kl-percpu

Per-CPU symbol address

Canonical: kernlens-percpu
Category: CPU topology
Synopsis: kl-percpu [-h] symbol [cpu]

Resolves a per-CPU symbol to the address the given CPU actually uses. This is the fix for the classic mistake of taking &current_task and getting a section offset that is not any CPU's copy.

Options

Option	Argument	Meaning
	—	a per-CPU symbol (e.g. current_task)
	(optional)	CPU id (default: current CPU)

The current CPU

Invocation

kl-percpu current_task

Output

Symbol: current_task (cpu 0)
Address: 0xffff90c347a1fb80
percpu_off: 0xffff90c347a00000

Reading it

The bare value of a per-CPU symbol is its offset within the per-CPU section, zero-based, so the runtime address is __per_cpu_offset[cpu] + offset. Both parts are printed, which is what makes a mismatch obvious.

Another CPU

Invocation

kl-percpu current_task 1

Output

Symbol: current_task (cpu 1)
Address: 0xffff90c347a9fb80
percpu_off: 0xffff90c347a80000

Reading it

Different CPU, different base, same offset. Comparing two CPUs' output is the quickest way to confirm the offsets are being applied rather than ignored.

Notes

The per-CPU base is read from __per_cpu_offset[] rather than derived, because the linker only guarantees the section is zero-based; where it lands at run time is the kernel's choice.

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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.