tcp: For hostcache performance, use atomics instead of counters

[rscheff]

Summary

As accessing the tcp hostcache happens frequently on some
classes of servers, it was recommended to use atomic_add/subtract
rather than (per-CPU distributed) counters, which have to be
summed up at high cost to cache efficiency.

This eliminates a counter_u64_fetch() from a hot path, introduced
by D29510.

PR: 254333
MFC after: 2 weeks
Sponsored by: NetApp, Inc.

[jtl]

Thanks for doing this so quickly!

sys/netinet/tcp_hostcache.c

377

I think you need an atomic_load_int() here.

643–644

I think you need an atomic_load_int() here, although I don't think it functionally probably matters too much. In fact, this code already allows races between when the sysctl function is called to get the length, and when the code is called with another buffer. But, it seems like it is probably good to be consistent in using the atomic* functions to access or set atomic variables.

[rscheff]

• use atomic access wrapper functions throughout

[rscheff]

also did a line length wrap.

[tuexen]

sys/netinet/tcp_hostcache.c

644

Please put the operator at the end of the previous line.

655

Please put the operator at the end of the previous line.

[rscheff]

• move operator to end of line at wrap