On 2021-03-08 14:24:10, Mark Tomlinson wrote: > The patches to change to using RCU synchronization in x_tables cause > updating tables to be slowed down by an order of magnitude. This has > been tried before, see https://lore.kernel.org/patchwork/patch/151796/ > and ultimately was rejected. As mentioned in the patch description, a > different method can be used to ensure ordering of reads/writes. This > can simply be done by changing from smp_wmb() to smp_mb(). I came across these patches while trying to root-cause a significant boot time regression we were seeing when comparing 5.4 to 5.10. The RCU changes were where I had started to investigate but hadn't yet made it to the point that Mark had reached when he sent out these fixes. Mark's description of the problem and fix looks correct to me. To touch on why the performance regression is a problem for us, we have three services that load various firewall rules during boot up. Here are the total run times that `systemd-analyze blame` reported for each one to run to completion: 5.4.88: 129ms fw-1.service 56ms fw-2.service 38ms fw-3.service 5.10.19: 586ms fw-1.service 847ms fw-2.service 193ms fw-3.service 5.10.19 + these fixes: 98ms fw-1.service 25ms fw-2.service 54ms fw-3.service You can see that the performance regression from 5.4.88 -> 5.10.19 considerably increased our boot time. I cannot explain why '5.10.19 + these fixes' shows improvements over 5.4.88 for two of the three services but I suspect it is due to systemd taking different code paths, units executing at different times, unrelated kernel performance improvements, etc. For all three patches, Tested-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx> Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx> Tyler > > changes in v2: > - Update commit messages only > > Mark Tomlinson (3): > Revert "netfilter: x_tables: Update remaining dereference to RCU" > Revert "netfilter: x_tables: Switch synchronization to RCU" > netfilter: x_tables: Use correct memory barriers. > > include/linux/netfilter/x_tables.h | 7 ++--- > net/ipv4/netfilter/arp_tables.c | 16 +++++----- > net/ipv4/netfilter/ip_tables.c | 16 +++++----- > net/ipv6/netfilter/ip6_tables.c | 16 +++++----- > net/netfilter/x_tables.c | 49 +++++++++++++++++++++--------- > 5 files changed, 60 insertions(+), 44 deletions(-) > > -- > 2.30.1 >
