workqueue: Report work funcs that trigger automatic CPU_INTENSIVE mechanism

Workqueue now automatically marks per-cpu work items that hog CPU for too
long as CPU_INTENSIVE, which excludes them from concurrency management and
prevents stalling other concurrency-managed work items. If a work function
keeps running over the thershold, it likely needs to be switched to use an
unbound workqueue.

This patch adds a debug mechanism which tracks the work functions which
trigger the automatic CPU_INTENSIVE mechanism and report them using
pr_warn() with exponential backoff.

v3: Documentation update.

v2: Drop bouncing to kthread_worker for printing messages. It was to avoid
    introducing circular locking dependency through printk but not effective
    as it still had pool lock -> wci_lock -> printk -> pool lock loop. Let's
    just print directly using printk_deferred().

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>

lib/Kconfig.debug

@@ -1134,6 +1134,19 @@ config WQ_WATCHDOG
 	  state.  This can be configured through kernel parameter
 	  "workqueue.watchdog_thresh" and its sysfs counterpart.
 
+config WQ_CPU_INTENSIVE_REPORT
+	bool "Report per-cpu work items which hog CPU for too long"
+	depends on DEBUG_KERNEL
+	help
+	  Say Y here to enable reporting of concurrency-managed per-cpu work
+	  items that hog CPUs for longer than
+	  workqueue.cpu_intensive_threshold_us. Workqueue automatically
+	  detects and excludes them from concurrency management to prevent
+	  them from stalling other per-cpu work items. Occassional
+	  triggering may not necessarily indicate a problem. Repeated
+	  triggering likely indicates that the work item should be switched
+	  to use an unbound workqueue.
+
 config TEST_LOCKUP
 	tristate "Test module to generate lockups"
 	depends on m