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

Wed Nov 18 12:42:58 UTC 2020

On Wed, Nov 18, 2020 at 05:40:04PM +0800, Zhong, Luyao wrote:
>
>
>On 11/12/2020 5:27 PM, Martin Kletzander wrote:
>> On Thu, Nov 12, 2020 at 02:19:06PM +0800, Zhong, Luyao wrote:
>>>
>>>
>>> On 11/9/2020 7:21 PM, Martin Kletzander wrote:
>>>> On Sat, Nov 07, 2020 at 10:41:52AM +0800, Zhong, Luyao wrote:
>>>>>
>>>>>
>>>>> On 11/4/2020 9:02 PM, Martin Kletzander wrote:
>>>>>> On Fri, Oct 16, 2020 at 10:38:51PM +0800, Zhong, Luyao wrote:
>>>>>>> On 10/16/2020 9:32 PM, Zang, Rui wrote:
>>>>>>>>
>>>>>>>> How about if “migratable” is set, “mode” should be ignored/omitted?
>>>>>>>> So any setting of “mode” will be rejected with an error
>>>>>>>> indicating an
>>>>>>>> invalid configuration.
>>>>>>>> We can say in the doc that “migratable” and “mode” shall not be set
>>>>>>>> together. So even the default value of “mode” is not taken.
>>>>>>>>
>>>>>>> If "mode" is not set, it's the same as setting "strict" value
>>>>>>> ('strict'
>>>>>>> is the default value). It involves some code detail, it will be
>>>>>>> translated to enumerated type, the value is 0 when mode not set or
>>>>>>> set
>>>>>>> to 'strict'. The code is in some fixed skeleton, so it's not easy to
>>>>>>> modify.
>>>>>>>
>>>>>>
>>>>>> Well I see it as it is "strict". It does not mean "strict cgroup
>>>>>> setting",
>>>>>> because cgroups are just one of the ways to enforce this.  Look at it
>>>>>> this way:
>>>>>>
>>>>>> mode can be:
>>>>>>   - strict: only these nodes can be used for the memory
>>>>>>   - preferred: there nodes should be preferred, but allocation should
>>>>>> not fail
>>>>>>   - interleave: interleave the memory between these nodes
>>>>>>
>>>>>> Due to the naming this maps to cgroup settings 1:1.
>>>>>>
>>>>
>>>> Sorry, I misspoke, this does not map to cgroup settings at all, in
>>>> cgroups you
>>>> can only set "strict" (by using cpuset.mems) and that's it.  There is no
>>>> way to
>>>> set preferred or interleaved mapping, sorry.
>>>>
>>> memory policy is independent of cpuset.mems
>>>
>>
>> Yes.
>>
>>> I quote here "Memory policies should not be confused with cpusets
>>> (Documentation/admin-guide/cgroup-v1/cpusets.rst) which is an
>>> administrative mechanism for restricting the nodes from which memory may
>>> be allocated by a set of processes. Memory policies are a programming
>>> interface that a NUMA-aware application can take advantage of.
>>
>> Pay attention to this part:
>>
>>> When both cpusets and policies are applied to a task, the restrictions
>>> of the
>>> cpuset takes priority.
>>
>>> See Memory Policies and cpusets below for more details."[1]
>>>
>>> So using cpuset.mems does not mean set "strict" memory policy if I
>>> understand it correctly, we can set cpuset.mems with any memory policy.
>>>
>>
>> That's not how I understand that.  Sure, it's independent of memory
>> policy, but
>> if you do not specify memory policy (which keeps it as "default") and set
>> cpuset.mems, then the process will only be permitted to allocate memory
>> on NUMA
>> nodes specified in the file.
>>
>yes it's not conflict with what I was saying, it's one kind of combinations.
>
>For instance, we can also set cpuset.mems to "1,2" and use mbind() set
>memory policy to MPOL_PREFERRED and preferred node is "1", that means we
>will allocate pages from the node 1 first then fall back to other
>nodes(only node 2 under this case since cpuset.mems restrict the memory
>nodes) if the preferred nodes is low on free memory. If the prefered
>node is "0", we will not allocate pages from node 0 since cpuset.mems
>takes priority.
>
>Do you mean cpuset.mems + MPOL_DEFAULT policy == MPOL_BIND policy? They
>might be functionally similar if not specific policies implemented in
>kernel. But I don't think they are exactly the same.
>

It's not the same, but has similar outcome.  If you do numa_set_membind() or
set_mempolicy(MPOL_BIND) on nodes 2-3, then the memory allocations will only be
done on those nodes.  If there is no space on them the allocation will fail.  If
you write 2-3 into cpuset.mems, then similar thing happens: allocations will
only be done on the mentioned nodes and will fail if there is not enough space.

If you are talking about mbind() in qemu, that is done *after* the allocation,
but *before* any write to that.  If that is anonymous memory or a private
file-backed memory, then writes that allocate will be allocated on the specified
nodes.  If you write to cpuset.mems *after* the allocations were done, then they
might be moved to those nodes based on cpuset.memory_migrate and couple of other
things.

>Because default policy indicate that we are using policy implemented in
>kernel(transparent to process), just like the memory
>tiering(https://lwn.net/Articles/802544/) case I stated before. So
>cpuset.mems + MPOL_DEFAULT policy is not MPOL_BIND policy under this case.
>

Yes, there are some other minor details between mpol_default and mpol_bind.  Are
you referring to the fact that mpol_default prefers same node as the thread is
running on whereas mpol_bind allocates on the node with the lowest ID from the
set, no matter what node the thread requesting the allocation is running on,
etc.?

If yes, then sure, I completely agree that someone might want to use cpuset.mems
with mpol_default.  That's what we're both trying to achieve here, right?  I
think the misunderstanding here comes from the fact we both understand what
libvirt XML is trying to describe.

In libvirt XML we have the setting for "mode" which accepts possible values of
"strict", "interleave" and "preferred".  It *does not* say *how* that is
achieved (in fact in the past we were using different mechanisms for that).  If
you say "default" in the terms of libvirt XML it means he value used if no
setting was specified.  But since we do not expose "memory policy" per se it is
very misleading to add such value.  In my opinion it would also be very
difficult to describe its outcome with regard to the "mode".

Given what I described earlier, the main difference between cgroups and libnuma
APIs is (at least from what I remember) that cgroup settings can be changed
(even if The API settings could be changed we'd need cooperation from QEMU to
change them at runtime).  And if cpuset.memory_migrate is set it can be migrated
to another node.  That was also requested couple of times in the past and that's
what I based my suggestion on.

I now understand better what you mean when you are looking at it from the other
side needing to keep MPOL_DEFAULT, but with restricted set of NUMA nodes allowed
for allocation.  You don't really care whether it is going to be migratable or
not so that naming does not make sense for your use case (which also means it
would not make sense for all of our users).  So my naming is bad too, probably
worse than your suggestion.

So let's finish this sooner rather than later.  Let's remove the
"migratable/movable" attribute and add another mode for the way memory
allocations are going to be treated.  But let's name it something else than
"default" and let's explain the name properly in man pages and documentation.
Since naming is hard and names I come up with are usually bad I can only suggest
a lowest bottom fallback if we can't come up with anything else =) Let's say
something like "restrictive".

Sure, I don't like that the only difference between "strict" and "the_new_name"
would be whether we pass the options to qemu or not, but saying that would not
really help users very much.

Does that sound OK to you?

>Regards,
>Luyao
>
>>> [1]https://www.infradead.org/~mchehab/kernel_docs/admin-guide/mm/numa_memory_policy.html
>>>
>>>>>> But now we have another way of enforcing this, using qemu cmdline
>>>>>> option.  The
>>>>>> names actually map 1:1 to those as well:
>>>>>>
>>>>>>
>>>>>> https://gitlab.com/qemu-project/qemu/-/blob/master/qapi/machine.json#L901
>>>>>>
>>>>>>
>>>>>>
>>>>>> So my idea was that we would add a movable/migratable/whatever
>>>>>> attribute
>>>>>> that
>>>>>> would tell us which way for enforcing we use because there does not
>>>>>> seem
>>>>>> to be
>>>>>> "one size fits all" solution.  Am I misunderstanding this discussion?
>>>>>> Please
>>>>>> correct me if I am.  Thank you.
>>>>>>
>>>>> Actually I need a default memory policy(memory policy is 'hard coded'
>>>>> into the kernel) support, I thought "migratable" was enough to indicate
>>>>
>>>> So I am getting on your track, yes.  What you mean is basically
>>>> MPOL_DEFAULT and
>>>> that's where the naming probably comes from, right?  Anyway, what we're
>>>> trying
>>>> to do is not restrict us from other options, even if they are only
>>>> possible in
>>>> the future.  So instead of adding "default" which would actually mean
>>>> "strict"
>>>> (because you still use cpuset.mems) which would restrict us from
>>>> potentially
>>>> being able to migrate with a different policy than "strict" (even
>>>> though it
>>>> might not make sense for "preferred", for example) and it's also a bit
>>>> confusing
>>> as I mentioned above, using "cpuset.mems" does not mean "strict" memory
>>> policy.
>>>> for users, I suggested we add "migratable" which restricts just the qemu
>>>> options.  Of course, "migratable" only makes sense with "strict" now,
>>>> but that's
>>>> fine.  The XML provides a possibility for something we don't support,
>>>> but we can
>>>> forbid that combination for the sake of clarity of the other option that
>>>> _is_
>>>> supported.
>>>>
>>>> I'll try to propose my idea based on your patch from Nov 3rd and it
>>>> might
>>>> improve my communication.  I feels difficult for me to explain myself
>>>> without
>>>> the code.  I just need to deal with a lot of other emails first.
>>>>
>>> Thank you in advance. Let's discuss later based on the patch.
>>>
>>> Regards,
>>> Luyao
>>>>> that we rely on operating system to operate memory policy. So when
>>>>> "migratable" is set, "mode" should not be set. But when I was coding, I
>>>>> found "mode" default value is "strict", it is always "strict" even if
>>>>> "migratable" is yes, that means we configure two different memory
>>>>> policies at the same time. Then I still need a new option for "mode" to
>>>>> make it not conflicting with the "migratable", then if we have the new
>>>>> option("default") for "mode", it seems we can drop "migratable".
>>>>>
>>>>> Besides, we can make "mode" being a "one size fits all" solution., just
>>>>> reject the different "mode" value config in memnode element when "mode"
>>>>> is "default" in memory element.
>>>>>
>>>>> I summary it in the new email
>>>>> https://www.redhat.com/archives/libvir-list/2020-November/msg00084.html
>>>>>
>>>>> Sorry I didn't make it easy to understand.
>>>>>
>>>>> Regards,
>>>>> Luyao
>>>>>>> So I need a option to indicate "I don't specify any mode.".
>>>>>>>
>>>>>>>>> 在 2020年10月16日，20:34，Zhong, Luyao <luyao.zhong at intel.com>
>>>>>>>>> 写道：
>>>>>>>>>
>>>>>>>>> Hi Martin, Peter and other experts,
>>>>>>>>>
>>>>>>>>> We got a consensus that we need introducing a new "migratable"
>>>>>>>>> attribute before. But in implementation, I found introducing a new
>>>>>>>>> 'default' option for existing mode attribute is still neccessary.
>>>>>>>>>
>>>>>>>>> I have a initial patch for 'migratable' and Peter gave some
>>>>>>>>> comments
>>>>>>>>> already.
>>>>>>>>> https://www.redhat.com/archives/libvir-list/2020-October/msg00396.html
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Current issue is, if I set 'migratable', any 'mode' should be
>>>>>>>>> ignored. Peter commented that I can't rely on docs to tell users
>>>>>>>>> some config is invalid, I need to reject the config in the code, I
>>>>>>>>> completely agree with that. But the 'mode' default value is
>>>>>>>>> 'strict', it will always conflict with the 'migratable', at the end
>>>>>>>>> I still need introducing a new option for 'mode' which can be a
>>>>>>>>> legal config when 'migratable' is set.
>>>>>>>>>
>>>>>>>>> If we have 'default' option, is 'migratable' still needed then?
>>>>>>>>>
>>>>>>>>> FYI.
>>>>>>>>> The 'mode' is corresponding to memory policy, there already a
>>>>>>>>> notion
>>>>>>>>> of default memory policy.
>>>>>>>>>   quote:
>>>>>>>>>     System Default Policy:  this policy is "hard coded" into the
>>>>>>>>> kernel.
>>>>>>>>> (https://www.kernel.org/doc/Documentation/vm/numa_memory_policy.txt)
>>>>>>>>>
>>>>>>>>> So it might be easier to understand if we introduce a 'default'
>>>>>>>>> option directly.
>>>>>>>>>
>>>>>>>>> Regards,
>>>>>>>>> Luyao
>>>>>>>>>
>>>>>>>>>> On 8/26/2020 6:20 AM, Martin Kletzander wrote:
>>>>>>>>>>> On Tue, Aug 25, 2020 at 09:42:36PM +0800, Zhong, Luyao wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> 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>
>>>>>>>>>>>>>>>>>>> Ã‚Â ...
>>>>>>>>>>>>>>>>>>> Ã‚Â <numatune>
>>>>>>>>>>>>>>>>>>> Ã‚Â Ã‚Â  <memory mode="strict" nodeset="1-4,^3"/> Ã‚Â Ã‚Â
>>>>>>>>>>>>>>>>>>> <memnode cellid="0" mode="strict" nodeset="1"/> Ã‚Â Ã‚Â
>>>>>>>>>>>>>>>>>>> <memnode
>>>>>>>>>>>>>>>>>>> cellid="2" mode="preferred" nodeset="2"/> Ã‚Â </numatune>
>>>>>>>>>>>>>>>>>>> Ã‚Â ...
>>>>>>>>>>>>>>>>>>> </domain>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Currently, attribute mode is either 'interleave',
>>>>>>>>>>>>>>>>>>> 'strict', or
>>>>>>>>>>>>>>>>>>> 'preferred', I propose to add a new 'default'Ã‚Â  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>
>>>>>>>>>>>>>>>>>>> Ã‚Â ...
>>>>>>>>>>>>>>>>>>> Ã‚Â <numatune>
>>>>>>>>>>>>>>>>>>> Ã‚Â Ã‚Â  <memory mode="default" nodeset="1-2"/> Ã‚Â Ã‚Â
>>>>>>>>>>>>>>>>>>> <memnode
>>>>>>>>>>>>>>>>>>> cellid="0" mode="default" nodeset="1"/> Ã‚Â Ã‚Â  <memnode
>>>>>>>>>>>>>>>>>>> cellid="1" mode="default" nodeset="2"/> Ã‚Â </numatune>
>>>>>>>>>>>>>>>>>>> Ã‚Â ...
>>>>>>>>>>>>>>>>>>> </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>
>>>>>>>>>>>>>>>>>>> Ã‚Â ...
>>>>>>>>>>>>>>>>>>> Ã‚Â <numa>
>>>>>>>>>>>>>>>>>>> Ã‚Â Ã‚Â  <cell id='0' cpus='0-3' memory='512000'
>>>>>>>>>>>>>>>>>>> unit='KiB' /> Ã‚Â
>>>>>>>>>>>>>>>>>>> Ã‚Â  <cell id='1' cpus='4-7' memory='512000'
>>>>>>>>>>>>>>>>>>> unit='KiB' />
>>>>>>>>>>>>>>>>>>> Ã‚Â
>>>>>>>>>>>>>>>>>>> </numa> Ã‚Â ...
>>>>>>>>>>>>>>>>>>> </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.Ã‚Â  The memory is not
>>>>>>>>>>>>>>>>>> (always)
>>>>>>>>>>>>>>>>>> allocated by the vCPU threads.Ã‚Â  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.Â  We can set the memory nodes that a
>>>>>>>>>>>>>>>> process
>>>>>>>>>>>>>>>> will allocate from.Â  However to set the node for the
>>>>>>>>>>>>>>>> process
>>>>>>>>>>>>>>>> (thread) QEMU needs to be started with the vCPU threads
>>>>>>>>>>>>>>>> already
>>>>>>>>>>>>>>>> spawned (albeit stopped).Â  And for that QEMU already
>>>>>>>>>>>>>>>> allocates some
>>>>>>>>>>>>>>>> memory.Â  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).Â  In all these cases cgroups will not do
>>>>>>>>>>>>>>>> what you
>>>>>>>>>>>>>>>> want them to do.Â  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).Â  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.Â  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?
>>>>>>>>>>>
>>>>>>>>>> Yeah, I was thinking something along the lines of:
>>>>>>>>>> <numatune>
>>>>>>>>>>     <memory mode="strict" nodeset="1-4,^3"
>>>>>>>>>> movable/migratable="yes/no" />
>>>>>>>>>>     <memnode cellid="0" mode="strict" nodeset="1"/>
>>>>>>>>>>     <memnode cellid="2" mode="preferred" nodeset="2"/>
>>>>>>>>>> </numatune>
>>>>>>>>>>> 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.
>>>>>>>>>>>
>>>>>>>>>> Yes, the example above gives the impression of the attribute being
>>>>>>>>>> available
>>>>>>>>>> per-node.  But that could be handled in the documentation.
>>>>>>>>>> Specifying it per-node seems very weird, why would you want the
>>>>>>>>>> memory to be
>>>>>>>>>> hard-locked, but for some guest nodes only?
>>>>>>>>>>> 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
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>
>>>>>
>>>
>
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