Re: [Bug 214523] New: RDMA Mellanox RoCE drivers are unresponsive to ARP updates during a reconnect

[Chuck Lever III <chuck.lever@oracle.com>]

> On Sep 27, 2021, at 8:09 AM, Leon Romanovsky <leon@kernel.org> wrote:
> 
> On Sun, Sep 26, 2021 at 05:36:01PM +0000, Chuck Lever III wrote:
>> Hi Leon-
>> 
>> Thanks for the suggestion! More below.
>> 
>>> On Sep 26, 2021, at 4:02 AM, Leon Romanovsky <leon@kernel.org> wrote:
>>> 
>>> On Fri, Sep 24, 2021 at 03:34:32PM +0000, bugzilla-daemon@bugzilla.kernel.org wrote:
>>>> https://bugzilla.kernel.org/show_bug.cgi?id=214523
>>>> 
>>>>           Bug ID: 214523
>>>>          Summary: RDMA Mellanox RoCE drivers are unresponsive to ARP
>>>>                   updates during a reconnect
>>>>          Product: Drivers
>>>>          Version: 2.5
>>>>   Kernel Version: 5.14
>>>>         Hardware: All
>>>>               OS: Linux
>>>>             Tree: Mainline
>>>>           Status: NEW
>>>>         Severity: normal
>>>>         Priority: P1
>>>>        Component: Infiniband/RDMA
>>>>         Assignee: drivers_infiniband-rdma@kernel-bugs.osdl.org
>>>>         Reporter: kolga@netapp.com
>>>>       Regression: No
>>>> 
>>>> RoCE RDMA connection uses CMA protocol to establish an RDMA connection. During
>>>> the setup the code uses hard coded timeout/retry values. These values are used
>>>> for when Connect Request is not being answered to to re-try the request. During
>>>> the re-try attempts the ARP updates of the destination server are ignored.
>>>> Current timeout values lead to 4+minutes long attempt at connecting to a server
>>>> that no longer owns the IP since the ARP update happens. 
>>>> 
>>>> The ask is to make the timeout/retry values configurable via procfs or sysfs.
>>>> This will allow for environments that use RoCE to reduce the timeouts to a more
>>>> reasonable values and be able to react to the ARP updates faster. Other CMA
>>>> users (eg IB or others) can continue to use existing values.
>> 
>> I would rather not add a user-facing tunable. The fabric should
>> be better at detecting addressing changes within a reasonable
>> time. It would be helpful to provide a history of why the ARP
>> timeout is so lax -- do certain ULPs rely on it being long?
> 
> I don't know about ULPs and ARPs, but how to calculate TimeWait is
> described in the spec.
> 
> Regarding tunable, I agree. Because it needs to be per-connection, most
> likely not many people in the world will success to configure it properly.

Exactly.


>>>> The problem exist in all kernel versions but bugzilla is filed for 5.14 kernel.
>>>> 
>>>> The use case is (RoCE-based) NFSoRDMA where a server went down and another
>>>> server was brought up in its place. RDMA layer introduces 4+ minutes in being
>>>> able to re-establish an RDMA connection and let IO resume, due to inability to
>>>> react to the ARP update.
>>> 
>>> RDMA-CM has many different timeouts, so I hope that my answer is for the
>>> right timeout.
>>> 
>>> We probably need to extend rdma_connect() to receive remote_cm_response_timeout
>>> value, so NFSoRDMA will set it to whatever value its appropriate.
>>> 
>>> The timewait will be calculated based it in ib_send_cm_req().
>> 
>> I hope a mechanism can be found that behaves the same or nearly the
>> same way for all RDMA fabrics.
> 
> It depends on the fabric itself, in every network
> remote_cm_response_timeout can be different.

What I mean is I hope a way can be found so that RDMA consumers do
not have to be aware of the fabric differences.


>> For those who are not NFS-savvy:
>> 
>> Simple NFS server failover is typically implemented with a heartbeat
>> between two similar platforms that both access the same backend
>> storage. When one platform fails, the other detects it and takes over
>> the failing platform's IP address. Clients detect connection loss
>> with the failing platform, and upon reconnection to that IP address
>> are transparently directed to the other platform.
>> 
>> NFS server vendors have tried to extend this behavior to RDMA fabrics,
>> with varying degrees of success.
>> 
>> In addition to enforcing availability SLAs, the time it takes to
>> re-establish a working connection is critical for NFSv4 because each
>> client maintains a lease to prevent the server from purging open and
>> lock state. If the reconnect takes too long, the client's lease is
>> jeopardized because other clients can then access files that client
>> might still have locked or open.
>> 
>> 
>> --
>> Chuck Lever

--
Chuck Lever