On Mon, 04 Jan 2016 11:47:12 +0100
Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
>
> > On Tue, 22 Dec 2015 17:32:30 +0100 Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> >
> >> Currently, all newly added memory blocks remain in 'offline' state unless
> >> someone onlines them, some linux distributions carry special udev rules
> >> like:
> >>
> >> SUBSYSTEM=="memory", ACTION=="add", ATTR{state}=="offline", ATTR{state}="online"
> >>
> >> to make this happen automatically. This is not a great solution for virtual
> >> machines where memory hotplug is being used to address high memory pressure
> >> situations as such onlining is slow and a userspace process doing this
> >> (udev) has a chance of being killed by the OOM killer as it will probably
> >> require to allocate some memory.
> >>
> >> Introduce default policy for the newly added memory blocks in
> >> /sys/devices/system/memory/hotplug_autoonline file with two possible
> >> values: "offline" which preserves the current behavior and "online" which
> >> causes all newly added memory blocks to go online as soon as they're added.
> >> The default is "online" when MEMORY_HOTPLUG_AUTOONLINE kernel config option
> >> is selected.
> >
> > I think the default should be "offline" so vendors can ship kernels
> > which have CONFIG_MEMORY_HOTPLUG_AUTOONLINE=y while being
> > back-compatible with previous kernels.
> >
>
> (sorry for the delayed response, just picking things up after holidays)
>
> I was under an (wrong?) impression that in the majority of use cases
> users want to start using their newly added memory right away and that's
> what distros will ship. As an alternative to making the feature off by
> default I can suggest making CONFIG_MEMORY_HOTPLUG_AUTOONLINE a tristate
> switch (no feature, default offline, default online).
That what probably would satisfy every distro,
only question is why do you need 'no feature',
wouldn't 'default offline' cover current state?
>
> >> --- a/Documentation/kernel-parameters.txt
> >> +++ b/Documentation/kernel-parameters.txt
> >> @@ -2537,6 +2537,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >> shutdown the other cpus. Instead use the REBOOT_VECTOR
> >> irq.
> >>
> >> + nomemhp_autoonline Don't automatically online newly added memory.
> >> +
> >
> > This wasn't mentioned in the changelog. Why do we need a boot
> > parameter as well as the sysfs knob?
if 'default online' policy is set then we need a kernel option to disable
auto-onlining at kernel boot time (when it parses ACPI tables for x86) if needed
and vice verse for 'default offline' to enable auto-onlining at kernel boot time.
For RHEL we would probably use 'default online' policy like
we do in RHEL6 with custom patch.
> >
>
> I was thinking about some faulty hardware (e.g. reporting new memory
> blocks which for some reason are not really usable) and an easy way to
> make such hardware work.
>
> >> +config MEMORY_HOTPLUG_AUTOONLINE
> >> + bool "Automatically online hot-added memory"
> >> + depends on MEMORY_HOTPLUG_SPARSE
> >> + help
> >> + When memory is hot-added, it is not at ready-to-use state, a special
> >
> > "When memory is hot-added it is not in a ready-to-use state. A special"
> >
> >> + userspace action is required to online the newly added blocks. With
> >> + this option enabled, the kernel will try to online all newly added
> >> + memory automatically.
> >> +
> >>
> >> ...
> >>
>
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