[PATCH v2 -tip] sched/rt: Fix locality of threaded interrupt handlers

[Alexander Gordeev]

When a interrupt affinity mask targets multiple CPUs, the
RT scheduler selects a runqueue for RT task corresponding
to a threaded interrupt handler without consideration of
where the interrupt is actually gets delivered. It leads
to a suboptimal condition when a hardware interrupt handler
executes on one CPU while the threaded interrupt handler
executes on another CPU.

This fix alters the behaviour of threaded handler wake-ups
by getting priority to a CPU where the hardware interrupt
handler is executing. As result, most of the time both
halves of interrupt handling are kept local.

Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxx>
---
 include/linux/sched.h |    2 ++
 kernel/irq/handle.c   |    2 +-
 kernel/sched/core.c   |    5 +++++
 kernel/sched/rt.c     |   27 +++++++++++++++++++--------
 4 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 206bb08..1d59efa 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1061,6 +1061,7 @@ struct sched_domain;
 #define WF_SYNC		0x01		/* waker goes to sleep after wakup */
 #define WF_FORK		0x02		/* child wakeup after fork */
 #define WF_MIGRATED	0x04		/* internal use, task got migrated */
+#define WF_LOCAL	0x08		/* try to wake up locally */
 
 #define ENQUEUE_WAKEUP		1
 #define ENQUEUE_HEAD		2
@@ -2207,6 +2208,7 @@ extern void xtime_update(unsigned long ticks);
 
 extern int wake_up_state(struct task_struct *tsk, unsigned int state);
 extern int wake_up_process(struct task_struct *tsk);
+extern int wake_up_local(struct task_struct *tsk);
 extern void wake_up_new_task(struct task_struct *tsk);
 #ifdef CONFIG_SMP
  extern void kick_process(struct task_struct *tsk);
diff --git a/kernel/irq/handle.c b/kernel/irq/handle.c
index 131ca17..fe97d0c 100644
--- a/kernel/irq/handle.c
+++ b/kernel/irq/handle.c
@@ -126,7 +126,7 @@ static void irq_wake_thread(struct irq_desc *desc, struct irqaction *action)
 	 */
 	atomic_inc(&desc->threads_active);
 
-	wake_up_process(action->thread);
+	wake_up_local(action->thread);
 }
 
 irqreturn_t
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 257002c..38413f6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1527,6 +1527,11 @@ int wake_up_process(struct task_struct *p)
 }
 EXPORT_SYMBOL(wake_up_process);
 
+int wake_up_local(struct task_struct *p)
+{
+	return try_to_wake_up(p, TASK_ALL, WF_LOCAL);
+}
+
 int wake_up_state(struct task_struct *p, unsigned int state)
 {
 	return try_to_wake_up(p, state, 0);
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 418feb0..de16e16 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1234,7 +1234,7 @@ static void yield_task_rt(struct rq *rq)
 }
 
 #ifdef CONFIG_SMP
-static int find_lowest_rq(struct task_struct *task);
+static int find_lowest_rq(struct task_struct *task, bool prefer_this_cpu);
 
 static int
 select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
@@ -1242,6 +1242,7 @@ select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
 	struct task_struct *curr;
 	struct rq *rq;
 	int cpu;
+	bool prefer_this_cpu = flags & WF_LOCAL;
 
 	cpu = task_cpu(p);
 
@@ -1258,6 +1259,11 @@ select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
 	curr = ACCESS_ONCE(rq->curr); /* unlocked access */
 
 	/*
+	 * If this RT task is a threaded interrupt handler, then
+	 * it is being awaken from the hardware interrupt handler.
+	 * In this case try to keep hardware and threaded interrupt
+	 * handlers as close as possible and wake it up on this CPU.
+	 *
 	 * If the current task on @p's runqueue is an RT task, then
 	 * try to see if we can wake this RT task up on another
 	 * runqueue. Otherwise simply start this RT task
@@ -1279,11 +1285,12 @@ select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
 	 * This test is optimistic, if we get it wrong the load-balancer
 	 * will have to sort it out.
 	 */
-	if (curr && unlikely(rt_task(curr)) &&
-	    (curr->nr_cpus_allowed < 2 ||
-	     curr->prio <= p->prio) &&
-	    (p->nr_cpus_allowed > 1)) {
-		int target = find_lowest_rq(p);
+	if (prefer_this_cpu ||
+	    (curr && unlikely(rt_task(curr)) &&
+	     (curr->nr_cpus_allowed < 2 ||
+	      curr->prio <= p->prio) &&
+	     (p->nr_cpus_allowed > 1))) {
+		int target = find_lowest_rq(p, prefer_this_cpu);
 
 		if (target != -1)
 			cpu = target;
@@ -1473,7 +1480,7 @@ next_idx:
 
 static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);
 
-static int find_lowest_rq(struct task_struct *task)
+static int find_lowest_rq(struct task_struct *task, bool prefer_this_cpu)
 {
 	struct sched_domain *sd;
 	struct cpumask *lowest_mask = __get_cpu_var(local_cpu_mask);
@@ -1495,9 +1502,13 @@ static int find_lowest_rq(struct task_struct *task)
 	 * lowest priority tasks in the system.  Now we want to elect
 	 * the best one based on our affinity and topology.
 	 *
+	 * If asked explicitly, try to pick up this cpu.
+	 *
 	 * We prioritize the last cpu that the task executed on since
 	 * it is most likely cache-hot in that location.
 	 */
+	if (prefer_this_cpu && cpumask_test_cpu(this_cpu, lowest_mask))
+		return this_cpu;
 	if (cpumask_test_cpu(cpu, lowest_mask))
 		return cpu;
 
@@ -1555,7 +1566,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 	int cpu;
 
 	for (tries = 0; tries < RT_MAX_TRIES; tries++) {
-		cpu = find_lowest_rq(task);
+		cpu = find_lowest_rq(task, false);
 
 		if ((cpu == -1) || (cpu == rq->cpu))
 			break;
-- 
1.7.7.6


-- 
Regards,
Alexander Gordeev
agordeev@xxxxxxxxxx
--
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