Hello, On Fri, 20 Aug 2010, Simon Horman wrote: > Signed-off-by: Simon Horman <horms@xxxxxxxxxxxx> > > --- > > I'm still getting my head around RCU, so review would be greatly appreciated. > > It occurs to me that this code is not performance critical, so > perhaps simply replacing the rwlock with a spinlock would be better? This specific code does not need RCU conversion, see below > Index: nf-next-2.6/net/netfilter/ipvs/ip_vs_sched.c > =================================================================== > --- nf-next-2.6.orig/net/netfilter/ipvs/ip_vs_sched.c 2010-08-20 22:21:01.000000000 +0900 > +++ nf-next-2.6/net/netfilter/ipvs/ip_vs_sched.c 2010-08-20 22:21:51.000000000 +0900 > @@ -35,7 +35,7 @@ > static LIST_HEAD(ip_vs_schedulers); > > /* lock for service table */ > -static DEFINE_RWLOCK(__ip_vs_sched_lock); > +static DEFINE_SPINLOCK(ip_vs_sched_mutex); Here is what I got as list of locking points: __ip_vs_conntbl_lock_array: - can benefit from RCU, main benefits come from here - ip_vs_conn_unhash() followed by ip_vs_conn_hash() is tricky with RCU, needs more thinking, eg. when cport is changed cp->lock, cp->refcnt: - not a problem tcp_app_lock, udp_app_lock, sctp_app_lock: - can benefit from RCU (once per connection) svc->sched_lock: - only 1 read_lock, mostly writers that need exclusive access - so, not suitable for RCU, can be switched to spin_lock for speed __ip_vs_sched_lock: - not called by packet handlers, no need for RCU - used only by one ip_vs_ctl user (configuration) and the scheduler modules - can remain RWLOCK, no changes in locking are needed __ip_vs_svc_lock: - spin_lock, use RCU - restrictions for schedulers with .update_service method because svc->sched_lock is write locked, see below __ip_vs_rs_lock: - spin_lock, use RCU Schedulers: - every .schedule method has its own locking, two examples: - write_lock: to protect the scheduler state (can be changed to spin_lock), see WRR. Difficult for RCU. - no lock: relies on IP_VS_WAIT_WHILE, no state is protected explicitly, fast like RCU, see WLC Scheduler state, eg. mark->cl: - careful RCU assignment, may be all .update_service methods should use copy-on-update (WRR). OTOH, ip_vs_wlc_schedule (WLC) has no locks at all, thanks to the IP_VS_WAIT_WHILE, so it is fast as RCU. Statistics: dest->stats.lock, svc->stats.lock, ip_vs_stats.lock: - called for every packet, BAD for SMP, see ip_vs_in_stats(), ip_vs_out_stats(), ip_vs_conn_stats() curr_sb_lock: - called for every packet depending on conn state - No benefits from RCU, should be spin_lock To summarize: - the main problem remains stats: dest->stats.lock, svc->stats.lock, ip_vs_stats.lock - RCU benefits when connection processes many packets per connection, eg. for TCP, SCTP, not much for UDP. No gains for the 1st packet in connection. - svc: no benefits from RCU, some schedulers protect state and need exclusive access, others have no state (and they do not use locks even now) Regards -- Julian Anastasov <ja@xxxxxx> -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
