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

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Message-ID: <1413055964.9362.50.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sat, 11 Oct 2014 12:32:44 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	paulmck@...ux.vnet.ibm.com
Cc:	Thomas Graf <tgraf@...g.ch>,
	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, 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


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