On 2012/12/4 8:10, Toshi Kani wrote:

On Mon, 2012-12-03 at 12:25 +0800, Hanjun Guo wrote:

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On 2012/11/30 6:27, Toshi Kani wrote:

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On Thu, 2012-11-29 at 12:48 +0800, Hanjun Guo wrote:

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On 2012/11/29 2:41, Toshi Kani wrote:

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On Wed, 2012-11-28 at 19:05 +0800, Hanjun Guo wrote:

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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.

Hi Toshi,
Thanks for your comments!

Boot    : .add()

Actually, at boot time: .add(), .start()

Hot-Add : .add(), .pre_configure(), configure(), etc.

Yes, we did it as you said in the framework. We use .pre_configure(), configure(),
and post_configure() to instead of .start() for better error handling and recovery.

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.

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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.

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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.

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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...

Hmm, how about trigger a hot add operation by OS ? we have eject interface for OS, but
have no add interface now, do you think this feature is useful? If it is, I think OS
should analyze the dependency first and tell the user.

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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?

Yes, we delete all I/O devices under the node. we delete I/O devices as
following steps:
1) Offline PCI devices;
2) Offline IOAPIC and IOMMU;
and offline I/O devices no matter in use or not.

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

Great! we also have a plan to implement this feature.

use.  Not sure if Linux's PCI hot-remove has such check, but I thought
I'd mention it. :)

Have no such check, I'm sure :)

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In this way, we can check that all the devices are hot-plugble or not under the
container device before execute phase, and further more, we can remove devices
in order to avoid some crash problems.

Yes, we should check if all the resources under the node can be
off-lined at validate phase.  (note, all the devices do not have to have
_EJ0 if that's what you meant by hot-pluggable.)

Yes, agreed. For node hotplug, no need for all the devices have _EJ0 method.

Thanks
 Hanjun

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3) Race condition check. if the device and its dependent device is in hot-plug
   process, another request will be denied.

I agree that hot-plug operation should be serialized.  I think another
request should be either queued or denied based on the caller's intent
(i.e. wait-ok or no-wait). 

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No rollback is needed for the above checks.

Great.

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2. Execute phase - Perform hot-add / hot-remove operation that can be
rolled-back in case of error or cancel.

In this phase, we introduce a state machine for the hot-plugble device,
please refer to:
https://lkml.org/lkml/2012/11/4/79

I think we have the same idea for the major framework, but the ACPI based
hot-plug framework implement it differently in detail, right ?

Yes, I am surprised with the similarity.  What I described is something
we had implemented for other OS.  I am still studying how best we can
improve the Linux hotplug code. :)

Great! your experience is very appreciable for me. I think we can share ideas
to achieve a better solution for Linux hotplug code. :)

Sounds great.

Thanks,
-Toshi

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