[openstack-dev] [nova] [cyborg] Race condition in the Cyborg/Nova flow

Eric Fried openstack at fried.cc
Fri Mar 23 19:44:56 UTC 2018


	First thought is to simplify by NOT keeping inventory information in
the cyborg db at all.  The provider record in the placement service
already knows the device (the provider ID, which you can look up in the
cyborg db) the host (the root_provider_uuid of the provider representing
the device) and the inventory, and (I hope) you'll be augmenting it with
traits indicating what functions it's capable of.  That way, you'll
always get allocation candidates with devices that *can* load the
desired function; now you just have to engage your weigher to prioritize
the ones that already have it loaded so you can prefer those.

	Am I missing something?


On 03/22/2018 11:27 PM, Nadathur, Sundar wrote:
> Hi all,
>     There seems to be a possibility of a race condition in the
> Cyborg/Nova flow. Apologies for missing this earlier. (You can refer to
> the proposed Cyborg/Nova spec
> <https://review.openstack.org/#/c/554717/1/doc/specs/rocky/cyborg-nova-sched.rst>
> for details.)
> Consider the scenario where the flavor specifies a resource class for a
> device type, and also specifies a function (e.g. encrypt) in the extra
> specs. The Nova scheduler would only track the device type as a
> resource, and Cyborg needs to track the availability of functions.
> Further, to keep it simple, say all the functions exist all the time (no
> reprogramming involved).
> To recap, here is the scheduler flow for this case:
>   * A request spec with a flavor comes to Nova conductor/scheduler. The
>     flavor has a device type as a resource class, and a function in the
>     extra specs.
>   * Placement API returns the list of RPs (compute nodes) which contain
>     the requested device types (but not necessarily the function).
>   * Cyborg will provide a custom filter which queries Cyborg DB. This
>     needs to check which hosts contain the needed function, and filter
>     out the rest.
>   * The scheduler selects one node from the filtered list, and the
>     request goes to the compute node.
> For the filter to work, the Cyborg DB needs to maintain a table with
> triples of (host, function type, #free units). The filter checks if a
> given host has one or more free units of the requested function type.
> But, to keep the # free units up to date, Cyborg on the selected compute
> node needs to notify the Cyborg API to decrement the #free units when an
> instance is spawned, and to increment them when resources are released.
> Therein lies the catch: this loop from the compute node to controller is
> susceptible to race conditions. For example, if two simultaneous
> requests each ask for function A, and there is only one unit of that
> available, the Cyborg filter will approve both, both may land on the
> same host, and one will fail. This is because Cyborg on the controller
> does not decrement resource usage due to one request before processing
> the next request.
> This is similar to this previous Nova scheduling issue
> <https://specs.openstack.org/openstack/nova-specs/specs/pike/implemented/placement-claims.html>.
> That was solved by having the scheduler claim a resource in Placement
> for the selected node. I don't see an analog for Cyborg, since it would
> not know which node is selected.
> Thanks in advance for suggestions and solutions.
> Regards,
> Sundar
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