Netgear R7800 exploration (IPQ8065, QCA9984)

Hi. I Installed OpenWRT on my R7800. But I found strange thing.

When Idle state, CPU Load is too high. Always keep 0.1 ~ 0.4

I used htop (install: opkg install htop) and monitored processes.

htop -d2

(refresh rate: 0.2 sec) or htop -d1

In every 2sec, kworker/x:x process runs 100% and go down. (CPU spikes every 2sec, 100% and go disappear.)

I tried various firmwares and found something different CPU Usage:

OpenWRT SNAPSHOT r9773: Every 2sec, CPU spike 100%
OpenWRT hnyman's Build: Every 2sec, CPU spike 100%
OpenWRT 18.06.2 Stable: Every 2sec, CPU spike 20~30%
DD-WRT kongs Build (2018-10?, It uses Linux 3.x Kernel): CPU IDLE (Load avg: 0.0x)
DD-WRT v3.0-r39296 (03/27/19) Linux 4.9.166-rc1: CPU IDLE (Load avg: 0.0x)

But I cannot figure out actually what process is cause of this problem.

htop just shows kworker/x:x (x is varient I think).

I removed wireless driver/firmware and rebooted, not worked.
Also disabled IPv6, DHCP, Firewall, NAT... not worked.
This is not related Luci Auto-refresh CPU load.
I tried RESET settings... etc..

DD-WRT is OK.

My other Routers (EA4500 (E4200v2), WRT32X) are OK. (Load avg: 0.00 ~ 0.0x, OpenWRT recent SNAPSHOTs and 18.06.x Stable)

Any ideas?