* perf regression from ipv6: use net->rt_genid to check dst validity
@ 2014-06-16 21:31 Chris Mason
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From: Chris Mason @ 2014-06-16 21:31 UTC (permalink / raw)
To: netdev
Hi everyone,
I tracked down a perf regression last week in our 3.10-stable based
kernel. fib6_lookup_1 was at the top of the profiles, being called very
frequently during sends. The 2.6.38 kernel we were comparing
against only called fib6_lookup_1 during recv.
The call chain was doing the lookups because we were always tossing the
destination cache. A little trial and error led me to this commit:
commit 6f3118b571b8a4c06c7985dc3172c3526cb86253
Author: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Date: Mon Sep 10 22:09:46 2012 +0000
ipv6: use net->rt_genid to check dst validity
The workload was our in memory database, and dropping this commit gave
us a 10% boost to overall queries per second.
Moving up to mainline, it looks like we're still failing the validity
check most of the time. A few printks show when we do fail, it's always
this line:
if (rt->rt6i_genid != rt_genid_ipv6(dev_net(rt->dst.dev)))
The cached dst had a genid of 2 and the dev_net version was 3.
What I haven't done yet is fully reproduce the 10% hit on mainline. I
have a few patches to port in and I'll get a workload running on 3.15.
But it doesn't look like this part has changed. Somehow we're hanging
onto a destination cache with an old genid and we're hammering on lookups
because of it.
Any ideas before I shower things with printk?
-chris
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2014-06-16 21:31 perf regression from ipv6: use net->rt_genid to check dst validity Chris Mason
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