path: root/debian/patches-rt/0205-irqwork-push-most-work-into-softirq-context.patch

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Tue, 23 Jun 2015 15:32:51 +0200
Subject: [PATCH 205/342] irqwork: push most work into softirq context
Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=3b91447711938ba1aee8c66249408ab12ba1b8f2

Initially we defered all irqwork into softirq because we didn't want the
latency spikes if perf or another user was busy and delayed the RT task.
The NOHZ trigger (nohz_full_kick_work) was the first user that did not work
as expected if it did not run in the original irqwork context so we had to
bring it back somehow for it. push_irq_work_func is the second one that
requires this.

This patch adds the IRQ_WORK_HARD_IRQ which makes sure the callback runs
in raw-irq context. Everything else is defered into softirq context. Without
-RT we have the orignal behavior.

This patch incorporates tglx orignal work which revoked a little bringing back
the arch_irq_work_raise() if possible and a few fixes from Steven Rostedt and
Mike Galbraith,

[bigeasy: melt tglx's irq_work_tick_soft() which splits irq_work_tick() into a
          hard and soft variant]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/irq_work.h |  8 +++++
 kernel/irq_work.c        | 75 ++++++++++++++++++++++++++++++----------
 kernel/rcu/tree.c        |  1 +
 kernel/sched/topology.c  |  1 +
 kernel/time/tick-sched.c |  1 +
 kernel/time/timer.c      |  2 ++
 6 files changed, 70 insertions(+), 18 deletions(-)

diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
index b11fcdfd0770..0c50559987c5 100644
--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -18,6 +18,8 @@
 
 /* Doesn't want IPI, wait for tick: */
 #define IRQ_WORK_LAZY		BIT(2)
+/* Run hard IRQ context, even on RT */
+#define IRQ_WORK_HARD_IRQ	BIT(3)
 
 #define IRQ_WORK_CLAIMED	(IRQ_WORK_PENDING | IRQ_WORK_BUSY)
 
@@ -52,4 +54,10 @@ static inline bool irq_work_needs_cpu(void) { return false; }
 static inline void irq_work_run(void) { }
 #endif
 
+#if defined(CONFIG_IRQ_WORK) && defined(CONFIG_PREEMPT_RT_FULL)
+void irq_work_tick_soft(void);
+#else
+static inline void irq_work_tick_soft(void) { }
+#endif
+
 #endif /* _LINUX_IRQ_WORK_H */
diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 73288914ed5e..2940622da5b3 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -17,6 +17,7 @@
 #include <linux/cpu.h>
 #include <linux/notifier.h>
 #include <linux/smp.h>
+#include <linux/interrupt.h>
 #include <asm/processor.h>
 
 
@@ -57,29 +58,35 @@ void __weak arch_irq_work_raise(void)
 }
 
 /* Enqueue on current CPU, work must already be claimed and preempt disabled */
-static void __irq_work_queue_local(struct irq_work *work)
+static void __irq_work_queue_local(struct irq_work *work, struct llist_head *list)
 {
-	/* If the work is "lazy", handle it from next tick if any */
-	if (work->flags & IRQ_WORK_LAZY) {
-		if (llist_add(&work->llnode, this_cpu_ptr(&lazy_list)) &&
-		    tick_nohz_tick_stopped())
-			arch_irq_work_raise();
-	} else {
-		if (llist_add(&work->llnode, this_cpu_ptr(&raised_list)))
-			arch_irq_work_raise();
-	}
+	bool empty;
+
+	empty = llist_add(&work->llnode, list);
+
+	if (empty &&
+	    (!(work->flags & IRQ_WORK_LAZY) ||
+	     tick_nohz_tick_stopped()))
+		arch_irq_work_raise();
 }
 
 /* Enqueue the irq work @work on the current CPU */
 bool irq_work_queue(struct irq_work *work)
 {
+	struct llist_head *list;
+
 	/* Only queue if not already pending */
 	if (!irq_work_claim(work))
 		return false;
 
 	/* Queue the entry and raise the IPI if needed. */
 	preempt_disable();
-	__irq_work_queue_local(work);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL) && !(work->flags & IRQ_WORK_HARD_IRQ))
+		list = this_cpu_ptr(&lazy_list);
+	else
+		list = this_cpu_ptr(&raised_list);
+
+	__irq_work_queue_local(work, list);
 	preempt_enable();
 
 	return true;
@@ -98,6 +105,9 @@ bool irq_work_queue_on(struct irq_work *work, int cpu)
 	return irq_work_queue(work);
 
 #else /* CONFIG_SMP: */
+	struct llist_head *list;
+	bool lazy_work, realtime = IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
+
 	/* All work should have been flushed before going offline */
 	WARN_ON_ONCE(cpu_is_offline(cpu));
 
@@ -106,13 +116,21 @@ bool irq_work_queue_on(struct irq_work *work, int cpu)
 		return false;
 
 	preempt_disable();
+
+	lazy_work = work->flags & IRQ_WORK_LAZY;
+
+	if (lazy_work || (realtime && !(work->flags & IRQ_WORK_HARD_IRQ)))
+		list = &per_cpu(lazy_list, cpu);
+	else
+		list = &per_cpu(raised_list, cpu);
+
 	if (cpu != smp_processor_id()) {
 		/* Arch remote IPI send/receive backend aren't NMI safe */
 		WARN_ON_ONCE(in_nmi());
-		if (llist_add(&work->llnode, &per_cpu(raised_list, cpu)))
+		if (llist_add(&work->llnode, list))
 			arch_send_call_function_single_ipi(cpu);
 	} else {
-		__irq_work_queue_local(work);
+		__irq_work_queue_local(work, list);
 	}
 	preempt_enable();
 
@@ -128,9 +146,8 @@ bool irq_work_needs_cpu(void)
 	raised = this_cpu_ptr(&raised_list);
 	lazy = this_cpu_ptr(&lazy_list);
 
-	if (llist_empty(raised) || arch_irq_work_has_interrupt())
-		if (llist_empty(lazy))
-			return false;
+	if (llist_empty(raised) && llist_empty(lazy))
+		return false;
 
 	/* All work should have been flushed before going offline */
 	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
@@ -144,8 +161,12 @@ static void irq_work_run_list(struct llist_head *list)
 	struct llist_node *llnode;
 	unsigned long flags;
 
+#ifndef CONFIG_PREEMPT_RT_FULL
+	/*
+	 * nort: On RT IRQ-work may run in SOFTIRQ context.
+	 */
 	BUG_ON(!irqs_disabled());
-
+#endif
 	if (llist_empty(list))
 		return;
 
@@ -177,7 +198,16 @@ static void irq_work_run_list(struct llist_head *list)
 void irq_work_run(void)
 {
 	irq_work_run_list(this_cpu_ptr(&raised_list));
-	irq_work_run_list(this_cpu_ptr(&lazy_list));
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) {
+		/*
+		 * NOTE: we raise softirq via IPI for safety,
+		 * and execute in irq_work_tick() to move the
+		 * overhead from hard to soft irq context.
+		 */
+		if (!llist_empty(this_cpu_ptr(&lazy_list)))
+			raise_softirq(TIMER_SOFTIRQ);
+	} else
+		irq_work_run_list(this_cpu_ptr(&lazy_list));
 }
 EXPORT_SYMBOL_GPL(irq_work_run);
 
@@ -187,8 +217,17 @@ void irq_work_tick(void)
 
 	if (!llist_empty(raised) && !arch_irq_work_has_interrupt())
 		irq_work_run_list(raised);
+
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
+		irq_work_run_list(this_cpu_ptr(&lazy_list));
+}
+
+#if defined(CONFIG_IRQ_WORK) && defined(CONFIG_PREEMPT_RT_FULL)
+void irq_work_tick_soft(void)
+{
 	irq_work_run_list(this_cpu_ptr(&lazy_list));
 }
+#endif
 
 /*
  * Synchronize against the irq_work @entry, ensures the entry is not
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f162a4f54b05..278fe66bfb70 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1296,6 +1296,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 		    !rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
 		    (rnp->ffmask & rdp->grpmask)) {
 			init_irq_work(&rdp->rcu_iw, rcu_iw_handler);
+			rdp->rcu_iw.flags = IRQ_WORK_HARD_IRQ;
 			rdp->rcu_iw_pending = true;
 			rdp->rcu_iw_gp_seq = rnp->gp_seq;
 			irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 02e85cd233d4..208af1181bf5 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -279,6 +279,7 @@ static int init_rootdomain(struct root_domain *rd)
 	rd->rto_cpu = -1;
 	raw_spin_lock_init(&rd->rto_lock);
 	init_irq_work(&rd->rto_push_work, rto_push_irq_work_func);
+	rd->rto_push_work.flags |= IRQ_WORK_HARD_IRQ;
 #endif
 
 	init_dl_bw(&rd->dl_bw);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 2b0ddd50e879..4d31ec98e968 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -236,6 +236,7 @@ static void nohz_full_kick_func(struct irq_work *work)
 
 static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) = {
 	.func = nohz_full_kick_func,
+	.flags = IRQ_WORK_HARD_IRQ,
 };
 
 /*
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 3cb79167852f..c7bd68db6f63 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1747,6 +1747,8 @@ static __latent_entropy void run_timer_softirq(struct softirq_action *h)
 {
 	struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
 
+	irq_work_tick_soft();
+
 	__run_timers(base);
 	if (IS_ENABLED(CONFIG_NO_HZ_COMMON))
 		__run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]));