[PATCH RFC] workqueue: Add system_dfl_long_wq for long unbound works

[Marco Crivellari <marco.crivellari@suse.com>]

Currently there are users of queue_delayed_work() who specify
system_long_wq, the per-cpu workqueue. This workqueue should
be used for long per-cpu works, but queue_delayed_work()
queue the work using:

  queue_delayed_work_on(WORK_CPU_UNBOUND, ...);

This would end up calling __queue_delayed_work() that does:

	if (housekeeping_enabled(HK_TYPE_TIMER)) {
	//	[....]
	} else {
		if (likely(cpu == WORK_CPU_UNBOUND))
			add_timer_global(timer);
		else
			add_timer_on(timer, cpu);
	}

So when cpu == WORK_CPU_UNBOUND the timer is global and is
not using a specific CPU. Later, when __queue_work() is called:

	if (req_cpu == WORK_CPU_UNBOUND) {
		if (wq->flags & WQ_UNBOUND)
			cpu = wq_select_unbound_cpu(raw_smp_processor_id());
		else
			cpu = raw_smp_processor_id();
	}

Because the wq is not unbound, it takes the CPU where the timer
fired and enqueue the work on that CPU.
The consequence of all of this is that the work can run anywhere,
depending on where the timer fired.

Introduce system_dfl_long_wq in order to change, in a future step,
users that are still calling:

  queue_delayed_work(system_long_wq, ...);

with the new system_dfl_long_wq instead, so that the work may
benefit from scheduler task placement.

Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 include/linux/workqueue.h | 6 ++++++
 kernel/workqueue.c        | 5 ++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index a4749f56398f..ab3853c54230 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -440,6 +440,9 @@ enum wq_consts {
  * system_long_wq is similar to system_percpu_wq but may host long running
  * works.  Queue flushing might take relatively long.
  *
+ * system_dfl_long_wq is similar to system_dfl_wq but it may host long running
+ * works.
+ *
  * system_dfl_wq is unbound workqueue.  Workers are not bound to
  * any specific CPU, not concurrency managed, and all queued works are
  * executed immediately as long as max_active limit is not reached and
@@ -468,6 +471,7 @@ extern struct workqueue_struct *system_power_efficient_wq;
 extern struct workqueue_struct *system_freezable_power_efficient_wq;
 extern struct workqueue_struct *system_bh_wq;
 extern struct workqueue_struct *system_bh_highpri_wq;
+extern struct workqueue_struct *system_dfl_long_wq;
 
 void workqueue_softirq_action(bool highpri);
 void workqueue_softirq_dead(unsigned int cpu);
@@ -783,6 +787,8 @@ extern void __warn_flushing_systemwide_wq(void)
 	     _wq == system_highpri_wq) ||				\
 	    (__builtin_constant_p(_wq == system_long_wq) &&		\
 	     _wq == system_long_wq) ||					\
+	    (__builtin_constant_p(_wq == system_dfl_long_wq) &&		\
+	     _wq == system_dfl_long_wq) ||					\
 	    (__builtin_constant_p(_wq == system_dfl_wq) &&		\
 	     _wq == system_dfl_wq) ||				\
 	    (__builtin_constant_p(_wq == system_freezable_wq) &&	\
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index aeaec79bc09c..cc137c907c31 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -530,6 +530,8 @@ struct workqueue_struct *system_bh_wq;
 EXPORT_SYMBOL_GPL(system_bh_wq);
 struct workqueue_struct *system_bh_highpri_wq;
 EXPORT_SYMBOL_GPL(system_bh_highpri_wq);
+struct workqueue_struct *system_dfl_long_wq __ro_after_init;
+EXPORT_SYMBOL_GPL(system_dfl_long_wq);
 
 static int worker_thread(void *__worker);
 static void workqueue_sysfs_unregister(struct workqueue_struct *wq);
@@ -7942,11 +7944,12 @@ void __init workqueue_init_early(void)
 	system_bh_wq = alloc_workqueue("events_bh", WQ_BH | WQ_PERCPU, 0);
 	system_bh_highpri_wq = alloc_workqueue("events_bh_highpri",
 					       WQ_BH | WQ_HIGHPRI | WQ_PERCPU, 0);
+	system_dfl_long_wq = alloc_workqueue("events_dfl_long", WQ_UNBOUND, WQ_MAX_ACTIVE);
 	BUG_ON(!system_wq || !system_percpu_wq|| !system_highpri_wq || !system_long_wq ||
 	       !system_unbound_wq || !system_freezable_wq || !system_dfl_wq ||
 	       !system_power_efficient_wq ||
 	       !system_freezable_power_efficient_wq ||
-	       !system_bh_wq || !system_bh_highpri_wq);
+	       !system_bh_wq || !system_bh_highpri_wq || !system_dfl_long_wq);
 }
 
 static void __init wq_cpu_intensive_thresh_init(void)
-- 
2.53.0