The way we need to do that is, 
>>> probably, per-
>>> domain, because adding yet another switch for each place in the XML 
>>> where we
>>> can select a NUMA memory binding would be a suicide.  There should 
>>> also be
>>> no need for this to be enabled per memory-(module, node), so it 
>>> should work
>>> fine.
>>>
>>
>> Thanks for letting us know your vision about this.
>> From what I understood, the "changeable fashion" means that the guest 
>> numa
>> cell binding can be changed out of band after initial binding, either 
>> by system
>> admin or the operating system (memory tiering in our case), or 
>> whatever the
>> third party is.  Is that perception correct?
> 
> Yes.  If the user wants to have the possibility of changing the binding, 
> then we
> use *only* cgroups.  Otherwise we use the qemu parameters that will make 
> qemu
> call mbind() (as that has other pros mentioned above).  The other option 
> would
> be extra communication between QEMU and libvirt during start to let us 
> know when
> to set what cgroups etc., but I don't think that's worth it.
> 
>> It seems to me mbind() or set_mempolicy() system calls do not offer that
>> flexibility of changing afterwards. So in case of QEMU/KVM, I can only 
>> think
>> of cgroups.
>> So to be specific, if we had this additional "memory_binding_changeable"
>> option specified, we will try to do the guest numa constraining via 
>> cgroups
>> whenever possible. There will probably also be conflicts in options or 
>> things
>> that cgroups can not do. For such cases we'd fail the domain.
> 
> Basically we'll do what we're doing now and skip the qemu `host-nodes` and
> `policy` parameters with the new option.  And of course we can fail with 
> a nice
> error message if someone wants to move the memory without the option 
> selected
> and so on.

Thanks for your comments.

I'd like get it more clear about defining the interface in domain xml, 
then I could go into the implementation further.

As you mentioned, per-domain option will be better than per-node. I go 
through the libvirt doamin format to look for a proper position to place 
this option. Then I'm thinking we could still utilizing numatune element 
to configure.

<numatune>
   <memory mode="strict" nodeset="1-4,^3"/>
   <memnode cellid="0" mode="strict" nodeset="1"/>
   <memnode cellid="2" mode="preferred" nodeset="2"/>
</numatune>

coincidentally, the optional memory element specifies how to allocate 
memory for the domain process on a NUMA host. So can we utilizing this
element, and introducing a new mode like "changeable" or whatever? Do 
you have a better name?

If the memory mode is set to 'changeable', we could ignore the mode 
setting for each memnode, and then we only configure by cgroups. I have 
not diven into code for now, expecting it could work.

Thanks,
Luyao

> 
>> If you agree with the direction, I think we can dig deeper to see what 
>> will
>> come out.
>>
>> Regards,
>> Zang, Rui
>>
>>
>>> Ideally we'd discuss it with others, but I think I am only one of a 
>>> few people
>>> who dealt with issues in this regard.  Maybe Michal (Cc'd) also dealt 
>>> with some
>>> things related to the binding, so maybe he can chime in.
>>>
>>> >regards,
>>> >Luyao
>>> >
>>> >>>> Have a nice day,
>>> >>>> Martin
>>> >>>>
>>> >>>>> Regards,
>>> >>>>> Luyao
>>> >>>>>
>>> >>>>>
>>> [1]https://github.com/qemu/qemu/blob/f2a1cf9180f63e88bb38ff21c169d
>>> >>>>> a97c3f2bad5/backends/hostmem.c#L379
>>> >>>>>
>>> >>>>>
>>> >>>>> [2]https://man7.org/linux/man-pages/man2/mbind.2.html
>>> >>>>>
>>> >>>>> --
>>> >>>>> 2.25.1
>>> >>>>>
>>> >>>
>>> >