This LFCS practice question tests your understanding of operation of running systems. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A system has a process stuck in uninterruptible sleep (D state). The administrator wants to identify which kernel function it is waiting on. Which tool should be used?
cat /proc/PID/stack
Shows kernel stack trace of the blocked process.
gdb -p PID
Why wrong: For userspace debugging, not kernel waits.
perf top -p PID
Why wrong: Shows CPU profiling, not blocked state.
strace -p PID
Why wrong: Cannot trace processes in D state.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
cat /proc/PID/stack
Option A is correct because reading /proc/PID/stack directly shows the kernel stack trace of the process, revealing the exact kernel function or wait queue the process is blocked on while in uninterruptible sleep (D state). This is the only tool listed that can inspect the kernel-side call stack without attaching a debugger or altering process state.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
cat /proc/PID/stack
Why this is correct
Shows kernel stack trace of the blocked process.
Related concept
Read the scenario before looking for a memorised answer.
gdb -p PID
Why it's wrong here
For userspace debugging, not kernel waits.
perf top -p PID
Why it's wrong here
Shows CPU profiling, not blocked state.
strace -p PID
Why it's wrong here
Cannot trace processes in D state.
Common exam traps
The trap here is that candidates often confuse strace (user-space syscall tracing) with kernel stack inspection, assuming strace can show kernel internals, but strace only traces syscall entry/exit and cannot reveal the internal kernel function where the process is blocked.
Trap categories for this question
Command / output trap
Shows CPU profiling, not blocked state.
Detailed technical explanation
The /proc/PID/stack file is a virtual file provided by the kernel's stack tracer (CONFIG_STACKTRACE), which dumps the kernel-mode call stack of the process at the moment of reading. This is invaluable for diagnosing I/O waits, NFS lockups, or device driver hangs where the process is stuck in TASK_UNINTERRUPTIBLE (D state) and cannot be killed. In real-world scenarios, an administrator might see a process stuck in 'wait_on_page_bit' or '__lock_page' and use this to identify a failing storage device.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the LFCS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Operation of Running Systems — This question tests Operation of Running Systems — Read the scenario before looking for a memorised answer..
The correct answer is: cat /proc/PID/stack — Option A is correct because reading /proc/PID/stack directly shows the kernel stack trace of the process, revealing the exact kernel function or wait queue the process is blocked on while in uninterruptible sleep (D state). This is the only tool listed that can inspect the kernel-side call stack without attaching a debugger or altering process state.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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.
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