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

From: Eric Dumazet
Date: Sat Oct 11 2014 - 15:32:51 EST

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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