I rebooted it via BMC web interface and k1px1 came back.
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This issue may be related to the issue about long-running gentoo front-ends (see also LLO#17765). We may need to reboot many front-ends to mitigate same issue in near future. Uptime of each front-end is as follows. Some of them are running +300 days thanks to a longer interval of planned power outage in the mine. Such a long run is the first time in KAGRA.
k1lsc0: 16:56:52 up 369 days, 23:17, 0 users, load average: 0.21, 0.43, 0.37
k1asc0: 16:56:52 up 317 days, 2:06, 0 users, load average: 0.11, 1.52, 2.02
k1als0: 16:56:52 up 17 days, 3:06, 0 users, load average: 0.00, 0.01, 0.05
k1ioo: 16:56:52 up 370 days, 3:33, 0 users, load average: 0.00, 0.01, 0.05
k1ioo1: 16:56:52 up 355 days, 2:15, 0 users, load average: 0.00, 0.01, 0.05
k1imc0: 16:56:53 up 370 days, 3:26, 0 users, load average: 0.00, 0.10, 0.15
k1omc1: 16:56:53 up 383 days, 0 min, 0 users, load average: 0.00, 0.01, 0.05
k1omc0: 16:56:53 up 370 days, 3:09, 0 users, load average: 1.28, 0.77, 0.58
k1pr2: 16:56:53 up 370 days, 3:22, 0 users, load average: 0.03, 0.04, 0.05
k1pr0: 16:56:53 up 100 days, 2:47, 0 users, load average: 0.00, 0.01, 0.05
k1prm: 16:56:53 up 5 days, 23:51, 0 users, load average: 0.05, 0.04, 0.05
k1bs: 16:56:53 up 382 days, 21:49, 0 users, load average: 0.01, 0.03, 0.05
k1sr2: 16:56:53 up 370 days, 4:28, 0 users, load average: 0.00, 0.01, 0.05
k1sr3: 16:56:54 up 370 days, 3:40, 0 users, load average: 0.00, 0.01, 0.05
k1srm: 16:56:54 up 370 days, 3:13, 0 users, load average: 0.00, 0.01, 0.05
k1ix1: 16:56:54 up 383 days, 0 min, 0 users, load average: 0.01, 0.26, 0.33
k1iy1: 16:56:54 up 382 days, 23:59, 0 users, load average: 0.00, 0.14, 0.22
k1ex1: 16:56:54 up 47 days, 4:54, 0 users, load average: 0.91, 0.37, 0.37
k1ey1: 16:56:54 up 382 days, 23:58, 0 users, load average: 0.05, 0.11, 0.20
k1px1: 16:56:54 up 19 min, 0 users, load average: 0.00, 0.01, 0.04
k1iy0: 16:56:55 up 152 days, 4:32, 0 users, load average: 0.00, 0.01, 0.05
k1mcf0: 16:56:55 up 35 days, 6:35, 0 users, load average: 0.02, 0.02, 0.05
k1ex0: 16:56:55 up 382 days, 23:57, 0 users, load average: 0.08, 0.09, 0.07
k1ey0: 16:56:55 up 382 days, 23:57, 0 users, load average: 0.15, 0.26, 0.22
k1test0: 16:56:55 up 19 days, 5:31, 0 users, load average: 0.11, 0.22, 0.30
