[RFC PATCH] irq_work: wakeup irq_workd when queued first rt_lazy work

[Schspa Shi <schspa@gmail.com>]

Commit b4c6f86ec2f64
("irq_work: Handle some irq_work in a per-CPU thread on PREEMPT_RT")
treat all irq_work without IRQ_WORK_HARD_IRQ flags to the lazy_list
in PREEMPT_RT. But this kind of irq_work still has some difference
with IRQ work set IRQ_WORK_LAZY. The difference as fellowing:

- With IRQ_WORK_LAZY: (type1)
  This kind of work will be executed after the next time tick by wakeup
  irq_workd, there will be more scheduling delays.

  Let's mark this as type1

- Without IRQ_WORK_LAZY and IRQ_WORK_HARD_IRQ: (type2)
  This kind of irq_work will have a faster response speed by wakeup
  irq_workd from IPI interrupt.

  Let's mark it as type2

I want to know if this difference is by design.

If this is by design, we have a problem that the irq_work of type2
will not execute as quickly as expected, it may be delayed by the
irq_work of type1.

Please consider the following scenarios:

If the CPU queued a type1 irq_work A, and then a type2 irq_work B.
But we won't make B executed quickly, because we won't issue the IPI
interrupt to wakeup irq_workd (the llist_add call will return false).

This PATCH will issue the IPI_IRQ_WORK to make B execute quickly.

One thing that needs to be optimized is that we now have
lazy_list.node.llist and lazy_work_raised which need to be granted
to be atomicity, disabled the local CPU IRQ to make this atomic.
There should be a better way to make these two variants to be atomically
and I can go in deep if this little problem is not by design, and need
to be fixed.

If these two types of irq_work should be the same with the priority.
maybe we should change.

if (!lazy_work || tick_nohz_tick_stopped()) {
	arch_irq_work_raise();
}

to

if (!(lazy_work || rt_lazy_work) || tick_nohz_tick_stopped()) {
	arch_irq_work_raise();
}

I'm uploading this patch just to explain the problem, hopefully
don't pay too much attention to the ugly changes below.

Signed-off-by: Schspa Shi <schspa@gmail.com>
---
 kernel/irq_work.c | 40 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 7afa40fe5cc43..d5d0b720fac15 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -25,6 +25,7 @@
 static DEFINE_PER_CPU(struct llist_head, raised_list);
 static DEFINE_PER_CPU(struct llist_head, lazy_list);
 static DEFINE_PER_CPU(struct task_struct *, irq_workd);
+static DEFINE_PER_CPU(bool, lazy_work_raised);
 
 static void wake_irq_workd(void)
 {
@@ -81,6 +82,7 @@ static void __irq_work_queue_local(struct irq_work *work)
 	bool rt_lazy_work = false;
 	bool lazy_work = false;
 	int work_flags;
+	unsigned long flags;
 
 	work_flags = atomic_read(&work->node.a_flags);
 	if (work_flags & IRQ_WORK_LAZY)
@@ -94,12 +96,35 @@ static void __irq_work_queue_local(struct irq_work *work)
 	else
 		list = this_cpu_ptr(&raised_list);
 
-	if (!llist_add(&work->node.llist, list))
+	local_irq_save(flags);
+	if (!llist_add(&work->node.llist, list)) {
+		bool irq_raised;
+		/*
+		 * In PREEMPT_RT, if we add a lazy work added to the list
+		 * before, the work maybe not raised. We need a extra check
+		 * for PREEMPT_RT.
+		 */
+		irq_raised = !xchg(this_cpu_ptr(&lazy_work_raised), true);
+		local_irq_restore(flags);
+		if (unlikely(!irq_raised))
+			arch_irq_work_raise();
+
 		return;
+	}
 
 	/* If the work is "lazy", handle it from next tick if any */
-	if (!lazy_work || tick_nohz_tick_stopped())
+	if (!lazy_work || tick_nohz_tick_stopped()) {
+		(void) xchg(this_cpu_ptr(&lazy_work_raised), true);
+		local_irq_restore(flags);
 		arch_irq_work_raise();
+	} else if (lazy_work || rt_lazy_work) {
+		/*
+		 * The first added irq work not raise a irq work, we need to
+		 * raise one for the next added irq work.
+		 */
+		(void) xchg(this_cpu_ptr(&lazy_work_raised), false);
+		local_irq_restore(flags);
+	}
 }
 
 /* Enqueue the irq work @work on the current CPU */
@@ -151,9 +176,18 @@ bool irq_work_queue_on(struct irq_work *work, int cpu)
 		 */
 		if (IS_ENABLED(CONFIG_PREEMPT_RT) &&
 		    !(atomic_read(&work->node.a_flags) & IRQ_WORK_HARD_IRQ)) {
+			unsigned long flags;
+
+			local_irq_save(flags);
+			if (!llist_add(&work->node.llist, &per_cpu(lazy_list, cpu))) {
+				if (!xchg(this_cpu_ptr(&lazy_work_raised), true))
+					arch_irq_work_raise();
 
-			if (!llist_add(&work->node.llist, &per_cpu(lazy_list, cpu)))
+				local_irq_restore(flags);
 				goto out;
+			}
+			(void) xchg(this_cpu_ptr(&lazy_work_raised), true);
+			local_irq_restore(flags);
 
 			work = &per_cpu(irq_work_wakeup, cpu);
 			if (!irq_work_claim(work))
-- 
2.37.0