[Bug 256217] [tcp] High system load because of interrupts with RACK

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256217

--- Comment #4 from Michael Tuexen <tuexen@freebsd.org> ---
So loading the RACK module, but not loading it, does not trigger the issue.

Getting rid of all RACK based TCP connections seems to resolve the issue.

Can you do the following experiment?

1) Load RACK and use net.inet.tcp.functions_default=freebsd at boot time. You
should not experience the problem.
2) Switch the stack for new connections to RACK by using sysctl
net.inet.tcp.functions_default=rack. You can check the stack being used by
using sockstat -SPtcp. The problem should now show up once new connections are
established.
3) Switch the stack for new connections to the base stack by using sysctl
net.inet.tcp.functions_default=freebsd. Either wait until the connections using
RACK have been closed or kill them by using tcpdrop -S rack. This should
resolve the issue.

If the behaviour is as I think it is, the problem is only there if you have
active RACK connections.

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