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When IPoIB receives an SM LID change event, it reacts by flushing its path record cache and rejoining multicast groups. This is the same behavior it performs when it receives a reregistration event. This behavior is unnecessary as an SM may have database backup or synchronization mechanisms which permit the SM location or LID to change without loss of multicast membership and without impact to path records. Both opensm and the OPA FM issue reregistration events if a new SM is started (or restarted with a new config) or an SM event occurs which results in loss of multicast membership records by the SM (such as opensm failover) or the SM encounters new nodes with Active ports (such as after joining 2 fabrics by connecting switches via ISLs). Hence this event can be depended on as the trigger for IPoIB cache and multicast flushing. It appears that some drivers, such as qib, and hfi1 issue the IB_EVENT_SM_CHANGE but other drivers such as mlx4 and mlx5 do not. Empirical testing on Mellanox EDR using ibv_asyncwatch has confirmed that Mellanox EDR HCAs do not generate SM change events and that opensm does generate reregistration. An SM LID change event is generated by the mentioned drivers to reflect that sm_lid and/or sm_sl in the local port info has changed. The intent of this event is to permit applications and ULPs which have a local copy of this information (or an address handle using it) to update their information. The intent is that the reregistration event (caused by the SM via a bit in Set(PortInfo)) be used to inform nodes that they need to rejoin multicast groups, resubscribe for notices and potentially update path records. When an SM migrates or fails over, a SM LID change event can occur. In response IPoIB discards path records and multicast membership and loses connectivity until these records are restored via SA requests. In very large fabrics, it may take minutes for the SM to be ready and for the SA responses to be supplied. This can result in undesirable and unnecessary IPoIB connectivity impacts. It also can result in an unnecessary storm of SA queries from all nodes in a cluster potentially followed by yet another storm if the SM issues the reregistration request. The fact the Mellanox HCAs do not even generate this event, is further evidence that on modern IB fabrics there will be no ill side effects from the proposed changes below to reduce the reaction by 3 kernel components to this event. So these changes should be benign for Mellanox IB fabrics and will benefit OPA fabrics while also making ib_core and ULP behavor "correct" as intended by the IBTA spec and kernel RDMA event APIs. Address these issues by removing IB_EVENT_SM_CHANGE handling from ipoib. IPoIB does not locally store sm_lid nor sm_sl, so it does not need to do anything on SM LID change. IPoIB makes use of other ib_core components to issue SA requests for it and those components correctly track SM LID and SM LID changes. Also in ib_core multicast handling, remove the test for IB_EVENT_SM_CHANGE. This code is moving all multicast groups to the error state, which will trigger rejoins. This code is used by IPoIB as well as the connection manager and other clients of multicast groups. This kernel module centralizes group membership status and joins since a node can only join a given group once but multiple ULPs or applications may want to join the same group. It makes use of the sa_query.c component in ib_core, which correctly trackes SM LID and SL. This component does not track SM LID nor SL itself and hence need not react to their changes. Similarly in the ib_core cache code remove the handling for the IB_EVENT_SM_CHANGE. In this function. The ib_cache_update function which is ultimately called is updating local copies of the pkey table, gid table and lmc. It does not update nor retain sm_lid nor sm_sl. As such it does not need to be called on an SM LID change. It technically also does not need to be called on a reregistration. The LID_CHANGE, PKEY_CHANGE, GID_CHANGE and port state change events (PORT_ERR, PORT_ACTICE) should be sufficient triggers. It is worth noting that the alternative of simply having the hfi1 and qib drivers not generate the SM LID change event was explored. While this would duplicate what Mellanox drivers do now, it is not the correct behavior and removes the ability for an SM to migrate without requiring reregistration. Since both opensm and OPA SM have mechanisms to backup or synchronize registration information, it is desirable to let them perform SM migrations (with LID or SL changes) without requiring reregistration when they deem it appropriate. Suggested-by: Todd Rimmer <todd.rimmer@intel.com> Tested-by: Michael Brooks <michael.brooks@intel.com> Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com> Reviewed-by: Todd Rimmer <todd.rimmer@intel.com> Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.