In order to avoid some OOM triggered by a flood of call_rcu() calls, we 
increased in linux 2.6.14 maxbatch from 10 to 10000, and conditionally call 
set_need_resched() in call_rcu().

This solution doesnt solve all the problems and has drawbacks.

1) Using a big maxbatch has a bad impact on latency.
2) A flood of call_rcu_bh() still can OOM

I have some servers that once in a while crashes when the ip route cache is 
flushed. After raising /proc/sys/net/ipv4/route/secret_interval (so that *no* 
flush is done), I got better uptime for these servers. But in some cases I 
think the network stack can floods call_rcu_bh(), and a fatal OOM occurs.

I suggest in this patch :

1) To lower maxbatch to a more reasonable value (as far as the latency is 
concerned)

2) To be able to guard a RCU cpu queue against a maximal count (10.000 for 
example). If this limit is reached, free the oldest entry of this queue.

I assume that if a CPU queued 10.000 items in its RCU queue, then the oldest 
entry cannot still be in use by another CPU. This might sounds as a violation 
of RCU rules, (I'm not an RCU expert) but seems quite reasonable.


Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>