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Subject: hotplug: Lightweight get online cpus
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 15 Jun 2011 12:36:06 +0200
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.18/older/patches-4.18.7-rt5.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(+)
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -110,6 +110,8 @@ 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 */
@@ -120,6 +122,9 @@ 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 */
/* Wrappers which go away once all code is converted */
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -283,6 +283,21 @@ 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)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7219,6 +7219,7 @@ void migrate_disable(void)
}
preempt_disable();
+ pin_current_cpu();
migrate_disable_update_cpus_allowed(p);
p->migrate_disable = 1;
@@ -7284,12 +7285,15 @@ 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);
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