Reduce the number of calls to reset the keepalive timer

[jtl]

Summary

The code currently resets the keepalive timer each time a packet is received on a TCP session that has entered the ESTABLISHED state. This results in a lot of calls to reset the keepalive timer.

This patch changes the behavior so we set the keepalive timer for the keepalive idle time (TP_KEEPIDLE). When the keepalive timer fires, it will first check to see if the session has been idle for TP_KEEPIDLE ticks. If not, it will reschedule the keepalive timer for the time the session will have been idle for TP_KEEPALIVE ticks.

For a session with regular communication, the keepalive timer should fire approximately once every TP_KEEPIDLE ticks. For sessions with irregular communication, the keepalive timer might fire more often. But, the disruption from a periodic keepalive timer should be less than the regular cost of resetting the keepalive timer on every packet.

MFC after: 2 weeks

Test Plan

(Not done yet.) Grab performance metrics from a live system.

[rrs]

I think that a grand idea :)

[gallatin]

This seems like a great idea. I don't see any downside.

[glebius]

Like.

head/sys/netinet/tcp_timer.c

473

I'd suggest comment change. It should explain not only why we do this check, rather than why we don't reset.

Suggested text to be checked by native speaker:
"We don't reschedule the keepalive callout on every packet, because that costs a lot of CPU cycles. Cheaper to actually
execute the callout once it expires and check against tp->t_rcvtime. If the check doesn't pass, schedule next keepalive
and return."