iptables-restore can take quite a long time when sytem is busy, in order of half a minute or more. The main reason for this is the way ip(6)tables performs table swap, or, more precisely, expensive sequence lock synchronizations when reading counters.
When xt_replace_table assigns the new ruleset pointer, it does not wait for other processors to finish with old ruleset. Instead it relies on the counter sequence lock in get_counters() to do this. This works but this is very costly if system is busy as each counter read operation can possibly be restarted indefinitely. Instead, make xt_replace_table wait until all processors are known to not use the old ruleset anymore. This allows to read the old counters without any locking, no cpu is using the ruleset anymore so counters can't change either. ipv4/netfilter/arp_tables.c | 22 ++++++++++++++++++++-- ipv4/netfilter/ip_tables.c | 23 +++++++++++++++++++++-- ipv6/netfilter/ip6_tables.c | 22 ++++++++++++++++++++-- netfilter/x_tables.c | 15 ++++++++++++--- 4 files changed, 73 insertions(+), 9 deletions(-)
