Searching for the cause of hung tasks in the Linux kernel

a year ago

Linux kernel produces hung task warnings when processes are stuck in an uninterruptible state (TASK_UNINTERRUPTIBLE) for too long.
The hung task warning is triggered by the khungtaskd daemon, which checks processes in the D state (TASK_UNINTERRUPTIBLE).
Processes in the D state cannot be interrupted by signals and may indicate system resource issues or overwhelmed subsystems.
The TASK_KILLABLE state was introduced to allow termination with fatal signals while still protecting process memory.
Example #1: XFS file system slowdown caused hung task warnings due to missing no_read_workqueue/no_write_workqueue flags.
Example #2: A process generating a core dump can trigger hung task warnings as the kernel preserves memory in the D state.
Example #3: The rtnl_mutex lock in the kernel networking subsystem caused multiple processes to hang, identified via BPF tracing.
Debugging hung tasks involves analyzing stack traces, checking system metrics, and using tools like bpftrace and drgn.
Hung task warnings are valuable for identifying system issues but may point to victims rather than the root cause.

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