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

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 redhat com>
Sent: Monday, August 17, 2020 4:58 PM
To: Zhong, Luyao <luyao zhong intel com>
Cc: libvir-list redhat com; Zang, Rui <rui zang intel com>; Michal Privoznik
<mprivozn 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.
>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 +
>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
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

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.

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

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.


If you agree with the direction, I think we can dig deeper to see what will
come out.

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.

>>>> Have a nice day,
>>>> Martin
>>>>> Regards,
>>>>> Luyao
>>>>> a97c3f2bad5/backends/hostmem.c#L379
>>>>> [2]https://man7.org/linux/man-pages/man2/mbind.2.html
>>>>> --
>>>>> 2.25.1

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