[PATCH v7 0/3] batman-adv: increase DAT DHT timeout

["Linus Lüssing" <linus.luessing@c0d3.blue>]

This patchset increases the DAT DHT timeout to reduce the amount 
of broadcasted ARP Replies.

To increase the timeout only for DAT DHT entries added via DHT-PUT but
not for any other entry in the DAT cache the DAT cache and DAT DHT
concepts are split into two separate hash tables (PATCH 2/3).

PATCH 3/3 then increases the timeout for DAT DHT entries from 5 to
30 minutes.


The motivation for this patchset is based on the observations made here:
https://www.open-mesh.org/projects/batman-adv/wiki/DAT_DHCP_Snooping


In tests this year at Freifunk Lübeck with ~180 mesh nodes and Gluon
this reduced the ARP broadcast overhead, measured over 7 days, as
follows:

- Total:           6677.66 bits/s -> 677.26 bits/s => -89.86%
                   11.92 pkts/s   -> 1.21 pkts/s   => -89.85%

  - from gateways: 5618.02 bits/s -> 212.28        => -96.22%
                   10.03 pkts/s   -> 0.38 pkts/s   => -96.21%

Also see graphics and a few more test details here:
- https://www.open-mesh.org/projects/batman-adv/wiki/DAT_DHCP_Snooping#Result-2

These patches (v5) have been applied in this mesh network without issues
for 3 months now.

Regards,
Linus

---

Changelog v7:
- adding PATCH 1/3 to add the batadv_netlink_get_softif() wrapper to
  reduce the amount of duplicate code, both in the current code base
  but also for the next PATCH 2/3

Changelog v6:
- removed renaming+deprecation of BATADV_P_DAT_CACHE_REPLY in PATCH 1/2
- small commit message rewording in PATCH 1/2

Changelog v5:
- rebased to current main branch
  -> removed now obsolete debugfs code

Changelog v4:
- rebased to: acfc9a214d01695
  ("batman-adv: genetlink: make policy common to family")

Changelog v3:

formerly:
 "batman-adv: Increase purge timeout on DAT DHT candidates"
 https://patchwork.open-mesh.org/patch/17728/
- fixed the potential jiffies overflow and jiffies initialization
  issues by replacing the last_dht_update timeout variable with
  a split of DAT cache and DAT DHT into two separate hash tables
  -> instead of maintaining two timeouts in one DAT entry two DAT
     entries are created and maintained in their respective DAT
     cache and DAT DHT hash tables

Changelog v2:

formerly:
 "batman-adv: Increase DHCP snooped DAT entry purge timeout in DHT"
 (https://patchwork.open-mesh.org/patch/17364/)
- removed the extended timeouts flag in the DHT-PUT messages introduced
  in v1 again
- removed DHCP dependency