netdev - Re: [PATCH ipvs-next] ipvs: Remove rcu_read_unlock();rcu_read

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Date:	Thu, 25 Apr 2013 11:05:26 +0200
From:	Hans Schillstrom <hans@...illstrom.com>
To:	Julian Anastasov <ja@....bg>
Cc:	Simon Horman <horms@...ge.net.au>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	lvs-devel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org,
	Wensong Zhang <wensong@...ux-vs.org>
Subject: Re: [PATCH ipvs-next] ipvs: Remove
 rcu_read_unlock();rcu_read_lock();

Hello
On Thu, 2013-04-25 at 11:15 +0300, Julian Anastasov wrote:
> 	Hello,
> 
> On Thu, 25 Apr 2013, Simon Horman wrote:
> 
> > It is unclear to me that there is any utility in the following:
> > 
> > 	 rcu_read_unlock();
> > 	 rcu_read_lock();
> 
> 	I thought it is a good idea for fixed hash table
> of IP_VS_TAB_BITS=20. May be if guarded by
> 
> 	if (!((++idx) & 4095))
> 
> to reduce its rate to 256 (with idx++ removed from the for loop) ?
> 
> 	Netfilter has no such logic for nf_conntrack because
> it has limit of 16384 rows. Not sure how fatal is to try 1048576
> empty rows under RCU lock for such rare operations as
> connection listing. OTOH, ip_vs_conn_array() needs to
> seek at some initial position, so it can skip many
> entries if reading table with many conns, for example,
> 1048576 rows * 16 conns per row, we will need to
> touch 16777216 conns under lock. Not sure what is the
> best practice for such cases.

My opinion is to keep it, people tends to do such "rare" things.
It's not unusual with 256k - 1M rows... 

Regards 
Hans


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