Re: [Xen-devel] [PATCH v2] memory-hotplug: add automatic onlining policy for the newly added memory

[Igor Mammedov <imammedo@xxxxxxxxxx>]

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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