use a hash for looking up delegation v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

currently recalling delegations has to walk the server->delegations
list, and then take the lock for each delegation.  This can take a lot of
time and has adverse effects to the rest of the system due to the number
atomic operations, cache lines touched and a long RCU critical section.

This series first converts the delegation watermark to be per-server, as
all the state guarded by it is per-server and the commit message adding
it talks about server side overhead as well, and then adds a very simple
hash for finding the delegation for a given file handle in
nfs_delegation_find_inode_server.

With this hash sample microbenchmarks that cause delegation recalls in
reverse list order are sped up ~5 percent, although the time is still
very variable due to other factors.

Changes since v1:
 - only allocate the delegation hash for v4 mounts

Diffstat:
 fs/nfs/client.c           |   15 +++++-
 fs/nfs/delegation.c       |  110 ++++++++++++++++++++++++++++------------------
 fs/nfs/delegation.h       |    3 +
 fs/nfs/nfs4client.c       |   10 +++-
 include/linux/nfs_fs_sb.h |    3 +
 5 files changed, 96 insertions(+), 45 deletions(-)