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

From: Michal Hocko
Date: Mon Mar 13 2017 - 08:28:45 EST

On Mon 13-03-17 11:55:54, Igor Mammedov wrote:
> On Thu, 9 Mar 2017 13:54:00 +0100
> Michal Hocko <mhocko@xxxxxxxxxx> 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