sunrpc: reduce pool->sp_lock contention when queueing a xprt for servicing

Hi Bruce!

Here are the patches that I had mentioned earlier that reduce the contention for the pool->sp_lock when the server is heavily loaded.

The basic problem is that whenever a svc_xprt needs to be queued up for servicing, we have to take the pool->sp_lock to try and find an idle thread to service it. On a busy server, that lock becomes highly contended and that limits the throughput.

This patchset fixes this by changing how we search for an idle thread. First, we convert svc_rqst and the sp_all_threads list to be RCU-managed. Then we change the search for an idle thread to use the sp_all_threads list, which now can be done under the rcu_read_lock. When there is an available thread, queueing an xprt to it can now be done without any spinlocking.

With this, we see a pretty substantial increase in performance on a larger-scale server that is heavily loaded. Chris has some preliminary numbers, but they need to be cleaned up a bit before we can present them. I'm hoping to have those by early next week.

Jeff Layton (4):
sunrpc: add a rcu_head to svc_rqst and use kfree_rcu to free it
sunrpc: fix potential races in pool_stats collection
sunrpc: convert to lockless lookup of queued server threads
sunrpc: add some tracepoints around enqueue and dequeue of svc_xprt

 include/linux/sunrpc/svc.h    |  12 +-
 include/trace/events/sunrpc.h |  98 +++++++++++++++-
 net/sunrpc/svc.c              |  17 +--
 net/sunrpc/svc_xprt.c         | 252 ++++++++++++++++++++++++------------------
 4 files changed, 258 insertions(+), 121 deletions(-)

2.1.0