From 760fbae52ee672d35a40f60ad9b510f1f7ef686d Mon Sep 17 00:00:00 2001
Message-Id: <760fbae52ee672d35a40f60ad9b510f1f7ef686d.1592846147.git.zanussi@kernel.org>
In-Reply-To: <07cd0dbc80b976663c80755496a03f288decfe5a.1592846146.git.zanussi@kernel.org>
References: <07cd0dbc80b976663c80755496a03f288decfe5a.1592846146.git.zanussi@kernel.org>
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 15 Jun 2011 12:36:06 +0200
Subject: [PATCH 115/330] hotplug: Lightweight get online cpus
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.127-rt55.tar.xz

get_online_cpus() is a heavy weight function which involves a global
mutex. migrate_disable() wants a simpler construct which prevents only
a CPU from going doing while a task is in a migrate disabled section.

Implement a per cpu lockless mechanism, which serializes only in the
real unplug case on a global mutex. That serialization affects only
tasks on the cpu which should be brought down.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/cpu.h |  5 +++++
 kernel/cpu.c        | 15 +++++++++++++++
 kernel/sched/core.c |  4 ++++
 3 files changed, 24 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index aab4273810e3..e67645924404 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -118,6 +118,8 @@ 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 */
 
@@ -129,6 +131,9 @@ static inline int  cpus_read_trylock(void) { return true; }
 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 */
 
 /* Wrappers which go away once all code is converted */
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6d6c106a495c..dc5d1e940e25 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -281,6 +281,21 @@ static int cpu_hotplug_disabled;
 
 #ifdef CONFIG_HOTPLUG_CPU
 
+/**
+ * pin_current_cpu - Prevent the current cpu from being unplugged
+ */
+void pin_current_cpu(void)
+{
+
+}
+
+/**
+ * unpin_current_cpu - Allow unplug of current cpu
+ */
+void unpin_current_cpu(void)
+{
+}
+
 DEFINE_STATIC_PERCPU_RWSEM(cpu_hotplug_lock);
 
 void cpus_read_lock(void)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 57617777c4ba..42b42ebf52bc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7249,6 +7249,7 @@ void migrate_disable(void)
 	}
 
 	preempt_disable();
+	pin_current_cpu();
 
 	migrate_disable_update_cpus_allowed(p);
 	p->migrate_disable = 1;
@@ -7314,12 +7315,15 @@ void migrate_enable(void)
 			arg.task = p;
 			arg.dest_cpu = dest_cpu;
 
+			unpin_current_cpu();
 			preempt_enable();
 			stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
 			tlb_migrate_finish(p->mm);
+
 			return;
 		}
 	}
+	unpin_current_cpu();
 	preempt_enable();
 }
 EXPORT_SYMBOL(migrate_enable);
-- 
2.17.1

