Messages in this thread | | | Date | Sat, 11 Oct 2014 23:25:14 +0100 | From | Thomas Graf <> | Subject | Re: [bisected] e341694e3eb5 netlink_lookup() rcu conversion causes latencies |
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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.
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