workqueue: add cmdline parameter workqueue.unbound_cpus to further constrain wq_unbound_cpumask at boot time

Motivation of doing this is to better improve boot times for devices when
we want to prevent our workqueue works from running on some specific CPUs,
e,g, some CPUs are busy with interrupts.

Signed-off-by: tiozhang <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: tiozhang

Documentation/admin-guide/kernel-parameters.txt

@@ -6964,6 +6964,13 @@
 			disables both lockup detectors. Default is 10
 			seconds.
 
+	workqueue.unbound_cpus=
+			[KNL,SMP] Specify to constrain one or some CPUs
+			to use in unbound workqueues.
+			Format: <cpu-list>
+			By default, all online CPUs are available for
+			unbound workqueues.
+
 	workqueue.watchdog_thresh=
 			If CONFIG_WQ_WATCHDOG is configured, workqueue can
 			warn stall conditions and dump internal state to

kernel/workqueue.c

@@ -368,6 +368,9 @@ static bool workqueue_freezing;		/* PL: have wqs started freezing? */
 /* PL&A: allowable cpus for unbound wqs and work items */
 static cpumask_var_t wq_unbound_cpumask;
 
+/* for further constrain wq_unbound_cpumask by cmdline parameter*/
+static struct cpumask wq_cmdline_cpumask __initdata;
+
 /* CPU where unbound work was last round robin scheduled from this CPU */
 static DEFINE_PER_CPU(int, wq_rr_cpu_last);
 
@@ -6455,6 +6458,9 @@ void __init workqueue_init_early(void)
 	cpumask_copy(wq_unbound_cpumask, housekeeping_cpumask(HK_TYPE_WQ));
 	cpumask_and(wq_unbound_cpumask, wq_unbound_cpumask, housekeeping_cpumask(HK_TYPE_DOMAIN));
 
+	if (!cpumask_empty(&wq_cmdline_cpumask))
+		cpumask_and(wq_unbound_cpumask, wq_unbound_cpumask, &wq_cmdline_cpumask);
+
 	pwq_cache = KMEM_CACHE(pool_workqueue, SLAB_PANIC);
 
 	/* initialize CPU pools */
@@ -6577,3 +6583,14 @@ void __warn_flushing_systemwide_wq(void)
 	dump_stack();
 }
 EXPORT_SYMBOL(__warn_flushing_systemwide_wq);
+
+static int __init workqueue_unbound_cpus_setup(char *str)
+{
+	if (cpulist_parse(str, &wq_cmdline_cpumask) < 0) {
+		cpumask_clear(&wq_cmdline_cpumask);
+		pr_warn("workqueue.unbound_cpus: incorrect CPU range, using default\n");
+	}
+
+	return 1;
+}
+__setup("workqueue.unbound_cpus=", workqueue_unbound_cpus_setup);