Fix kernel panic error "task * blocked for more than 120 seconds"

If your server goes down, and you get a message like:

Feb 16 03:00:12 server kernel: INFO: task httpd:16101 blocked for more than 120 seconds.

Verify memory usage:

sar -r

Verify CPU usage:

sar -u

For me the culprit was CPU reaching %idle reaching 99.18

01:40:01 PM     CPU     %user     %nice   %system   %iowait    %steal     %idle
...
06:30:01 PM     all     42.52      0.00      3.52      0.32      0.00     53.65
06:40:01 PM     all     43.72      0.00      3.60      0.32      0.00     52.36
06:50:01 PM     all     42.32      0.00      3.47      0.38      0.00     53.82
07:00:01 PM     all     41.38      0.00      3.47      0.25      0.00     54.91
07:10:01 PM     all     44.30      0.00      3.50      0.65      0.00     51.54
07:20:01 PM     all     36.22      0.00      2.89      0.38      0.00     60.50
07:30:01 PM     all     31.56      0.00      2.72      0.26      0.00     65.47
07:40:01 PM     all     23.77      0.00      2.03      0.41      0.00     73.79
07:50:01 PM     all      2.01      0.00      0.20      0.14      0.00     97.65
08:00:01 PM     all      0.59      0.00      0.07      0.15      0.00     99.18
Average:        all     36.88      0.10      3.22      0.39      0.00     59.41

08:06:39 PM       LINUX RESTART

08:10:01 PM     CPU     %user     %nice   %system   %iowait    %steal     %idle
08:20:01 PM     all     50.79      0.00      3.62      0.08      0.00     45.51
Average:        all     50.79      0.00      3.62      0.08      0.00     45.51

Temporarily apply new settings for a couple of days. See here for a detailed explanation.

sudo sysctl -w vm.dirty_ratio=10
sudo sysctl -w vm.dirty_background_ratio=5
sudo sysctl -p

If everything runs smoothly, make the changes permanent:

vi /etc/sysctl.conf

Enter the following:

vm.dirty_background_ratio = 5
vm.dirty_ratio = 10

Now reboot.

reboot

Thanks to blackMORE Ops for the detailed tutorial.