kl-bt

Backtrace

Canonical: kernlens-bt
Category: memory & symbols
Synopsis: kl-bt [-h] [--chain] [depth]

A kernel-aware backtrace: each frame's return address resolved to a symbol and source line, with the noise of the interrupted context handled. --chain collapses the same stack to one line, which is the form kl-chain used to print.

Options

Option	Argument	Meaning
	(optional)	maximum frames, 1-128 (default 8)
--chain	—	print the compact caller <- callee chain instead of the frames

The whole stack

Invocation

kl-bt

Output

bt:
                   #0  0xffffffffafa1263b  native_safe_halt  at ./arch/x86/include/asm/irqflags.h:52
                   #1  0xffffffffafa1248e  default_idle  at ./arch/x86/include/asm/paravirt.h:173
                   #2  0xffffffffafa1248e  default_idle  at ./arch/x86/include/asm/paravirt.h:173
                   #3  0xffffffffae85a245  arch_cpu_idle  at arch/x86/kernel/process.c:728
                   #4  0xffffffffafa127aa  default_idle_call  at ./arch/x86/include/asm/paravirt.h:699
                   #5  0xffffffffaea119bf  do_idle  at kernel/sched/idle.c:192
                   #6  0xffffffffaea119bf  do_idle  at kernel/sched/idle.c:192
                   #7  0xffffffffaea11c1a  cpu_startup_entry  at kernel/sched/idle.c:400

Reading it

Frames are printed #N address symbol at file:line. Frames 1 and 2 being the same symbol (default_idle) is normal in an idle stack: the compiler emitted inlined copies, and both map back to the same out-of-line body. The stack ends in the idle loop, which tells you the CPU is not running anything interesting.

Limiting the depth

Invocation

kl-bt 4

Output

bt:
                   #0  0xffffffffafa1263b  native_safe_halt  at ./arch/x86/include/asm/irqflags.h:52
                   #1  0xffffffffafa1248e  default_idle  at ./arch/x86/include/asm/paravirt.h:173
                   #2  0xffffffffafa1248e  default_idle  at ./arch/x86/include/asm/paravirt.h:173
                   #3  0xffffffffae85a245  arch_cpu_idle  at arch/x86/kernel/process.c:728

Reading it

A depth limit keeps the output readable on a deep stack. The inner frames are the same, so the limit is applied from the innermost frame outward, not by truncating the printed text.

The chain form

Invocation

kl-bt --chain

Output

native_safe_halt <- default_idle <- default_idle <- arch_cpu_idle <- default_idle_call <- do_idle <- do_idle <- cpu_startup_entry

Reading it

Innermost first, joined by <-, symbol names only -- no addresses, no source lines -- which is what makes it fit on one line. Repeated inlined symbols are kept rather than deduplicated, because collapsing them would misrepresent the depth. init_thread_union appearing between frames is a known artefact of the kernel's stack layout, not a bad unwind.

Notes

A frame showing ?? is one where the return address did not resolve -- often a stack-corruption artefact, and worth noting rather than hiding. For anything where the addresses matter, do not use --chain: it deliberately drops them.

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Part of the KernLens tutorial. Every output block on this page was captured from the kernel described in the tutorial index.