[PATCH v2 0/3] Add telemetry tracepoints for CPU hogs, distress, and BH budget yields
From: Aaron Tomlin <atomlin@atomlin.com>
Date: 2026-09-04 19:34:54
Hi Tejun, Lai, While the workqueue subsystem maintains rich internal telemetry via pwq->stats[], wq_cpu_intensive_thresh_us, and distress mechanisms, several critical state transitions and execution anomalies currently lack real-time event notifications. Across production fleets and low-latency networking workloads, polling pwq->stats[] or running drgn scripts is impractical for detecting intermittent stalls. Tail-latency spikes and packet drops often stem from softirq overruns or latency-critical work items queueing behind CPU-bound tasks. These event-driven tracepoints allow zero-overhead eBPF tools and latency profilers to capture stack traces and kernel context at the exact moment a starvation event or softirq budget exhaustion occurs. This patch series introduces lightweight tracepoints for these key operational boundaries: Patch 1 adds workqueue_cpu_intensive tracepoint. When a concurrency-managed worker runs for longer than wq_cpu_intensive_thresh_us without sleeping, wq_worker_tick() marks it as WORKER_CPU_INTENSIVE and kicks the pool to prevent queue starvation. The tracepoint captures the offending work function, workqueue name, CPU, and elapsed duration. Patch 2 adds workqueue_mayday and workqueue_rescued tracepoints. One when worker allocation stalls trigger mayday distress, and another when pending work items are handed off to the rescuer thread to ensure forward progress. Patch 3 adds workqueue_bh_budget_yield tracepoint. Bottom-Half (BH) workqueues enforce execution limits in softirq context (BH_WORKER_JIFFIES and BH_WORKER_RESTARTS). When a BH worker hits these limits while pending work remains, it yields and re-raises the softirq. The tracepoint can be used to identify softirq budget saturation and track whether yielding occurred due to time slice expiration or restart counts. Changes since v1: - Fixed 32-bit architecture build/link failures reported by including <linux/math64.h> and using div_u64() (Tejun Heo) - Kept the fast-path threshold comparison in wq_worker_tick() untouched to avoid performing division on every timer tick - Calculated elapsed duration using div_u64() on the cold path only after marking the worker CPU_INTENSIVE and releasing pool->lock - Eliminated the race where re-reading jiffies after the loop could misattribute an exhausted restart budget to a timeout; timeout is now derived directly inside the loop from the condition that actually terminated it (Tejun Heo) - Corrected restart counting. Explicitly tracked and reported the actual number of restarts executed rather than total loop iterations (Tejun Heo) - Link to v1: https://lore.kernel.org/lkml/20260829230517.42468-1-atomlin@atomlin.com/ Aaron Tomlin (3): workqueue: Add workqueue_cpu_intensive tracepoint workqueue: Add workqueue_mayday and workqueue_rescued tracepoints workqueue: Add workqueue_bh_budget_yield tracepoint include/trace/events/workqueue.h | 147 +++++++++++++++++++++++++++++++ kernel/workqueue.c | 35 +++++++- 2 files changed, 180 insertions(+), 2 deletions(-) -- 2.55.0