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At this moment, there is only one type of next-hop group: an mpath group, which implements the hash-threshold algorithm. To select a next hop, hash-threshold algorithm first assigns a range of hashes to each next hop in the group, and then selects the next hop by comparing the SKB hash with the individual ranges. When a next hop is removed from the group, the ranges are recomputed, which leads to reassignment of parts of hash space from one next hop to another. While there will usually be some overlap between the previous and the new distribution, some traffic flows change the next hop that they resolve to. That causes problems e.g. as established TCP connections are reset, because the traffic is forwarded to a server that is not familiar with the connection. Resilient hashing is a technique to address the above problem. Resilient next-hop group has another layer of indirection between the group itself and its constituent next hops: a hash table. The selection algorithm uses a straightforward modulo operation to choose a hash bucket, and then reads the next hop that this bucket contains, and forwards traffic there. This indirection brings an important feature. In the hash-threshold algorithm, the range of hashes associated with a next hop must be continuous. With a hash table, mapping between the hash table buckets and the individual next hops is arbitrary. Therefore when a next hop is deleted the buckets that held it are simply reassigned to other next hops. When weights of next hops in a group are altered, it may be possible to choose a subset of buckets that are currently not used for forwarding traffic, and use those to satisfy the new next-hop distribution demands, keeping the "busy" buckets intact. This way, established flows are ideally kept being forwarded to the same endpoints through the same paths as before the next-hop group change. In a nutshell, the algorithm works as follows. Each next hop has a number of buckets that it wants to have, according to its weight and the number of buckets in the hash table. In case of an event that might cause bucket allocation change, the numbers for individual next hops are updated, similarly to how ranges are updated for mpath group next hops. Following that, a new "upkeep" algorithm runs, and for idle buckets that belong to a next hop that is currently occupying more buckets than it wants (it is "overweight"), it migrates the buckets to one of the next hops that has fewer buckets than it wants (it is "underweight"). If, after this, there are still underweight next hops, another upkeep run is scheduled to a future time. Chances are there are not enough "idle" buckets to satisfy the new demands. The algorithm has knobs to select both what it means for a bucket to be idle, and for whether and when to forcefully migrate buckets if there keeps being an insufficient number of idle buckets. There are three users of the resilient data structures. - The forwarding code accesses them under RCU, and does not modify them except for updating the time a selected bucket was last used. - Netlink code, running under RTNL, which may modify the data. - The delayed upkeep code, which may modify the data. This runs unlocked, and mutual exclusion between the RTNL code and the delayed upkeep is maintained by canceling the delayed work synchronously before the RTNL code touches anything. Later it restarts the delayed work if necessary. The RTNL code has to implement next-hop group replacement, next hop removal, etc. For removal, the mpath code uses a neat trick of having a backup next hop group structure, doing the necessary changes offline, and then RCU-swapping them in. However, the hash tables for resilient hashing are about an order of magnitude larger than the groups themselves (the size might be e.g. 4K entries), and it was felt that keeping two of them is an overkill. Both the primary next-hop group and the spare therefore use the same resilient table, and writers are careful to keep all references valid for the forwarding code. The hash table references next-hop group entries from the next-hop group that is currently in the primary role (i.e. not spare). During the transition from primary to spare, the table references a mix of both the primary group and the spare. When a next hop is deleted, the corresponding buckets are not set to NULL, but instead marked as empty, so that the pointer is valid and can be used by the forwarding code. The buckets are then migrated to a new next-hop group entry during upkeep. The only times that the hash table is invalid is the very beginning and very end of its lifetime. Between those points, it is always kept valid. This patch introduces the core support code itself. It does not handle notifications towards drivers, which are kept as if the group were an mpath one. It does not handle netlink either. The only bit currently exposed to user space is the new next-hop group type, and that is currently bounced. There is therefore no way to actually access this code. Signed-off-by: Petr Machata <petrm@nvidia.com> Reviewed-by: Ido Schimmel <idosch@nvidia.com> Reviewed-by: David Ahern <dsahern@kernel.org> Signed-off-by: David S. Miller <davem@davemloft.net> |
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