LFCS Operation of Running Systems Practice Question
Exhibit
Refer to the exhibit. [ 123.456789] INFO: task kworker/u:2:789 blocked for more than 120 seconds. [ 123.456790] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 123.456791] kworker/u:2 D 0 789 2 0x00000000 [ 123.456792] Call Trace: [ 123.456793] __schedule+0x2f0/0x800 [ 123.456794] schedule+0x28/0x80 [ 123.456795] io_schedule+0x16/0x40 [ 123.456796] wait_on_page_bit+0x10b/0x120 [ 123.456797] __filemap_fdatawait_range+0x100/0x150 [ 123.456798] filemap_fdatawait+0x1c/0x20 [ 123.456799] __sync_filesystem+0x4a/0x80 [ 123.456800] sync_one+0x2a/0x30 [ 123.456801] process_sync_work+0x22/0x50 [ 123.456802] worker_thread+0x4f/0x3f0 [ 123.456803] kthread+0x10b/0x140
Based on the exhibit, what is the most likely cause of the blocked task?
⚠ Common exam trap
Watch out — candidates often confuse a process in 'D' state (uninterruptible sleep, I/O wait) with a process that is simply sleeping or waiting on CPU, leading them to incorrectly choose CPU starvation or memory issues instead of recognizing the classic symptom of a disk I/O bottleneck.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Disk I/O bottleneck or hung storage
The exhibit shows a process in 'D' state (uninterruptible sleep), which typically indicates the process is waiting for I/O completion from a block device. When a task is blocked in this state for an extended period, it is most likely due to a disk I/O bottleneck or hung storage, as the kernel cannot interrupt this wait. CPU starvation (run queue) and memory leaks (OOM or swapping) produce different process states, making disk I/O the primary suspect.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
CPU starvation
Why it's wrong here
CPU starvation leaves tasks runnable but unscheduled, shown by high load with idle CPU; the exhibit's blocked state instead points to a task sleeping on a resource such as I/O or a lock. CPU starvation would be the cause when run-queue length climbs while processors stay busy.
- ✗
Memory leak
Why it's wrong here
A leak grows resident memory over time and eventually triggers the OOM killer or swapping; it does not produce the single blocked task in the exhibit. Memory leaks are the correct diagnosis when RSS climbs steadily across many processes until allocation fails.
- ✓
Disk I/O bottleneck or hung storage
Why this is correct
A blocked task in uninterruptible sleep (D state) typically indicates the process is waiting on storage I/O that never completes. Since the exhibit shows the task stuck rather than terminated, a hung disk or saturated I/O queue is the most likely cause, directly satisfying the blocked-task symptom.
- ✗
Network congestion
Why it's wrong here
Congestion delays packets but does not place a process in the uninterruptible D state shown; blocked tasks typically wait on I/O or locks. Network congestion is the right diagnosis when throughput drops and latency rises across many hosts, not when a single task hangs.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.