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Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks

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  Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Igor Mammedov <imammedo@redhat.com> - 2017-03-02 15:00 +0100
    Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Michal Hocko <mhocko@kernel.org> - 2017-03-02 16:10 +0100
      Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Igor Mammedov <imammedo@redhat.com> - 2017-03-02 18:20 +0100
        Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Michal Hocko <mhocko@kernel.org> - 2017-03-03 10:10 +0100
          Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Igor Mammedov <imammedo@redhat.com> - 2017-03-03 18:40 +0100
            Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Michal Hocko <mhocko@kernel.org> - 2017-03-06 16:10 +0100
              Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Igor Mammedov <imammedo@redhat.com> - 2017-03-07 13:50 +0100
                Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Michal Hocko <mhocko@kernel.org> - 2017-03-09 14:00 +0100
                  WTH is going on with memory hotplug sysf interface (was: Re: [RFC  PATCH] mm, hotplug: get rid of auto_online_blocks) Michal Hocko <mhocko@kernel.org> - 2017-03-10 15:00 +0100
                    Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Michal Hocko <mhocko@kernel.org> - 2017-03-10 17:00 +0100
                      Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Reza Arbab <arbab@linux.vnet.ibm.com> - 2017-03-10 20:10 +0100
                        Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Michal Hocko <mhocko@kernel.org> - 2017-03-13 10:30 +0100
                          Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Reza Arbab <arbab@linux.vnet.ibm.com> - 2017-03-13 16:00 +0100
                      Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Michal Hocko <mhocko@kernel.org> - 2017-03-13 16:20 +0100
                        Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Andi Kleen <ak@linux.intel.com> - 2017-03-14 00:20 +0100
                    Re: WTH is going on with memory hotplug sysf interface Yasuaki Ishimatsu <yasu.isimatu@gmail.com> - 2017-03-10 18:40 +0100
                      Re: WTH is going on with memory hotplug sysf interface Michal Hocko <mhocko@kernel.org> - 2017-03-13 10:30 +0100
                    Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Igor Mammedov <imammedo@redhat.com> - 2017-03-13 11:40 +0100
                      Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Michal Hocko <mhocko@kernel.org> - 2017-03-13 11:50 +0100
                        Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Igor Mammedov <imammedo@redhat.com> - 2017-03-13 15:00 +0100
                          Re: WTH is going on with memory hotplug sysf interface (was: Re:  [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Michal Hocko <mhocko@kernel.org> - 2017-03-13 15:40 +0100
                  Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Igor Mammedov <imammedo@redhat.com> - 2017-03-13 12:00 +0100
                    Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Michal Hocko <mhocko@kernel.org> - 2017-03-13 13:30 +0100
                      Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-03-13 14:00 +0100
                        Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Michal Hocko <mhocko@kernel.org> - 2017-03-13 14:20 +0100
                          Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-03-13 14:50 +0100
                            Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Michal Hocko <mhocko@kernel.org> - 2017-03-13 15:40 +0100
                              Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-03-13 16:20 +0100
              Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Daniel Kiper <daniel.kiper@oracle.com> - 2017-03-10 23:10 +0100

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#1599453 — Re: WTH is going on with memory hotplug sysf interface (was: Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks)

On Mon 13-03-17 14:57:12, Igor Mammedov wrote:
> On Mon, 13 Mar 2017 11:43:02 +0100
> Michal Hocko <mhocko@kernel.org> wrote:
> 
> > 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?
> 
> It's not forbidden by spec and has less complications
> when it comes to removable memory. Declaring it in E820
> would add following limitations/drawbacks:
>  - firmware should be able to exclude removable memory
>    from its usage (currently SeaBIOS nor EFI have to
>    know/care about it) => less qemu-guest ABI to maintain.
>  - OS should be taught to avoid/move (early) nonmovable
>    allocations from removable address ranges.
>    There were patches targeting that in recent kernels,
>    but it won't work with older kernels that don't have it.
>    So limiting a range of OSes that could run on QEMU
>    and do memory removal.
> 
> E820 less approach works reasonably well with wide range
> of guest OSes and less complex that if removable memory
> were present it E820. Hence I don't have a compelling
> reason to introduce removable memory in E820 as it
> only adds to hot(un)plug issues.

OK I see and that sounds like an argument to not put those ranges to
E820. I still fail to see why we haeve to online the memory early during
the boot and cannot wait for userspace to run?

-- 
Michal Hocko
SUSE Labs

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#1599203

On Thu, 9 Mar 2017 13:54:00 +0100
Michal Hocko <mhocko@kernel.org> wrote:

[...]
> > It's major regression if you remove auto online in kernels that
> > run on top of x86 kvm/vmware hypervisors, making API cleanups
> > while breaking useful functionality doesn't make sense.
> > 
> > I would ACK config option removal if auto online keeps working
> > for all x86 hypervisors (hyperv/xen isn't the only who needs it)
> > and keep kernel CLI option to override default.
> > 
> > That doesn't mean that others will agree with flipping default,
> > that's why config option has been added.
> > 
> > Now to sum up what's been discussed on this thread, there were 2
> > different issues discussed:
> >   1) memory hotplug: remove in kernel auto online for all
> >                      except of hyperv/xen
> > 
> >        - suggested RFC is not acceptable from virt point of view
> >          as it regresses guests on top of x86 kvm/vmware which
> >          both use ACPI based memory hotplug.
> > 
> >        - udev/userspace solution doesn't work in practice as it's
> >          too slow and unreliable when system is under load which
> >          is quite common in virt usecase. That's why auto online
> >          has been introduced in the first place.  
> 
> Please try to be more specific why "too slow" is a problem. Also how
> much slower are we talking about?
In virt case on host with lots VMs, userspace handler
processing could be scheduled late enough to trigger a race
between (guest memory going away/OOM handler) and memory
coming online.

>  
> >   2) memory unplug: online memory as movable
> > 
> >        - doesn't work currently with udev rule due to kernel
> >          issues https://bugzilla.redhat.com/show_bug.cgi?id=1314306#c7  
> 
> These should be fixed
>  
> >        - could be fixed both for in kernel auto online and udev
> >          with following patch:
> >          https://bugzilla.redhat.com/attachment.cgi?id=1146332
> >          but fixing it this way exposes zone disbalance issues,
> >          which are not present in current kernel as blocks are
> >          onlined in Zone Normal. So this is area to work and
> >          improve on.
> > 
> >        - currently if one wants to use online_movable,
> >          one has to either
> >            * disable auto online in kernel OR  
> 
> which might not just work because an unmovable allocation could have
> made the memblock pinned.
With memhp_default_state=offline on kernel CLI there won't be any
unmovable allocation as hotplugged memory won't be onlined and
user can online it manually. So it works for non default usecase
of playing with memory hot-unplug.
 
> >            * remove udev rule that distro ships
> >          AND write custom daemon that will be able to online
> >          block in right zone/order. So currently whole
> >          online_movable thing isn't working by default
> >          regardless of who onlines memory.  
> 
> my epxperience with onlining full nodes as movable shows this works just
> fine (with all the limitations of the movable zones but that is a
> separate thing). I haven't played with configurations where movable
> zones are sharing the node with other zones.
I don't have access to a such baremetal configuration to play
with anymore.


> >          I'm in favor of implementing that in kernel as it keeps
> >          kernel internals inside kernel and doesn't need
> >          kernel API to be involved (memory blocks in sysfs,
> >          online_kernel, online_movable)
> >          There would be no need in userspace which would have to
> >          deal with kernel zoo and maintain that as well.  
> 
> The kernel is supposed to provide a proper API and that is sysfs
> currently. I am not entirely happy about it either but pulling a lot of
> code into the kernel is not the rigth thing to do. Especially when
> different usecases require different treatment.
If it could be done from kernel side alone, it looks like a better way
to me not to involve userspace at all. And for ACPI based x86/ARM it's
possible to implement without adding a lot of kernel code.
That's one more of a reason to keep CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
so we could continue on improving kernel only auto-onlining
and fixing current memory hot(un)plug issues without affecting
other platforms/users that are no interested in it.
(PS: I don't care much about sysfs knob for setting auto-onlining,
as kernel CLI override with memhp_default_state seems
sufficient to me)

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#1599276

On Mon 13-03-17 11:55:54, Igor Mammedov wrote:
> On Thu, 9 Mar 2017 13:54:00 +0100
> Michal Hocko <mhocko@kernel.org> wrote:
> 
> [...]
> > > It's major regression if you remove auto online in kernels that
> > > run on top of x86 kvm/vmware hypervisors, making API cleanups
> > > while breaking useful functionality doesn't make sense.
> > > 
> > > I would ACK config option removal if auto online keeps working
> > > for all x86 hypervisors (hyperv/xen isn't the only who needs it)
> > > and keep kernel CLI option to override default.
> > > 
> > > That doesn't mean that others will agree with flipping default,
> > > that's why config option has been added.
> > > 
> > > Now to sum up what's been discussed on this thread, there were 2
> > > different issues discussed:
> > >   1) memory hotplug: remove in kernel auto online for all
> > >                      except of hyperv/xen
> > > 
> > >        - suggested RFC is not acceptable from virt point of view
> > >          as it regresses guests on top of x86 kvm/vmware which
> > >          both use ACPI based memory hotplug.
> > > 
> > >        - udev/userspace solution doesn't work in practice as it's
> > >          too slow and unreliable when system is under load which
> > >          is quite common in virt usecase. That's why auto online
> > >          has been introduced in the first place.  
> > 
> > Please try to be more specific why "too slow" is a problem. Also how
> > much slower are we talking about?
>
> In virt case on host with lots VMs, userspace handler
> processing could be scheduled late enough to trigger a race
> between (guest memory going away/OOM handler) and memory
> coming online.

Either you are mixing two things together or this doesn't really make
much sense. So is this a balloning based on memory hotplug (aka active
memory hotadd initiated between guest and host automatically) or a guest
asking for additional memory by other means (pay more for memory etc.)?
Because if this is an administrative operation then I seriously question
this reasoning.

[...]
> > >        - currently if one wants to use online_movable,
> > >          one has to either
> > >            * disable auto online in kernel OR  
> > 
> > which might not just work because an unmovable allocation could have
> > made the memblock pinned.
>
> With memhp_default_state=offline on kernel CLI there won't be any
> unmovable allocation as hotplugged memory won't be onlined and
> user can online it manually. So it works for non default usecase
> of playing with memory hot-unplug.

I was talking about the case when the auto_online was true, of course,
e.g. depending on the config option which you've said is enabled in
Fedora kernels.
  
[...] 
> > >          I'm in favor of implementing that in kernel as it keeps
> > >          kernel internals inside kernel and doesn't need
> > >          kernel API to be involved (memory blocks in sysfs,
> > >          online_kernel, online_movable)
> > >          There would be no need in userspace which would have to
> > >          deal with kernel zoo and maintain that as well.  
> > 
> > The kernel is supposed to provide a proper API and that is sysfs
> > currently. I am not entirely happy about it either but pulling a lot of
> > code into the kernel is not the rigth thing to do. Especially when
> > different usecases require different treatment.
>
> If it could be done from kernel side alone, it looks like a better way
> to me not to involve userspace at all. And for ACPI based x86/ARM it's
> possible to implement without adding a lot of kernel code.

But this is not how we do the kernel development. We provide the API so
that userspace can implement the appropriate policy on top. We do not
add random knobs to implement the same thing in the kernel. Different
users might want to implement different onlining strategies and that is
hardly describable by a single global knob. Just look at the s390
example provided earlier. Please try to think out of your usecase scope.

> That's one more of a reason to keep CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
> so we could continue on improving kernel only auto-onlining
> and fixing current memory hot(un)plug issues without affecting
> other platforms/users that are no interested in it.

I really do not see any reason to keep the config option. Setting up
this to enabled is _wrong_ thing to do in general purpose
(distribution) kernel and a kernel for the specific usecase can achieve
the same thing via boot command line.

> (PS: I don't care much about sysfs knob for setting auto-onlining,
> as kernel CLI override with memhp_default_state seems
> sufficient to me)

That is good to hear! I would be OK with keeping the kernel command line
option until we resolve all the current issues with the hotplug.

-- 
Michal Hocko
SUSE Labs

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#1599329

Michal Hocko <mhocko@kernel.org> writes:

> On Mon 13-03-17 11:55:54, Igor Mammedov wrote:
>> > > 
>> > >        - suggested RFC is not acceptable from virt point of view
>> > >          as it regresses guests on top of x86 kvm/vmware which
>> > >          both use ACPI based memory hotplug.
>> > > 
>> > >        - udev/userspace solution doesn't work in practice as it's
>> > >          too slow and unreliable when system is under load which
>> > >          is quite common in virt usecase. That's why auto online
>> > >          has been introduced in the first place.  
>> > 
>> > Please try to be more specific why "too slow" is a problem. Also how
>> > much slower are we talking about?
>>
>> In virt case on host with lots VMs, userspace handler
>> processing could be scheduled late enough to trigger a race
>> between (guest memory going away/OOM handler) and memory
>> coming online.
>
> Either you are mixing two things together or this doesn't really make
> much sense. So is this a balloning based on memory hotplug (aka active
> memory hotadd initiated between guest and host automatically) or a guest
> asking for additional memory by other means (pay more for memory etc.)?
> Because if this is an administrative operation then I seriously question
> this reasoning.

I'm probably repeating myself but it seems this point was lost:

This is not really a 'ballooning', it is just a pure memory
hotplug. People may have any tools monitoring their VM memory usage and
when a VM is running low on memory they may want to hotplug more memory
to it. With udev-style memory onlining they should be aware of page
tables and other in-kernel structures which require allocation so they
need to add memory slowly and gradually or they risk running into OOM
(at least getting some processes killed and these processes may be
important). With in-kernel memory hotplug everything happens
synchronously and no 'slowly and gradually' algorithm is required in
all tools which may trigger memory hotplug.

It's not about slowness, it's about being synchronous or
asynchronous. This is not related to the virtualization technology used,
the use-case is the same for all of them which support memory hotplug.

-- 
  Vitaly

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#1599346

On Mon 13-03-17 13:54:59, Vitaly Kuznetsov wrote:
> Michal Hocko <mhocko@kernel.org> writes:
> 
> > On Mon 13-03-17 11:55:54, Igor Mammedov wrote:
> >> > > 
> >> > >        - suggested RFC is not acceptable from virt point of view
> >> > >          as it regresses guests on top of x86 kvm/vmware which
> >> > >          both use ACPI based memory hotplug.
> >> > > 
> >> > >        - udev/userspace solution doesn't work in practice as it's
> >> > >          too slow and unreliable when system is under load which
> >> > >          is quite common in virt usecase. That's why auto online
> >> > >          has been introduced in the first place.  
> >> > 
> >> > Please try to be more specific why "too slow" is a problem. Also how
> >> > much slower are we talking about?
> >>
> >> In virt case on host with lots VMs, userspace handler
> >> processing could be scheduled late enough to trigger a race
> >> between (guest memory going away/OOM handler) and memory
> >> coming online.
> >
> > Either you are mixing two things together or this doesn't really make
> > much sense. So is this a balloning based on memory hotplug (aka active
> > memory hotadd initiated between guest and host automatically) or a guest
> > asking for additional memory by other means (pay more for memory etc.)?
> > Because if this is an administrative operation then I seriously question
> > this reasoning.
> 
> I'm probably repeating myself but it seems this point was lost:
> 
> This is not really a 'ballooning', it is just a pure memory
> hotplug. People may have any tools monitoring their VM memory usage and
> when a VM is running low on memory they may want to hotplug more memory
> to it.

What is the API those guests ask for the memory? And who is actually
responsible to ask for that memory? Is it a kernel or userspace
solution?

> With udev-style memory onlining they should be aware of page
> tables and other in-kernel structures which require allocation so they
> need to add memory slowly and gradually or they risk running into OOM
> (at least getting some processes killed and these processes may be
> important). With in-kernel memory hotplug everything happens
> synchronously and no 'slowly and gradually' algorithm is required in
> all tools which may trigger memory hotplug.

What prevents those APIs being used reasonably and only asks so much
memory as they can afford? I mean 1.5% available memory necessary for
the hotplug is not all that much. Or more precisely what prevents to ask
for this additional memory in a synchronous way?

And just to prevent from a further solution, I can see why _in-kernel_
hotplug based 'ballooning (or whatever you call an on demand memory hotplug
based on the memory pressure)' want's to be synchronous and that is why
my patch changed those onlined the memory explicitly. I am questioning
memory hotplug requested by admin/user space component to need any
special from kernel assistance becuase it is only a shortcut which can
be implemented from the userspace. I hope I've made myself clear
finally.
-- 
Michal Hocko
SUSE Labs

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#1599372

Michal Hocko <mhocko@kernel.org> writes:

> On Mon 13-03-17 13:54:59, Vitaly Kuznetsov wrote:
>> Michal Hocko <mhocko@kernel.org> writes:
>> 
>> > On Mon 13-03-17 11:55:54, Igor Mammedov wrote:
>> >> > > 
>> >> > >        - suggested RFC is not acceptable from virt point of view
>> >> > >          as it regresses guests on top of x86 kvm/vmware which
>> >> > >          both use ACPI based memory hotplug.
>> >> > > 
>> >> > >        - udev/userspace solution doesn't work in practice as it's
>> >> > >          too slow and unreliable when system is under load which
>> >> > >          is quite common in virt usecase. That's why auto online
>> >> > >          has been introduced in the first place.  
>> >> > 
>> >> > Please try to be more specific why "too slow" is a problem. Also how
>> >> > much slower are we talking about?
>> >>
>> >> In virt case on host with lots VMs, userspace handler
>> >> processing could be scheduled late enough to trigger a race
>> >> between (guest memory going away/OOM handler) and memory
>> >> coming online.
>> >
>> > Either you are mixing two things together or this doesn't really make
>> > much sense. So is this a balloning based on memory hotplug (aka active
>> > memory hotadd initiated between guest and host automatically) or a guest
>> > asking for additional memory by other means (pay more for memory etc.)?
>> > Because if this is an administrative operation then I seriously question
>> > this reasoning.
>> 
>> I'm probably repeating myself but it seems this point was lost:
>> 
>> This is not really a 'ballooning', it is just a pure memory
>> hotplug. People may have any tools monitoring their VM memory usage and
>> when a VM is running low on memory they may want to hotplug more memory
>> to it.
>
> What is the API those guests ask for the memory? And who is actually
> responsible to ask for that memory? Is it a kernel or userspace
> solution?

Whatever, this can even be a system administrator running
'free'. Hyper-V driver sends si_mem_available() and
vm_memory_committed() metrics to the host every second and this can be
later queried by any tool (e.g. powershell script).

>
>> With udev-style memory onlining they should be aware of page
>> tables and other in-kernel structures which require allocation so they
>> need to add memory slowly and gradually or they risk running into OOM
>> (at least getting some processes killed and these processes may be
>> important). With in-kernel memory hotplug everything happens
>> synchronously and no 'slowly and gradually' algorithm is required in
>> all tools which may trigger memory hotplug.
>
> What prevents those APIs being used reasonably and only asks so much
> memory as they can afford? I mean 1.5% available memory necessary for
> the hotplug is not all that much. Or more precisely what prevents to ask
> for this additional memory in a synchronous way?

The knowledge about the fact that we need to add memory slowly and
wait till it gets onlined is not obvious. AFAIR when you hotplug memory
to Windows VMs there is no such thing as 'onlining', and no brain is
required, a simple script 'low memory -> add mory memory' always
works. Asking all these script writers to think twice before issuing a
memory add command memory sounds like too much (to me).

-- 
  Vitaly

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#1599469

On Mon 13-03-17 14:42:37, Vitaly Kuznetsov wrote:
> Michal Hocko <mhocko@kernel.org> writes:
> 
> > On Mon 13-03-17 13:54:59, Vitaly Kuznetsov wrote:
> >> Michal Hocko <mhocko@kernel.org> writes:
> >> 
> >> > On Mon 13-03-17 11:55:54, Igor Mammedov wrote:
> >> >> > > 
> >> >> > >        - suggested RFC is not acceptable from virt point of view
> >> >> > >          as it regresses guests on top of x86 kvm/vmware which
> >> >> > >          both use ACPI based memory hotplug.
> >> >> > > 
> >> >> > >        - udev/userspace solution doesn't work in practice as it's
> >> >> > >          too slow and unreliable when system is under load which
> >> >> > >          is quite common in virt usecase. That's why auto online
> >> >> > >          has been introduced in the first place.  
> >> >> > 
> >> >> > Please try to be more specific why "too slow" is a problem. Also how
> >> >> > much slower are we talking about?
> >> >>
> >> >> In virt case on host with lots VMs, userspace handler
> >> >> processing could be scheduled late enough to trigger a race
> >> >> between (guest memory going away/OOM handler) and memory
> >> >> coming online.
> >> >
> >> > Either you are mixing two things together or this doesn't really make
> >> > much sense. So is this a balloning based on memory hotplug (aka active
> >> > memory hotadd initiated between guest and host automatically) or a guest
> >> > asking for additional memory by other means (pay more for memory etc.)?
> >> > Because if this is an administrative operation then I seriously question
> >> > this reasoning.
> >> 
> >> I'm probably repeating myself but it seems this point was lost:
> >> 
> >> This is not really a 'ballooning', it is just a pure memory
> >> hotplug. People may have any tools monitoring their VM memory usage and
> >> when a VM is running low on memory they may want to hotplug more memory
> >> to it.
> >
> > What is the API those guests ask for the memory? And who is actually
> > responsible to ask for that memory? Is it a kernel or userspace
> > solution?
> 
> Whatever, this can even be a system administrator running
> 'free'.

I am pretty sure that 'free' will not give you additional memory but
let's be serious here... If this is solely about monitoring from
userspace and requesting more memory from the userspace then I would
consider arguing about timely hotplug operation as void because there is
absolutely no guarantee to do the request itself in a timely fashion.

> Hyper-V driver sends si_mem_available() and
> vm_memory_committed() metrics to the host every second and this can be
> later queried by any tool (e.g. powershell script).

And how exactly is this related to the acpi hotplug which you were
arguing needs the timely handling as well?
 
> >> With udev-style memory onlining they should be aware of page
> >> tables and other in-kernel structures which require allocation so they
> >> need to add memory slowly and gradually or they risk running into OOM
> >> (at least getting some processes killed and these processes may be
> >> important). With in-kernel memory hotplug everything happens
> >> synchronously and no 'slowly and gradually' algorithm is required in
> >> all tools which may trigger memory hotplug.
> >
> > What prevents those APIs being used reasonably and only asks so much
> > memory as they can afford? I mean 1.5% available memory necessary for
> > the hotplug is not all that much. Or more precisely what prevents to ask
> > for this additional memory in a synchronous way?
> 
> The knowledge about the fact that we need to add memory slowly and
> wait till it gets onlined is not obvious.

yes it is and we cannot afford to give a better experience with the
implementation that requires to have memory to online a memory.

> AFAIR when you hotplug memory
> to Windows VMs there is no such thing as 'onlining', and no brain is
> required, a simple script 'low memory -> add mory memory' always
> works. Asking all these script writers to think twice before issuing a
> memory add command memory sounds like too much (to me).

Pardon me, but not requiring a brain while doing something on Windows
VMs is not really an argument...
-- 
Michal Hocko
SUSE Labs

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#1599504

Michal Hocko <mhocko@kernel.org> writes:

> On Mon 13-03-17 14:42:37, Vitaly Kuznetsov wrote:
>> >
>> > What is the API those guests ask for the memory? And who is actually
>> > responsible to ask for that memory? Is it a kernel or userspace
>> > solution?
>> 
>> Whatever, this can even be a system administrator running
>> 'free'.
>
> I am pretty sure that 'free' will not give you additional memory but
> let's be serious here... 
> If this is solely about monitoring from
> userspace and requesting more memory from the userspace then I would
> consider arguing about timely hotplug operation as void because there is
> absolutely no guarantee to do the request itself in a timely fashion.
>
>> Hyper-V driver sends si_mem_available() and
>> vm_memory_committed() metrics to the host every second and this can be
>> later queried by any tool (e.g. powershell script).
>
> And how exactly is this related to the acpi hotplug which you were
> arguing needs the timely handling as well?
>

What I meant to say is that there is no single 'right' way to get memory
usage from a VM, make a decision somewhere (in the hypervisor, on some
other host or even in someone's head) and issue a command to add more
memory. I don't know what particular tools people use with ESX/KVM VMs
but I think that multiple options are available.

>> >> With udev-style memory onlining they should be aware of page
>> >> tables and other in-kernel structures which require allocation so they
>> >> need to add memory slowly and gradually or they risk running into OOM
>> >> (at least getting some processes killed and these processes may be
>> >> important). With in-kernel memory hotplug everything happens
>> >> synchronously and no 'slowly and gradually' algorithm is required in
>> >> all tools which may trigger memory hotplug.
>> >
>> > What prevents those APIs being used reasonably and only asks so much
>> > memory as they can afford? I mean 1.5% available memory necessary for
>> > the hotplug is not all that much. Or more precisely what prevents to ask
>> > for this additional memory in a synchronous way?
>> 
>> The knowledge about the fact that we need to add memory slowly and
>> wait till it gets onlined is not obvious.
>
> yes it is and we cannot afford to give a better experience with the
> implementation that requires to have memory to online a memory.

Actually, we need memory to add memory, not to online it. And as I said
before, this is a real issue which requires addressing, it should always
be possible to add more memory (and to online already added memory if
these events are separated).

>
>> AFAIR when you hotplug memory
>> to Windows VMs there is no such thing as 'onlining', and no brain is
>> required, a simple script 'low memory -> add mory memory' always
>> works. Asking all these script writers to think twice before issuing a
>> memory add command memory sounds like too much (to me).
>
> Pardon me, but not requiring a brain while doing something on Windows
> VMs is not really an argument...

Why? Windows (or any other OS) is just an example that things can be
done in a different way, otherwise we'll end up with arguments like "it
was always like that in Linux so it's good".

-- 
  Vitaly

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#1598139

Hey,

On Mon, Mar 06, 2017 at 03:54:17PM +0100, Michal Hocko wrote:

[...]

> So let's discuss the current memory hotplug shortcomings and get rid of
> the crud which developed on top. I will start by splitting up the patch
> into 3 parts. Do the auto online thing from the HyperV and xen balloning
> drivers and dropping the config option and finally drop the sysfs knob.
> The last patch might be NAKed and I can live with that as long as the
> reasoning is proper and there is a general consensus on that.

If it is not too late please CC me on the patches and relevant threads.

Daniel

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