On Mar 4, 2014, at 22:00, NeilBrown <neilb@xxxxxxx> wrote: > The access cache is used during RCU-walk path lookups, so it is best > to avoid locking if possible as taking a lock kills concurrency. > > The rbtree is not rcu-safe and cannot easily be made so. > Instead we simply check the last (i.e. most recent) entry on the LRU > list. If this doesn't match, then we return -ECHILD and retry in > lock/refcount mode. > > This requires freeing the nfs_access_entry struct with rcu, and > requires using rcu access primates when adding entries to the lru, and > when examining the last entry. > > Calling put_rpccred before kfree_rcu looks a bit odd, but as > put_rpccred already provide rcu protection, we know that the cred will > not actually be freed until the next grace period, so any concurrent > access will be safe. > > This patch provides about 5% performance improvement on a stat-heavy > synthetic work load with 4 threads on a 2-core CPU. > Have you looked at converting the rb-tree into something a little more RCU-friendly? Since our lookup key is just a pointer value, couldn’t we use a radix-tree? _________________________________ Trond Myklebust Linux NFS client maintainer, PrimaryData trond.myklebust@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
