Taking a closer look at the timing behind the CPU soft lockup, turns out it happens every 21 minutes, varying between 21min33sec and 21min42sec for the times I've looked at. 

root@marcus-server:/var/log# cat kern.log | grep watchdog: BUG: soft lockup -
(standard input):Sep 21 08:15:52 marcus-server kernel: [25828.061389] watchdog: BUG: soft lockup - CPU#8 stuck for 26s! [kworker/u64:47:2349337]
(standard input):Sep 21 08:15:52 marcus-server kernel: [25856.060650] watchdog: BUG: soft lockup - CPU#8 stuck for 52s! [kworker/u64:47:2349337]
(standard input):Sep 21 08:37:25 marcus-server kernel: [27118.374205] watchdog: BUG: soft lockup - CPU#10 stuck for 26s! [kworker/u64:110:2400870]
(standard input):Sep 21 08:37:25 marcus-server kernel: [27146.372183] watchdog: BUG: soft lockup - CPU#10 stuck for 53s! [kworker/u64:110:2400870]
(standard input):Sep 21 08:58:58 marcus-server kernel: [28408.518001] watchdog: BUG: soft lockup - CPU#7 stuck for 26s! [kworker/u64:15:2451705]
(standard input):Sep 21 08:58:58 marcus-server kernel: [28436.518146] watchdog: BUG: soft lockup - CPU#7 stuck for 52s! [kworker/u64:15:2451705]
(standard input):Sep 21 09:20:32 marcus-server kernel: [29698.735150] watchdog: BUG: soft lockup - CPU#1 stuck for 26s! [kworker/u64:42:2504150]
(standard input):Sep 21 09:20:32 marcus-server kernel: [29726.735261] watchdog: BUG: soft lockup - CPU#1 stuck for 52s! [kworker/u64:42:2504150]
(standard input):Sep 21 09:42:05 marcus-server kernel: [30988.922555] watchdog: BUG: soft lockup - CPU#6 stuck for 26s! [kworker/u64:17:2571505]
(standard input):Sep 21 09:42:05 marcus-server kernel: [31016.924522] watchdog: BUG: soft lockup - CPU#6 stuck for 52s! [kworker/u64:17:2571505]
(standard input):Sep 21 10:03:38 marcus-server kernel: [32278.634545] watchdog: BUG: soft lockup - CPU#4 stuck for 26s! [kworker/u64:107:2624962]
(standard input):Sep 21 10:03:38 marcus-server kernel: [32306.631958] watchdog: BUG: soft lockup - CPU#4 stuck for 52s! [kworker/u64:107:2624962]
(standard input):Sep 21 10:25:11 marcus-server kernel: [33568.843698] watchdog: BUG: soft lockup - CPU#11 stuck for 26s! [kworker/u64:50:2679696]
(standard input):Sep 21 10:25:11 marcus-server kernel: [33596.841576] watchdog: BUG: soft lockup - CPU#11 stuck for 52s! [kworker/u64:50:2679696]
(standard input):Sep 21 10:46:44 marcus-server kernel: [34859.443424] watchdog: BUG: soft lockup - CPU#2 stuck for 26s! [kworker/u64:38:2735876]
(standard input):Sep 21 10:46:44 marcus-server kernel: [34887.443661] watchdog: BUG: soft lockup - CPU#2 stuck for 52s! [kworker/u64:38:2735876]
(standard input):Sep 21 11:08:17 marcus-server kernel: [36149.446509] watchdog: BUG: soft lockup - CPU#7 stuck for 26s! [kworker/u64:102:2791856]
(standard input):Sep 21 11:08:17 marcus-server kernel: [36177.444332] watchdog: BUG: soft lockup - CPU#7 stuck for 52s! [kworker/u64:102:2791856]
(standard input):Sep 21 11:29:52 marcus-server kernel: [37438.207072] watchdog: BUG: soft lockup - CPU#0 stuck for 27s! [kworker/u64:99:2846490]
(standard input):Sep 21 11:29:52 marcus-server kernel: [37466.204826] watchdog: BUG: soft lockup - CPU#0 stuck for 53s! [kworker/u64:99:2846490]
(standard input):Sep 21 11:29:52 marcus-server kernel: [37498.202216] watchdog: BUG: soft lockup - CPU#0 stuck for 82s! [kworker/u64:99:2846490]
(standard input):Sep 21 11:51:27 marcus-server kernel: [38731.780341] watchdog: BUG: soft lockup - CPU#7 stuck for 26s! [kworker/u64:27:2899692]
(standard input):Sep 21 11:51:27 marcus-server kernel: [38759.780468] watchdog: BUG: soft lockup - CPU#7 stuck for 52s! [kworker/u64:27:2899692]
(standard input):Sep 21 12:13:09 marcus-server kernel: [40031.224394] watchdog: BUG: soft lockup - CPU#4 stuck for 26s! [kworker/u64:112:2959948]
(standard input):Sep 21 12:13:09 marcus-server kernel: [40059.224543] watchdog: BUG: soft lockup - CPU#4 stuck for 53s! [kworker/u64:112:2959948]
(standard input):Sep 21 12:34:45 marcus-server kernel: [41322.879468] watchdog: BUG: soft lockup - CPU#4 stuck for 26s! [kworker/u64:16:3010254]
(standard input):Sep 21 12:34:45 marcus-server kernel: [41350.876932] watchdog: BUG: soft lockup - CPU#4 stuck for 53s! [kworker/u64:16:3010254]
(standard input):Sep 21 12:56:17 marcus-server kernel: [42613.149821] watchdog: BUG: soft lockup - CPU#9 stuck for 26s! [kworker/u64:110:3059593]
(standard input):Sep 21 12:56:17 marcus-server kernel: [42641.147794] watchdog: BUG: soft lockup - CPU#9 stuck for 52s! [kworker/u64:110:3059593]
(standard input):Sep 21 13:17:50 marcus-server kernel: [43903.557055] watchdog: BUG: soft lockup - CPU#6 stuck for 26s! [kworker/u64:30:3109619]
(standard input):Sep 21 13:17:50 marcus-server kernel: [43931.557265] watchdog: BUG: soft lockup - CPU#6 stuck for 52s! [kworker/u64:30:3109619]
(standard input):Sep 21 13:39:24 marcus-server kernel: [45193.795060] watchdog: BUG: soft lockup - CPU#5 stuck for 27s! [kworker/u64:110:3157668]
(standard input):Sep 21 13:39:24 marcus-server kernel: [45221.795159] watchdog: BUG: soft lockup - CPU#5 stuck for 53s! [kworker/u64:110:3157668]
(standard input):Sep 21 14:00:57 marcus-server kernel: [46483.788246] watchdog: BUG: soft lockup - CPU#9 stuck for 26s! [kworker/u64:29:3217705]
(standard input):Sep 21 14:00:57 marcus-server kernel: [46511.785742] watchdog: BUG: soft lockup - CPU#9 stuck for 52s! [kworker/u64:29:3217705]
(standard input):Sep 21 14:22:32 marcus-server kernel: [47775.425566] watchdog: BUG: soft lockup - CPU#11 stuck for 26s! [kworker/u64:31:3278001]
(standard input):Sep 21 14:22:32 marcus-server kernel: [47803.425675] watchdog: BUG: soft lockup - CPU#11 stuck for 52s! [kworker/u64:31:3278001]