Re: [RFC PATCH v3 0/3] acpi: Introduce prepare_remove device operation

[Toshi Kani <toshi.kani@hp.com>]

On Tue, 2012-12-11 at 22:34 +0800, Jiang Liu wrote:
> On 12/08/2012 09:08 AM, Toshi Kani wrote:
> > On Fri, 2012-12-07 at 13:57 +0800, Jiang Liu wrote:
> >> On 2012-12-7 10:57, Toshi Kani wrote:
> >>> On Fri, 2012-12-07 at 00:40 +0800, Jiang Liu wrote:
> >>>> On 12/04/2012 08:10 AM, Toshi Kani wrote:
> >>>>> On Mon, 2012-12-03 at 12:25 +0800, Hanjun Guo wrote:
> >>>>>> On 2012/11/30 6:27, Toshi Kani wrote:
> >>>>>>> On Thu, 2012-11-29 at 12:48 +0800, Hanjun Guo wrote:
 :
> >>> Yes, the framework should allow such future work.  I also think that the
> >>> framework itself should be independent from such ACPI issue.  Ideally,
> >>> it should be able to support non-ACPI platforms.
> >> The same point here. The ACPI based hotplug framework is designed as:
> >> 1) an ACPI based hotplug slot driver to handle platform specific logic.
> >>    Platform may provide platform specific slot drivers to discover, manage
> >>    hotplug slots. We have provided a default implementation of slot driver
> >>    according to the ACPI spec.
> > 
> > The ACPI spec does not define that _EJ0 is required to receive a hot-add
> > request, i.e. bus/device check.  This is a major issue.  Since Windows
> > only supports hot-add, I think there are platforms that only support
> > hot-add today.
> > 
> >> 2) an ACPI based hotplug manager driver, which is a platform independent
> >>    driver and manages all hotplug slot created by the slot driver.
> > 
> > It is surely impressive work, but I think is is a bit overdoing.  I
> > expect hot-pluggable servers come with management console and/or GUI
> > where a user can manage hardware units and initiate hot-plug operations.
> > I do not think the kernel needs to step into such area since it tends to
> > be platform-specific. 
> One of the major usages of this feature is for testing. 
> It will be hard for OSVs and OEMs to verify hotplug functionalities if it could
> only be tested by physical hotplug or through management console. So to pave the
> way for hotplug, we need to provide a mechanism for OEMs and OSVs to execute 
> auto stress tests for hotplug functionalities.

Yes, but such OS->FW interface is platform-specific.  Some platforms use
IPMI for the OS to communicate with the management console.  In this
case, an OEM-specific command can be used to request a hotplug through
IPMI.  Some platforms may also support test programs to run on the
management console for validations.

For early development testing, Yinghai's SCI emulation patch can be used
to emulate hotplug events from the OS.  It would be part of the kernel
debugging features once this patch is accepted. 

 
> >> We haven't gone further enough to provide an ACPI independent hotplug framework
> >> because we only have experience with x86 and Itanium, both are ACPI based.
> >> We may try to implement an ACPI independent hotplug framework by pushing all
> >> ACPI specific logic into the slot driver, I think it's doable. But we need
> >> suggestions from experts of other architectures, such as SPARC and Power.
> >> But seems Power already have some sorts of hotplug framework, right?
> > 
> > I do not know about the Linux hot-plug support on other architectures.
> > PA-RISC SuperDome also supports Node hot-plug, but it is not supported
> > by Linux.  Since ARM is getting used by servers, I would not surprise if
> > there will be an ARM based server with hot-plug support in future.
> Seems ARM is on the way to adopt ACPI, so may be we could support ARM servers
> in the future.

That's good to know.

 :
> >>>> So in our framework, we have an option to relay hotplug event from firmware
> >>>> to userspace, so the userspace has a chance to reject the hotplug operations
> >>>> if it may cause unacceptable disturbance to userspace services.
> >>>
> >>> I think validation from user-space is necessary for deleting I/O
> >>> devices.  For CPU and memory, the kernel check works fine.
> >> Agreed. But we may need help from userspace to handle cgroup/cpuset/cpuisol
> >> etc for cpu and memory hot-removal. Especially for telecom applications, they
> >> have strong dependency on cgroup/cpuisol to guarantee latency.
> > 
> > I have not looked at the code, but isn't these cpu attributes managed in
> > the kernel?
> Some Telecom applications want to run in an deterministic environment, so they
> depend on cpuisol/cpuset to provide such an environment. If hotplug event happens,
> these Telecom application should be notified so they have a chance to redistribute
> the workload.

I agree that we need to generate an event that can be subscribed by
those applications, so that they can react quickly on the change.

Thanks,
-Toshi


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