libbpf-tools/softirqslower: Add libbpf CO-RE port of softirqslower

[ismhong]

Description

Convert from legacy Python/bcc to modern libbpf-based C implementation. This tool traces slow softirq events by measuring two critical latency dimensions of softirq handling.

Features:

• Trace irq(hard)-to-softirq latency: delay from raise_softirq() until the softirq handler starts executing
• Trace softirq runtime: actual handler execution duration (entry to exit)
• Filter by minimum latency threshold (default 10000 us)
• Filter by CPU with -c/--cpu option
• Output columns: TIME, STAGE, SOFTIRQ, LAT(us), CPU, COMM
• Supports both tp_btf (preferred) and raw_tp fallback tracepoints
• Uses PERCPU_ARRAY maps indexed by softirq vector for lock-free tracking

Test on x86_64/Manjaro/5.15.60

❯ sudo ./softirqslower 100
Tracing softirq latency higher than 100 us... Hit Ctrl-C to end.
TIME     STAGE                SOFTIRQ  LAT(us)        CPU    COMM
18:43:53 irq(hard) to softirq rcu      116            0      ksoftirqd/0
18:43:53 irq(hard) to softirq net_rx   100            3      ksoftirqd/3
18:43:53 softirq runtime      net_rx   143            3      ksoftirqd/3
18:43:53 irq(hard) to softirq sched    387            3      ksoftirqd/3

❯ uname -a
Linux ism-manjaro 5.15.60-1-MANJARO #1 SMP PREEMPT Thu Aug 11 13:14:05 UTC 2022 x86_64 GNU/Linux

Test on arm64/Android/6.12.38

RTKRTD1325GTV_36:/ # /data/softirqslower 100
Tracing softirq latency higher than 100 us... Hit Ctrl-C to end.
TIME     STAGE                SOFTIRQ  LAT(us)        CPU    COMM
10:37:09 softirq runtime      net_rx   113            0      swapper/0
10:37:09 irq(hard) to softirq sched    104            3      swapper/3
10:37:09 softirq runtime      net_rx   101            0      swapper/0

RTKRTD1325GTV_36:/ # uname -a
Linux localhost 6.12.38-android16-5-gb9e1e77593f9-ab14266828-4k #1 SMP PREEMPT Tue Oct 14 10:02:12 UTC 2025 armv8l Toybox

Why this approach

Just port softirqslower from BCC to libbpf

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Checklist

• Commit prefix matches changed area (e.g., tools/toolname:, libbpf-tools/toolname:, src/cc:, docs:, build:, tests/python:)
• Commit body explains why this change is needed

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[Copilot]

Pull request overview

Ports the BCC Python softirqslower tool to a libbpf CO-RE implementation under libbpf-tools/. The new tool traces two latency dimensions of softirq processing — raise→entry and entry→exit — emitting events via a perf buffer when latency exceeds a configurable threshold, with optional per-CPU filtering and tp_btf/raw_tp fallback paths.

Changes:

• New softirqslower.bpf.c with per-CPU arrays indexed by vector and matched tp_btf/raw_tp programs for raise/entry/exit.
• New softirqslower.c userspace driver (argp, rodata configuration, BTF probe, perf buffer polling).
• Shared softirqslower.h defining event layout and stage enum; Makefile APPS updated.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
libbpf-tools/softirqslower.h	Shared event struct, stage enum, NR_SOFTIRQS constant.
libbpf-tools/softirqslower.bpf.c	BPF programs and helpers for raise/entry/exit tracking.
libbpf-tools/softirqslower.c	Userspace argp, BTF/raw_tp selection, perf buffer loop.
libbpf-tools/Makefile	Adds softirqslower to APPS.