add dbus commands for dynamic creation of instances

[adriaan42]

Commit taken from #580

[adriaan42]

@yarda @zacikpa to continue the discussion on dynamic creation (and deletion) of plugin instances (originally in #580):

I have updated the behavior of instance_destroy:

• any instance can be destroyed (not only the ones created dynamically)
• devices attached to the instance are released, and later the plugin's _add_device function is called, and existing logic will attach the device to a suitable instance

For instance_create it's a bit more complex:

• I can extract devices and devices_udev_regex from the options and pass it to the instance creation. So when new devices are plugged, the new instance is taken into consideration.
• But what should happen to existing devices already attached to another instance?
  • Current implementation: nothing happens, and the user explicitly calls instance_acquire_devices to move devices, OR
  • Automatically move devices, i.e., iterate through all instances and check if any of their devices should be moved to the new instance. Would be possible with _check_matching_devices, while also considering instance priorities, but this would override any manual assignments made by previous calls to instance_acquire_devices... so not sure about the side effects.

Also, I noticed that the code (existing instance_acquire_devices and also my new instance_create and instance_destroy) call methods only available in Hotplug plugins, but without checking if we're actually dealing with a Hotplug plugin. Is that a bug in the existing instance_acquire_devices? Should I just add an isinstance() check to all three functions?

[adriaan42]

• Automatically move devices, i.e., iterate through all instances and check if any of their devices should be moved to the new instance. Would be possible with _check_matching_devices, while also considering instance priorities, but this would override any manual assignments made by previous calls to instance_acquire_devices... so not sure about the side effects.

After thinking about it some more, I believe that we do want to automatically move matching devices. So the latest update implements that, acquiring all matching devices from plugin instances with an equal or lower priority (i.e. if the priority value of the new instance is lower of equal to the one of the existing instance).

[adriaan42]

After some more testing and fixing, I'm now quite happy with the current implementation.

However, I'm seeing some potential inconsistencies in the current behavior of the instance.active property, where instances are only active when they have attached devices:

• non-active instances are not listed by the get_instances dbus call, so it seems they don't exist, but instance_get_devices can be called successfully -> the view exposed via the dbus interface is not consistent.
• for non-active instances, instance_apply_tuning (https://github.com/redhat-performance/tuned/blob/master/tuned/plugins/base.py#L250, and related functions) immediately returns, so in particular, the static tuning is not executed -> it is not possible to have static instance-specific tuning that is independent of devices.

[adriaan42]

Thanks for the update, @adriaan42.

Also, I noticed that the code (existing instance_acquire_devices and also my new instance_create and instance_destroy) call methods only available in Hotplug plugins, but without checking if we're actually dealing with a Hotplug plugin. Is that a bug in the existing instance_acquire_devices? Should I just add an isinstance() check to all three functions?

I think this is indeed a bug. It hasn't caused any issues yet because there aren't many non-hotplug plugins with device support, and for plugins that do not support devices at all, the existing function returns earlier. I would add the type check.

I've added checks.

non-active instances are not listed by the get_instances dbus call, so it seems they don't exist, but instance_get_devices can be called successfully -> the view exposed via the dbus interface is not consistent.

The only issue I see here is the name of the API function get_instances, which probably should have been named get_active_instances. Nonetheless, it's well-documented that it returns only active instances, so I wouldn't worry about it much.

If inactive instances are "invisible" then any code using the interface to create a new instance would have to explicitly check (using instance_get_devices) if the chosen name is already taken, or just try to create and catch the error. But I can live with that, and you're right, it's the documented behavior.

for non-active instances, instance_apply_tuning (https://github.com/redhat-performance/tuned/blob/master/tuned/plugins/base.py#L250, and related functions) immediately returns, so in particular, the static tuning is not executed -> it is not possible to have static instance-specific tuning that is independent of devices.

Good catch - I also found a related issue, see one of my comments. I think resolving this would require some heavier refactoring where we would decouple the non-device tuning from the device tuning. I don't really think it's a blocker for this PR - I would open a separate issue for that if you're not up for it. @yarda, what do you think?

Functionality-wise, I tested this and it works quite well.

Thanks, I appreciate it! :)

[adriaan42]

@yarda anything missing/blocking this?

[yarda]

for non-active instances, instance_apply_tuning (https://github.com/redhat-performance/tuned/blob/master/tuned/plugins/base.py#L250, and related functions) immediately returns, so in particular, the static tuning is not executed -> it is not possible to have static instance-specific tuning that is independent of devices.

Good catch - I also found a related issue, see one of my comments. I think resolving this would require some heavier refactoring where we would decouple the non-device tuning from the device tuning. I don't really think it's a blocker for this PR - I would open a separate issue for that if you're not up for it. @yarda, what do you think?

OK, NP, it makes sense.

[yarda]

LGTM. I also tested basic functionality and it seems everything works OK, thanks for contribution.