linux-kernel - Re: [bisected] e341694e3eb5 netlink_lookup() rcu conversion causes latencies

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Message-ID: <20141020082107.GB4268@osiris>
Date:	Mon, 20 Oct 2014 10:21:08 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	paulmck@...ux.vnet.ibm.com,
	Nikolay Aleksandrov <nikolay@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ursula Braun <braunu@...ibm.com>
Subject: Re: [bisected] e341694e3eb5 netlink_lookup() rcu conversion causes
 latencies

On Sat, Oct 11, 2014 at 11:25:14PM +0100, Thomas Graf wrote:
> On 10/11/14 at 12:32pm, Eric Dumazet wrote:
> > On Sat, 2014-10-11 at 10:36 +0200, Heiko Carstens wrote:
> > > Hi all,
> > > 
> > > it just came to my attention that commit e341694e3eb5
> > > "netlink: Convert netlink_lookup() to use RCU protected hash table"
> > > causes network latencies for me on s390.
> > > 
> > > The testcase is quite simple and 100% reproducible on s390:
> > > 
> > > Simply login via ssh to a remote system which has the above mentioned
> > > patch applied. Any action like pressing return now has significant
> > > latencies. Or in other words, working via such a connection becomes
> > > a pain ;)
> > > 
> > > I haven't debugged it, however I assume the problem is that a) the
> > > commit introduces a synchronize_net() call und b) s390 kernels
> > > usually get compiled with CONFIG_HZ_100 while most other architectures
> > > use CONFIG_HZ_1000.
> > > If I change the kernel config to CONFIG_HZ_1000 the problem goes away,
> > > however I don't consider this a fix...
> > > 
> > > Another reason why this hasn't been observed on x86 may or may not be
> > > that we haven't implemented CONFIG_HAVE_CONTEXT_TRACKING on s390 (yet).
> > > But that's just guessing...
> > 
> > CC Paul and Sasha
> 
> I think the issue here is obvious and a fix is on the way to move
> the insertion and removal to a worker to no longer require the
> synchronize_rcu().
> 
> What bothers me is that the synchronize_rcu() should only occur
> on expand/shrink and not for every table update. The default table
> size is 64.

*ping* ... is there already any patch available?

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