Messages in this thread | | | Date | Thu, 16 Apr 2009 15:53:15 +0200 | From | Patrick McHardy <> | Subject | Re: [PATCH] netfilter: use per-cpu spinlock and RCU (v5) |
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Eric Dumazet wrote: > Stephen Hemminger a écrit : >> This is an alternative version of ip/ip6/arp tables locking using >> per-cpu locks. This avoids the overhead of synchronize_net() during >> update but still removes the expensive rwlock in earlier versions. >> >> The idea for this came from an earlier version done by Eric Dumazet. >> Locking is done per-cpu, the fast path locks on the current cpu >> and updates counters. The slow case involves acquiring the locks on >> all cpu's. This version uses RCU for the table->base reference >> but per-cpu-lock for counters.
> This version is a regression over 2.6.2[0-9], because of two points > > 1) Much more atomic ops : > > Because of additional > >> + spin_lock(&__get_cpu_var(ip_tables_lock)); >> ADD_COUNTER(e->counters, ntohs(ip->tot_len), 1); >> + spin_unlock(&__get_cpu_var(ip_tables_lock)); > > added on each counter updates. > > On many setups, each packet coming in or out of the machine has > to update between 2 to 20 rule counters. So to avoid *one* atomic ops > of read_unlock(), this v4 version adds 2 to 20 atomic ops...
I agree, this seems to be a step backwards.
> I still not see the problem between the previous version (2.6.2[0-8]) that had a central > rwlock, that hurted performance on SMP because of cache line ping pong, and the solution > having one rwlock per cpu. > > We wanted to reduce the cache line ping pong first. This *is* the hurting point, > by an order of magnitude.
Dave doesn't seem to like the rwlock approach. I don't see a way to do anything asynchronously like call_rcu() to improve this, so to bring up one of Stephens suggestions again:
>> > * use on_each_cpu() somehow to do grace periood?
We could use this to replace the counters, presuming it is indeed faster than waiting for a RCU grace period.
> 2) Second problem : Potential OOM > > About freeing old rules with call_rcu() and/or schedule_work(), this is going > to OOM pretty fast on small appliances with basic firewall setups loading > rules one by one, as done by original topic reporter. > > We had reports from guys using linux with 4MB of available ram (French provider free.fr on > their applicance box), and we had to use SLAB_DESTROY_BY_RCU thing on conntrack > to avoid OOM for their setups. We dont want to use call_rcu() and queue 100 or 200 vfree().
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