Operating Systems
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Detecting hung tasks in Linux

Sometimes tasks under Linux are blocked forever (essentially hung). Recent Linux kernels have an infrastructure to detect hung tasks. When this infrastructure is active it will periodically get activated to find out hung tasks and present a stack dump of those hung tasks (and maybe locks held). Additionally we can choose to panic the system when we detect atleast one hung task in the system.
I will try to explain how khungtaskd works.

The infrastructure is based on a single kernel thread named as “khungtaskd”. So if you do a ps in your system and see that there is entry like [khungtaskd] you know it is there. I have one in my system:
136 root SW [khungtaskd]

The loop of the khungtaskd daemon is a call to the scheduler for waking it up after ever 120 seconds (default value). The core algorithm is like this:

1. Iterate over all the tasks in the system which are marked as TASK_UNINTERRUPTIBLE (additionally it does not consider UNINTERRUPTIBLE frozen tasks & UNINTERRUPTIBLE tasks that are newly created and never been scheduled out).

2. If a task has not been switched out by the scheduler atleast once in the last 120 seconds it is considered as a hung task and its stack dump is displayed. If CONFIG_LOCKDEP is defined then it will also show all the locks the hung task is holding.

One can change the sampling interval of khungtaskd through the sysctl interface /proc/sys/kernel/hung_task_timeout_secs.


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