Merge tag 'wq-for-6.5-rc5-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq

Pull workqueue fixes from Tejun Heo:

- The recently added cpu_intensive auto detection and warning mechanism
was spuriously triggered on slow CPUs.

While not causing serious issues, it's still a nuisance and can cause
unintended concurrency management behaviors.

Relax the threshold on machines with lower BogoMIPS. While BogoMIPS
is not an accurate measure of performance by most measures, we don't
have to be accurate and it has rough but strong enough correlation.

- A correction in Kconfig help text

* tag 'wq-for-6.5-rc5-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq:
workqueue: Scale up wq_cpu_intensive_thresh_us if BogoMIPS is below 4000
workqueue: Fix cpu_intensive_thresh_us name in help text

kernel/workqueue.c

···
 #include <linux/sched/debug.h>
 #include <linux/nmi.h>
 #include <linux/kvm_para.h>
+#include <linux/delay.h>
 
 #include "workqueue_internal.h"
 
···
  * Per-cpu work items which run for longer than the following threshold are
  * automatically considered CPU intensive and excluded from concurrency
  * management to prevent them from noticeably delaying other per-cpu work items.
+ * ULONG_MAX indicates that the user hasn't overridden it with a boot parameter.
+ * The actual value is initialized in wq_cpu_intensive_thresh_init().
  */
-static unsigned long wq_cpu_intensive_thresh_us = 10000;
+static unsigned long wq_cpu_intensive_thresh_us = ULONG_MAX;
 module_param_named(cpu_intensive_thresh_us, wq_cpu_intensive_thresh_us, ulong, 0644);
 
 static bool wq_disable_numa;
···
 	       !system_freezable_power_efficient_wq);
 }
 
+static void __init wq_cpu_intensive_thresh_init(void)
+{
+	unsigned long thresh;
+	unsigned long bogo;
+
+	/* if the user set it to a specific value, keep it */
+	if (wq_cpu_intensive_thresh_us != ULONG_MAX)
+		return;
+
+	/*
+	 * The default of 10ms is derived from the fact that most modern (as of
+	 * 2023) processors can do a lot in 10ms and that it's just below what
+	 * most consider human-perceivable. However, the kernel also runs on a
+	 * lot slower CPUs including microcontrollers where the threshold is way
+	 * too low.
+	 *
+	 * Let's scale up the threshold upto 1 second if BogoMips is below 4000.
+	 * This is by no means accurate but it doesn't have to be. The mechanism
+	 * is still useful even when the threshold is fully scaled up. Also, as
+	 * the reports would usually be applicable to everyone, some machines
+	 * operating on longer thresholds won't significantly diminish their
+	 * usefulness.
+	 */
+	thresh = 10 * USEC_PER_MSEC;
+
+	/* see init/calibrate.c for lpj -> BogoMIPS calculation */
+	bogo = max_t(unsigned long, loops_per_jiffy / 500000 * HZ, 1);
+	if (bogo < 4000)
+		thresh = min_t(unsigned long, thresh * 4000 / bogo, USEC_PER_SEC);
+
+	pr_debug("wq_cpu_intensive_thresh: lpj=%lu BogoMIPS=%lu thresh_us=%lu\n",
+		 loops_per_jiffy, bogo, thresh);
+
+	wq_cpu_intensive_thresh_us = thresh;
+}
+
 /**
  * workqueue_init - bring workqueue subsystem fully online
  *
···
 	struct workqueue_struct *wq;
 	struct worker_pool *pool;
 	int cpu, bkt;
+
+	wq_cpu_intensive_thresh_init();
 
 	/*
 	 * It'd be simpler to initialize NUMA in workqueue_init_early() but

lib/Kconfig.debug

···
 	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
+	  workqueue.cpu_intensive_thresh_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