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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>
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