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

[Jiang Liu]

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:
>>>>>> On 2012/11/29 2:41, Toshi Kani wrote:
>>>>>>> On Wed, 2012-11-28 at 19:05 +0800, Hanjun Guo wrote:
>>>>>>>> On 2012/11/24 1:50, Vasilis Liaskovitis wrote:
>>>>>>>> As you may know, the ACPI based hotplug framework we are working on already addressed
>>>>>>>> this problem, and the way we slove this problem is a bit like yours.
>>>>>>>>
>>>>>>>> We introduce hp_ops in struct acpi_device_ops:
>>>>>>>> struct acpi_device_ops {
>>>>>>>> 	acpi_op_add add;
>>>>>>>> 	acpi_op_remove remove;
>>>>>>>> 	acpi_op_start start;
>>>>>>>> 	acpi_op_bind bind;
>>>>>>>> 	acpi_op_unbind unbind;
>>>>>>>> 	acpi_op_notify notify;
>>>>>>>> #ifdef	CONFIG_ACPI_HOTPLUG
>>>>>>>> 	struct acpihp_dev_ops *hp_ops;
>>>>>>>> #endif	/* CONFIG_ACPI_HOTPLUG */
>>>>>>>> };
>>>>>>>>
>>>>>>>> in hp_ops, we divide the prepare_remove into six small steps, that is:
>>>>>>>> 1) pre_release(): optional step to mark device going to be removed/busy
>>>>>>>> 2) release(): reclaim device from running system
>>>>>>>> 3) post_release(): rollback if cancelled by user or error happened
>>>>>>>> 4) pre_unconfigure(): optional step to solve possible dependency issue
>>>>>>>> 5) unconfigure(): remove devices from running system
>>>>>>>> 6) post_unconfigure(): free resources used by devices
>>>>>>>>
>>>>>>>> In this way, we can easily rollback if error happens.
>>>>>>>> How do you think of this solution, any suggestion ? I think we can achieve
>>>>>>>> a better way for sharing ideas. :)
>>>>>>>
>>>>>>> Yes, sharing idea is good. :)  I do not know if we need all 6 steps (I
>>>>>>> have not looked at all your changes yet..), but in my mind, a hot-plug
>>>>>>> operation should be composed with the following 3 phases.
>>>>>>
>>>>>> Good idea ! we also implement a hot-plug operation in 3 phases:
>>>>>> 1) acpihp_drv_pre_execute
>>>>>> 2) acpihp_drv_execute
>>>>>> 3) acpihp_drv_post_execute
>>>>>> you may refer to :
>>>>>> https://lkml.org/lkml/2012/11/4/79
>>>>>
>>>>> Great.  Yes, I will take a look.
>>>>
>>>> Thanks, any comments are welcomed :)
>>>
>>> If I read the code right, the framework calls ACPI drivers differently
>>> at boot-time and hot-add as follows.  That is, the new entry points are
>>> called at hot-add only, but .add() is called at both cases.  This
>>> requires .add() to work differently.
>>>
>>> Boot    : .add()
>>> Hot-Add : .add(), .pre_configure(), configure(), etc.
>>>
>>> I think the boot-time and hot-add initialization should be done
>>> consistently.  While there is difficulty with the current boot sequence,
>>> the framework should be designed to allow them consistent, not make them
>>> diverged.
>> Hi Toshi,
>> 	We have separated hotplug operations from driver binding/unbinding interface
>> due to following considerations.
>> 1) Physical CPU and memory devices are initialized/used before the ACPI subsystem
>>    is initialized. So under normal case, .add() of processor and acpi_memhotplug only
>>    figures out information about device already in working state instead of starting
>>    the device.
> 
> I agree that the current boot sequence is not very hot-plug friendly...
> 
>> 2) It's impossible to rmmod the processor and acpi_memhotplug driver at runtime 
>>    if .remove() of CPU and memory drivers do really remove the CPU/memory device
>>    from the system. And the ACPI processor driver also implements CPU PM funcitonality
>>    other than hotplug.
> 
> Agreed.
> 
>> And recently Rafael has mentioned that he has a long term view to get rid of the
>> concept of "ACPI device". If that happens, we could easily move the hotplug
>> logic from ACPI device drivers into the hotplug framework if the hotplug logic
>> is separated from the .add()/.remove() callbacks. Actually we could even move all
>> hotplug only logic into the hotplug framework and don't rely on any ACPI device
>> driver any more. So we could get rid of all these messy things. We could achieve
>> that by:
>> 1) moving code shared by ACPI device drivers and the hotplug framework into the core.
>> 2) moving hotplug only code to the framework.
> 
> 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.
2) an ACPI based hotplug manager driver, which is a platform independent
   driver and manages all hotplug slot created by the slot driver.

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?

> 
>> Hi Rafael, what's your thoughts here?
>>
>>>
>>>>>>> 1. Validate phase - Verify if the request is a supported operation.  All
>>>>>>> known restrictions are verified at this phase.  For instance, if a
>>>>>>> hot-remove request involves kernel memory, it is failed in this phase.
>>>>>>> Since this phase makes no change, no rollback is necessary to fail. 
>>>>>>
>>>>>> Yes, we have done this in acpihp_drv_pre_execute, and check following things:
>>>>>>
>>>>>> 1) Hot-plugble or not. the instance kernel memory you mentioned is also checked
>>>>>>    when memory device remove;
>>>>>
>>>>> Agreed.
>>>>>
>>>>>> 2) Dependency check involved. For instance, if hot-add a memory device,
>>>>>>    processor should be added first, otherwise it's not valid to this operation.
>>>>>
>>>>> I think FW should be the one that assures such dependency.  That is,
>>>>> when a memory device object is marked as present/enabled/functioning, it
>>>>> should be ready for the OS to use.
>>>>
>>>> Yes, BIOS should do something for the dependency, because BIOS knows the
>>>> actual hardware topology. 
>>>
>>> Right.
>>>
>>>> The ACPI specification provides _EDL method to
>>>> tell OS the eject device list, but still has no method to tell OS the add device
>>>> list now.
>>>
>>> Yes, but I do not think the OS needs special handling for add...
>> We have a plan to support triggering hot-adding events from OS provided interfaces,
>> so we also need to solve dependency issues when handling requests from those interfaces.
>> For need to power on the physical processor before powering on a memory device if
>> the memory device is attached to a physical processor.
> 
> I am afraid that this issue is platform-specific, and I am not sure if
> there is a common way to handle such things in general.  I'd recommend
> to work with FW folks to implement such platform-specific validation
> code in FW.
You are right, we may rely on firmware to validate the dependency.

> 
>>>> For some cases, OS should analyze the dependency in the validate phase. For example,
>>>> when hot remove a node (container device), OS should analyze the dependency to get
>>>> the remove order as following:
>>>> 1) Host bridge;
>>>> 2) Memory devices;
>>>> 3) Processor devices;
>>>> 4) Container device itself;
>>>
>>> This may be off-topic, but how do you plan to delete I/O devices under a
>>> node?  Are you planning to delete all I/O devices along with the node?
>>>
>>> On other OS, we made a separate step called I/O chassis delete, which
>>> off-lines all I/O devices under the node, and is required before a node
>>> hot-remove.  It basically triggers PCIe hot-remove to detach drivers
>>> from all devices.  It does not eject the devices so that they do not
>>> have to be on hot-plug slots.  This step runs user-space scripts to
>>> verify if the devices can be off-lined without disrupting user's
>>> applications, and provides comprehensive reports if any of them are in
>>> use.  Not sure if Linux's PCI hot-remove has such check, but I thought
>>> I'd mention it. :)
>> Yinghai is working on PCI host bridge hotplug, which just stops all PCI devices
>> under the host bridge. That's really a little dangerous and we do need help
>> from userspace to check whether the hot-removal operaitons is fatal, 
>> e.g. removing PCI device hosting the rootfs.
> 
> Agreed.
> 
>> 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.

Regards!
Gerry

> 
> Thanks,
> -Toshi
> 
> 
> .
> 


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