From 6713cc0b0d0483ae1e1a31b770fb6591a7b4c86c Mon Sep 17 00:00:00 2001
From: Scott Wood <swood@redhat.com>
Date: Sat, 12 Oct 2019 01:52:13 -0500
Subject: [PATCH 484/518] sched: Lazy migrate_disable processing

[ Upstream commit 425c5b38779a860062aa62219dc920d374b13c17 ]

Avoid overhead on the majority of migrate disable/enable sequences by
only manipulating scheduler data (and grabbing the relevant locks) when
the task actually schedules while migrate-disabled.  A kernel build
showed around a 10% reduction in system time (with CONFIG_NR_CPUS=512).

Instead of cpuhp_pin_lock, CPU hotplug is handled by keeping a per-CPU
count of the number of pinned tasks (including tasks which have not
scheduled in the migrate-disabled section); takedown_cpu() will
wait until that reaches zero (confirmed by take_cpu_down() in stop
machine context to deal with races) before migrating tasks off of the
cpu.

To simplify synchronization, updating cpus_mask is no longer deferred
until migrate_enable().  This lets us not have to worry about
migrate_enable() missing the update if it's on the fast path (didn't
schedule during the migrate disabled section).  It also makes the code
a bit simpler and reduces deviation from mainline.

While the main motivation for this is the performance benefit, lazy
migrate disable also eliminates the restriction on calling
migrate_disable() while atomic but leaving the atomic region prior to
calling migrate_enable() -- though this won't help with local_bh_disable()
(and thus rcutorture) unless something similar is done with the recently
added local_lock.

Signed-off-by: Scott Wood <swood@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>

 Conflicts:
	include/linux/sched.h
	init/init_task.c
	kernel/sched/core.c
---
 include/linux/cpu.h       |   4 -
 include/linux/init_task.h |   9 ++
 include/linux/sched.h     |  11 +--
 kernel/cpu.c              | 103 ++++++++-------------
 kernel/sched/core.c       | 182 ++++++++++++++++----------------------
 kernel/sched/sched.h      |   4 +
 lib/smp_processor_id.c    |   3 +
 7 files changed, 134 insertions(+), 182 deletions(-)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 5a0caaf6898a9..36a97555aa700 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -133,8 +133,6 @@ extern void cpu_hotplug_disable(void);
 extern void cpu_hotplug_enable(void);
 void clear_tasks_mm_cpumask(int cpu);
 int cpu_down(unsigned int cpu);
-extern void pin_current_cpu(void);
-extern void unpin_current_cpu(void);
 
 #else /* CONFIG_HOTPLUG_CPU */
 
@@ -145,8 +143,6 @@ static inline void cpus_read_unlock(void) { }
 static inline void lockdep_assert_cpus_held(void) { }
 static inline void cpu_hotplug_disable(void) { }
 static inline void cpu_hotplug_enable(void) { }
-static inline void pin_current_cpu(void) { }
-static inline void unpin_current_cpu(void) { }
 
 #endif	/* !CONFIG_HOTPLUG_CPU */
 
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index ee3ff961b84c9..3f7aa4dc7e1f6 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -225,6 +225,14 @@ extern struct cred init_cred;
 #define INIT_TASK_SECURITY
 #endif
 
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE) &&		\
+    defined(CONFIG_SCHED_DEBUG)
+# define INIT_LAZY_MIGRATE(tsk)						\
+	.pinned_on_cpu	= -1,
+#else
+# define INIT_LAZY_MIGRATE(tsk)
+#endif
+
 /*
  *  INIT_TASK is used to set up the first task table, touch at
  * your own risk!. Base=0, limit=0x1fffff (=2MB)
@@ -243,6 +251,7 @@ extern struct cred init_cred;
 	.cpus_ptr	= &tsk.cpus_mask,				\
 	.cpus_mask	= CPU_MASK_ALL,					\
 	.nr_cpus_allowed= NR_CPUS,					\
+	INIT_LAZY_MIGRATE(tsk)						\
 	.mm		= NULL,						\
 	.active_mm	= &init_mm,					\
 	.restart_block = {						\
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 205d8c0633733..b918b8a3248e0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -229,6 +229,8 @@ extern void io_schedule_finish(int token);
 extern long io_schedule_timeout(long timeout);
 extern void io_schedule(void);
 
+int cpu_nr_pinned(int cpu);
+
 /**
  * struct prev_cputime - snapshot of system and user cputime
  * @utime: time spent in user mode
@@ -628,16 +630,13 @@ struct task_struct {
 	cpumask_t			cpus_mask;
 #if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 	int				migrate_disable;
-	int				migrate_disable_update;
-	int				pinned_on_cpu;
+	bool				migrate_disable_scheduled;
 # ifdef CONFIG_SCHED_DEBUG
-	int				migrate_disable_atomic;
+	int				pinned_on_cpu;
 # endif
-
 #elif !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
 # ifdef CONFIG_SCHED_DEBUG
 	int				migrate_disable;
-	int				migrate_disable_atomic;
 # endif
 #endif
 #ifdef CONFIG_PREEMPT_RT_FULL
@@ -1882,4 +1881,6 @@ extern long sched_getaffinity(pid_t pid, struct cpumask *mask);
 #define TASK_SIZE_OF(tsk)	TASK_SIZE
 #endif
 
+extern struct task_struct *takedown_cpu_task;
+
 #endif
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 856ec89899605..85b231c91c8a6 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -77,11 +77,6 @@ static DEFINE_PER_CPU(struct cpuhp_cpu_state, cpuhp_state) = {
 	.fail = CPUHP_INVALID,
 };
 
-#if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PREEMPT_RT_FULL)
-static DEFINE_PER_CPU(struct rt_rw_lock, cpuhp_pin_lock) = \
-	__RWLOCK_RT_INITIALIZER(cpuhp_pin_lock);
-#endif
-
 #if defined(CONFIG_LOCKDEP) && defined(CONFIG_SMP)
 static struct lockdep_map cpuhp_state_up_map =
 	STATIC_LOCKDEP_MAP_INIT("cpuhp_state-up", &cpuhp_state_up_map);
@@ -295,57 +290,6 @@ static int cpu_hotplug_disabled;
 
 #ifdef CONFIG_HOTPLUG_CPU
 
-/**
- * pin_current_cpu - Prevent the current cpu from being unplugged
- */
-void pin_current_cpu(void)
-{
-#ifdef CONFIG_PREEMPT_RT_FULL
-	struct rt_rw_lock *cpuhp_pin;
-	unsigned int cpu;
-	int ret;
-
-again:
-	cpuhp_pin = this_cpu_ptr(&cpuhp_pin_lock);
-	ret = __read_rt_trylock(cpuhp_pin);
-	if (ret) {
-		current->pinned_on_cpu = smp_processor_id();
-		return;
-	}
-	cpu = smp_processor_id();
-	preempt_lazy_enable();
-	preempt_enable();
-
-	sleeping_lock_inc();
-	__read_rt_lock(cpuhp_pin);
-	sleeping_lock_dec();
-
-	preempt_disable();
-	preempt_lazy_disable();
-	if (cpu != smp_processor_id()) {
-		__read_rt_unlock(cpuhp_pin);
-		goto again;
-	}
-	current->pinned_on_cpu = cpu;
-#endif
-}
-
-/**
- * unpin_current_cpu - Allow unplug of current cpu
- */
-void unpin_current_cpu(void)
-{
-#ifdef CONFIG_PREEMPT_RT_FULL
-	struct rt_rw_lock *cpuhp_pin = this_cpu_ptr(&cpuhp_pin_lock);
-
-	if (WARN_ON(current->pinned_on_cpu != smp_processor_id()))
-		cpuhp_pin = per_cpu_ptr(&cpuhp_pin_lock, current->pinned_on_cpu);
-
-	current->pinned_on_cpu = -1;
-	__read_rt_unlock(cpuhp_pin);
-#endif
-}
-
 DEFINE_STATIC_PERCPU_RWSEM(cpu_hotplug_lock);
 
 void cpus_read_lock(void)
@@ -929,6 +873,15 @@ static int take_cpu_down(void *_param)
 	int err, cpu = smp_processor_id();
 	int ret;
 
+#ifdef CONFIG_PREEMPT_RT_BASE
+	/*
+	 * If any tasks disabled migration before we got here,
+	 * go back and sleep again.
+	 */
+	if (cpu_nr_pinned(cpu))
+		return -EAGAIN;
+#endif
+
 	/* Ensure this CPU doesn't handle any more interrupts. */
 	err = __cpu_disable();
 	if (err < 0)
@@ -956,11 +909,10 @@ static int take_cpu_down(void *_param)
 	return 0;
 }
 
+struct task_struct *takedown_cpu_task;
+
 static int takedown_cpu(unsigned int cpu)
 {
-#ifdef CONFIG_PREEMPT_RT_FULL
-	struct rt_rw_lock *cpuhp_pin = per_cpu_ptr(&cpuhp_pin_lock, cpu);
-#endif
 	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 	int err;
 
@@ -973,17 +925,38 @@ static int takedown_cpu(unsigned int cpu)
 	 */
 	irq_lock_sparse();
 
-#ifdef CONFIG_PREEMPT_RT_FULL
-	__write_rt_lock(cpuhp_pin);
+#ifdef CONFIG_PREEMPT_RT_BASE
+	WARN_ON_ONCE(takedown_cpu_task);
+	takedown_cpu_task = current;
+
+again:
+	/*
+	 * If a task pins this CPU after we pass this check, take_cpu_down
+	 * will return -EAGAIN.
+	 */
+	for (;;) {
+		int nr_pinned;
+
+		set_current_state(TASK_UNINTERRUPTIBLE);
+		nr_pinned = cpu_nr_pinned(cpu);
+		if (nr_pinned == 0)
+			break;
+		schedule();
+	}
+	set_current_state(TASK_RUNNING);
 #endif
 
 	/*
 	 * So now all preempt/rcu users must observe !cpu_active().
 	 */
 	err = stop_machine_cpuslocked(take_cpu_down, NULL, cpumask_of(cpu));
+#ifdef CONFIG_PREEMPT_RT_BASE
+	if (err == -EAGAIN)
+		goto again;
+#endif
 	if (err) {
-#ifdef CONFIG_PREEMPT_RT_FULL
-		__write_rt_unlock(cpuhp_pin);
+#ifdef CONFIG_PREEMPT_RT_BASE
+		takedown_cpu_task = NULL;
 #endif
 		/* CPU refused to die */
 		irq_unlock_sparse();
@@ -1003,8 +976,8 @@ static int takedown_cpu(unsigned int cpu)
 	wait_for_ap_thread(st, false);
 	BUG_ON(st->state != CPUHP_AP_IDLE_DEAD);
 
-#ifdef CONFIG_PREEMPT_RT_FULL
-	__write_rt_unlock(cpuhp_pin);
+#ifdef CONFIG_PREEMPT_RT_BASE
+	takedown_cpu_task = NULL;
 #endif
 	/* Interrupts are moved away from the dying cpu, reenable alloc/free */
 	irq_unlock_sparse();
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 757030bf86f8e..a1f749f4574e5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1103,7 +1103,8 @@ static int migration_cpu_stop(void *data)
 void set_cpus_allowed_common(struct task_struct *p, const struct cpumask *new_mask)
 {
 	cpumask_copy(&p->cpus_mask, new_mask);
-	p->nr_cpus_allowed = cpumask_weight(new_mask);
+	if (p->cpus_ptr == &p->cpus_mask)
+		p->nr_cpus_allowed = cpumask_weight(new_mask);
 }
 
 #if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
@@ -1114,8 +1115,7 @@ int __migrate_disabled(struct task_struct *p)
 EXPORT_SYMBOL_GPL(__migrate_disabled);
 #endif
 
-static void __do_set_cpus_allowed_tail(struct task_struct *p,
-				       const struct cpumask *new_mask)
+void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 {
 	struct rq *rq = task_rq(p);
 	bool queued, running;
@@ -1144,20 +1144,6 @@ static void __do_set_cpus_allowed_tail(struct task_struct *p,
 		set_curr_task(rq, p);
 }
 
-void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
-{
-#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
-	if (__migrate_disabled(p)) {
-		lockdep_assert_held(&p->pi_lock);
-
-		cpumask_copy(&p->cpus_mask, new_mask);
-		p->migrate_disable_update = 1;
-		return;
-	}
-#endif
-	__do_set_cpus_allowed_tail(p, new_mask);
-}
-
 /*
  * Change a given task's CPU affinity. Migrate the thread to a
  * proper CPU and schedule it away if the CPU it's executing on
@@ -1217,7 +1203,8 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
 	}
 
 	/* Can the task run on the task's current CPU? If so, we're done */
-	if (cpumask_test_cpu(task_cpu(p), new_mask) || __migrate_disabled(p))
+	if (cpumask_test_cpu(task_cpu(p), new_mask) ||
+	    p->cpus_ptr != &p->cpus_mask)
 		goto out;
 
 	if (task_running(rq, p) || p->state == TASK_WAKING) {
@@ -3330,6 +3317,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	BUG();
 }
 
+static void migrate_disabled_sched(struct task_struct *p);
+
 /*
  * __schedule() is the main scheduler function.
  *
@@ -3397,6 +3386,9 @@ static void __sched notrace __schedule(bool preempt)
 	rq_lock(rq, &rf);
 	smp_mb__after_spinlock();
 
+	if (__migrate_disabled(prev))
+		migrate_disabled_sched(prev);
+
 	/* Promote REQ to ACT */
 	rq->clock_update_flags <<= 1;
 	update_rq_clock(rq);
@@ -5620,6 +5612,8 @@ static void migrate_tasks(struct rq *dead_rq, struct rq_flags *rf)
 		BUG_ON(!next);
 		put_prev_task(rq, next);
 
+		WARN_ON_ONCE(__migrate_disabled(next));
+
 		/*
 		 * Rules for changing task_struct::cpus_mask are holding
 		 * both pi_lock and rq->lock, such that holding either
@@ -6922,14 +6916,9 @@ update_nr_migratory(struct task_struct *p, long delta)
 static inline void
 migrate_disable_update_cpus_allowed(struct task_struct *p)
 {
-	struct rq *rq;
-	struct rq_flags rf;
-
-	rq = task_rq_lock(p, &rf);
 	p->cpus_ptr = cpumask_of(smp_processor_id());
 	update_nr_migratory(p, -1);
 	p->nr_cpus_allowed = 1;
-	task_rq_unlock(rq, p, &rf);
 }
 
 static inline void
@@ -6947,54 +6936,35 @@ migrate_enable_update_cpus_allowed(struct task_struct *p)
 
 void migrate_disable(void)
 {
-	struct task_struct *p = current;
+	preempt_disable();
 
-	if (in_atomic() || irqs_disabled()) {
+	if (++current->migrate_disable == 1) {
+		this_rq()->nr_pinned++;
+		preempt_lazy_disable();
 #ifdef CONFIG_SCHED_DEBUG
-		p->migrate_disable_atomic++;
+		WARN_ON_ONCE(current->pinned_on_cpu >= 0);
+		current->pinned_on_cpu = smp_processor_id();
 #endif
-		return;
-	}
-#ifdef CONFIG_SCHED_DEBUG
-	if (unlikely(p->migrate_disable_atomic)) {
-		tracing_off();
-		WARN_ON_ONCE(1);
 	}
-#endif
 
-	if (p->migrate_disable) {
-		p->migrate_disable++;
-		return;
-	}
+	preempt_enable();
+}
+EXPORT_SYMBOL(migrate_disable);
 
-	preempt_disable();
-	preempt_lazy_disable();
-	pin_current_cpu();
+static void migrate_disabled_sched(struct task_struct *p)
+{
+	if (p->migrate_disable_scheduled)
+		return;
 
 	migrate_disable_update_cpus_allowed(p);
-	p->migrate_disable = 1;
-
-	preempt_enable();
+	p->migrate_disable_scheduled = 1;
 }
-EXPORT_SYMBOL(migrate_disable);
 
 void migrate_enable(void)
 {
 	struct task_struct *p = current;
-
-	if (in_atomic() || irqs_disabled()) {
-#ifdef CONFIG_SCHED_DEBUG
-		p->migrate_disable_atomic--;
-#endif
-		return;
-	}
-
-#ifdef CONFIG_SCHED_DEBUG
-	if (unlikely(p->migrate_disable_atomic)) {
-		tracing_off();
-		WARN_ON_ONCE(1);
-	}
-#endif
+	struct rq *rq = this_rq();
+	int cpu = task_cpu(p);
 
 	WARN_ON_ONCE(p->migrate_disable <= 0);
 	if (p->migrate_disable > 1) {
@@ -7004,67 +6974,69 @@ void migrate_enable(void)
 
 	preempt_disable();
 
+#ifdef CONFIG_SCHED_DEBUG
+	WARN_ON_ONCE(current->pinned_on_cpu != cpu);
+	current->pinned_on_cpu = -1;
+#endif
+
+	WARN_ON_ONCE(rq->nr_pinned < 1);
+
 	p->migrate_disable = 0;
+	rq->nr_pinned--;
+	if (rq->nr_pinned == 0 && unlikely(!cpu_active(cpu)) &&
+	    takedown_cpu_task)
+		wake_up_process(takedown_cpu_task);
+
+	if (!p->migrate_disable_scheduled)
+		goto out;
+
+	p->migrate_disable_scheduled = 0;
+
 	migrate_enable_update_cpus_allowed(p);
 
-	if (p->migrate_disable_update) {
-		struct rq *rq;
+	WARN_ON(smp_processor_id() != cpu);
+	if (!is_cpu_allowed(p, cpu)) {
+		struct migration_arg arg = { p };
 		struct rq_flags rf;
-		int cpu = task_cpu(p);
 
 		rq = task_rq_lock(p, &rf);
 		update_rq_clock(rq);
-
-		__do_set_cpus_allowed_tail(p, &p->cpus_mask);
+		arg.dest_cpu = select_fallback_rq(cpu, p);
 		task_rq_unlock(rq, p, &rf);
 
-		p->migrate_disable_update = 0;
-
-		WARN_ON(smp_processor_id() != cpu);
-		if (!cpumask_test_cpu(cpu, &p->cpus_mask)) {
-			struct migration_arg arg = { p };
-			struct rq_flags rf;
+		preempt_lazy_enable();
+		preempt_enable();
 
-			rq = task_rq_lock(p, &rf);
-			update_rq_clock(rq);
-			arg.dest_cpu = select_fallback_rq(cpu, p);
-			task_rq_unlock(rq, p, &rf);
-
-			unpin_current_cpu();
-			preempt_lazy_enable();
-			preempt_enable();
-
-			sleeping_lock_inc();
-			stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
-			sleeping_lock_dec();
-			tlb_migrate_finish(p->mm);
+		sleeping_lock_inc();
+		stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
+		sleeping_lock_dec();
+		tlb_migrate_finish(p->mm);
 
-			return;
-		}
+		return;
 	}
-	unpin_current_cpu();
+
+out:
 	preempt_lazy_enable();
 	preempt_enable();
 }
 EXPORT_SYMBOL(migrate_enable);
 
-#elif !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
-void migrate_disable(void)
+int cpu_nr_pinned(int cpu)
 {
-#ifdef CONFIG_SCHED_DEBUG
-	struct task_struct *p = current;
+	struct rq *rq = cpu_rq(cpu);
 
-	if (in_atomic() || irqs_disabled()) {
-		p->migrate_disable_atomic++;
-		return;
-	}
+	return rq->nr_pinned;
+}
 
-	if (unlikely(p->migrate_disable_atomic)) {
-		tracing_off();
-		WARN_ON_ONCE(1);
-	}
+#elif !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
+static void migrate_disabled_sched(struct task_struct *p)
+{
+}
 
-	p->migrate_disable++;
+void migrate_disable(void)
+{
+#ifdef CONFIG_SCHED_DEBUG
+	current->migrate_disable++;
 #endif
 	barrier();
 }
@@ -7075,20 +7047,14 @@ void migrate_enable(void)
 #ifdef CONFIG_SCHED_DEBUG
 	struct task_struct *p = current;
 
-	if (in_atomic() || irqs_disabled()) {
-		p->migrate_disable_atomic--;
-		return;
-	}
-
-	if (unlikely(p->migrate_disable_atomic)) {
-		tracing_off();
-		WARN_ON_ONCE(1);
-	}
-
 	WARN_ON_ONCE(p->migrate_disable <= 0);
 	p->migrate_disable--;
 #endif
 	barrier();
 }
 EXPORT_SYMBOL(migrate_enable);
+#else
+static void migrate_disabled_sched(struct task_struct *p)
+{
+}
 #endif
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index fdfd06872bd76..0a90469bd5a4a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -811,6 +811,10 @@ struct rq {
 	/* Must be inspected within a rcu lock section */
 	struct cpuidle_state *idle_state;
 #endif
+
+#if defined(CONFIG_PREEMPT_RT_BASE) && defined(CONFIG_SMP)
+	int			nr_pinned;
+#endif
 };
 
 static inline int cpu_of(struct rq *rq)
diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
index 9f3c8bb62e576..7a0c19c282cc0 100644
--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c
@@ -23,6 +23,9 @@ notrace static unsigned int check_preemption_disabled(const char *what1,
 	 * Kernel threads bound to a single CPU can safely use
 	 * smp_processor_id():
 	 */
+	if (current->migrate_disable)
+		goto out;
+
 	if (current->nr_cpus_allowed == 1)
 		goto out;
 
-- 
2.41.0

