Subject: Possible regression: Packet drops during iptables calls

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 14 Dec 2010 15:46:14 +0100

I'm experiencing RX packet drops during call to iptables, on my
production servers.

Further investigations showed, that its only the CPU executing the
iptables command that experience packet drops!?  Thus, a quick fix was
to force the iptables command to run on one of the idle CPUs (This can
be achieved with the "taskset" command).

I have a 2x Xeon 5550 CPU system, thus 16 CPUs (with HT enabled).  We
only use 8 CPUs due to a multiqueue limitation of 8 queues in the
1Gbit/s NICs (82576 chips).  CPUs 0 to 7 is assigned for packet
processing via smp_affinity.

Can someone explain why the packet drops only occur on the CPU
executing the iptables command?

What can we do to solve this issue?

I should note that I have a very large ruleset on this machine, and
the production machine is routing around 800 Mbit/s, in each
direction.  The issue occurs on a simple iptables rule listing.

I think (untested) the problem is related to kernel git commit:

 commit 942e4a2bd680c606af0211e64eb216be2e19bf61
 Author: Stephen Hemminger <[email-masked]>
 Date: Tue Apr 28 22:36:33 2009 -0700

 netfilter: revised locking for x_tables

 The x_tables are organized with a table structure and a per-cpu copies
 of the counters and rules. On older kernels there was a reader/writer
 lock per table which was a performance bottleneck. In 2.6.30-rc, this
 was converted to use RCU and the counters/rules which solved the performance
 problems for do_table but made replacing rules much slower because of
 the necessary RCU grace period.

 This version uses a per-cpu set of spinlocks and counters to allow to
 table processing to proceed without the cache thrashing of a global
 reader lock and keeps the same performance for table updates.

 Signed-off-by: Stephen Hemminger <[email-masked]>
 Acked-by: Linus Torvalds <[email-masked]>
 Signed-off-by: David S. Miller <[email-masked]>

-- 
Med venlig hilsen / Best regards
  Jesper Brouer
  ComX Networks A/S
  Linux Network Kernel Developer
  Cand. Scient Datalog / MSc.CS
  Author of http://adsl-optimizer.dk
  LinkedIn: http://www.linkedin.com/in/brouer