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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
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 | 7 +--
kernel/cpu.c | 118 ++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 122 insertions(+), 3 deletions(-)
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -192,9 +192,6 @@ static inline void cpu_notifier_register
#endif /* CONFIG_SMP */
extern struct bus_type cpu_subsys;
-static inline void pin_current_cpu(void) { }
-static inline void unpin_current_cpu(void) { }
-
#ifdef CONFIG_HOTPLUG_CPU
/* Stop CPUs going up and down. */
@@ -204,6 +201,8 @@ extern void get_online_cpus(void);
extern void put_online_cpus(void);
extern void cpu_hotplug_disable(void);
extern void cpu_hotplug_enable(void);
+extern void pin_current_cpu(void);
+extern void unpin_current_cpu(void);
#define hotcpu_notifier(fn, pri) cpu_notifier(fn, pri)
#define __hotcpu_notifier(fn, pri) __cpu_notifier(fn, pri)
#define register_hotcpu_notifier(nb) register_cpu_notifier(nb)
@@ -221,6 +220,8 @@ static inline void cpu_hotplug_done(void
#define put_online_cpus() do { } while (0)
#define cpu_hotplug_disable() do { } while (0)
#define cpu_hotplug_enable() do { } while (0)
+static inline void pin_current_cpu(void) { }
+static inline void unpin_current_cpu(void) { }
#define hotcpu_notifier(fn, pri) do { (void)(fn); } while (0)
#define __hotcpu_notifier(fn, pri) do { (void)(fn); } while (0)
/* These aren't inline functions due to a GCC bug. */
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -166,6 +166,100 @@ static struct {
#define cpuhp_lock_acquire() lock_map_acquire(&cpu_hotplug.dep_map)
#define cpuhp_lock_release() lock_map_release(&cpu_hotplug.dep_map)
+struct hotplug_pcp {
+ struct task_struct *unplug;
+ int refcount;
+ struct completion synced;
+};
+
+static DEFINE_PER_CPU(struct hotplug_pcp, hotplug_pcp);
+
+/**
+ * pin_current_cpu - Prevent the current cpu from being unplugged
+ *
+ * Lightweight version of get_online_cpus() to prevent cpu from being
+ * unplugged when code runs in a migration disabled region.
+ *
+ * Must be called with preemption disabled (preempt_count = 1)!
+ */
+void pin_current_cpu(void)
+{
+ struct hotplug_pcp *hp = this_cpu_ptr(&hotplug_pcp);
+
+retry:
+ if (!hp->unplug || hp->refcount || preempt_count() > 1 ||
+ hp->unplug == current) {
+ hp->refcount++;
+ return;
+ }
+ preempt_enable();
+ mutex_lock(&cpu_hotplug.lock);
+ mutex_unlock(&cpu_hotplug.lock);
+ preempt_disable();
+ goto retry;
+}
+
+/**
+ * unpin_current_cpu - Allow unplug of current cpu
+ *
+ * Must be called with preemption or interrupts disabled!
+ */
+void unpin_current_cpu(void)
+{
+ struct hotplug_pcp *hp = this_cpu_ptr(&hotplug_pcp);
+
+ WARN_ON(hp->refcount <= 0);
+
+ /* This is safe. sync_unplug_thread is pinned to this cpu */
+ if (!--hp->refcount && hp->unplug && hp->unplug != current)
+ wake_up_process(hp->unplug);
+}
+
+/*
+ * FIXME: Is this really correct under all circumstances ?
+ */
+static int sync_unplug_thread(void *data)
+{
+ struct hotplug_pcp *hp = data;
+
+ preempt_disable();
+ hp->unplug = current;
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ while (hp->refcount) {
+ schedule_preempt_disabled();
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ }
+ set_current_state(TASK_RUNNING);
+ preempt_enable();
+ complete(&hp->synced);
+ return 0;
+}
+
+/*
+ * Start the sync_unplug_thread on the target cpu and wait for it to
+ * complete.
+ */
+static int cpu_unplug_begin(unsigned int cpu)
+{
+ struct hotplug_pcp *hp = &per_cpu(hotplug_pcp, cpu);
+ struct task_struct *tsk;
+
+ init_completion(&hp->synced);
+ tsk = kthread_create(sync_unplug_thread, hp, "sync_unplug/%d\n", cpu);
+ if (IS_ERR(tsk))
+ return (PTR_ERR(tsk));
+ kthread_bind(tsk, cpu);
+ wake_up_process(tsk);
+ wait_for_completion(&hp->synced);
+ return 0;
+}
+
+static void cpu_unplug_done(unsigned int cpu)
+{
+ struct hotplug_pcp *hp = &per_cpu(hotplug_pcp, cpu);
+
+ hp->unplug = NULL;
+}
void get_online_cpus(void)
{
@@ -799,6 +893,8 @@ static int __ref _cpu_down(unsigned int
struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
int prev_state, ret = 0;
bool hasdied = false;
+ int mycpu;
+ cpumask_var_t cpumask;
if (num_online_cpus() == 1)
return -EBUSY;
@@ -806,7 +902,27 @@ static int __ref _cpu_down(unsigned int
if (!cpu_present(cpu))
return -EINVAL;
+ /* Move the downtaker off the unplug cpu */
+ if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
+ return -ENOMEM;
+ cpumask_andnot(cpumask, cpu_online_mask, cpumask_of(cpu));
+ set_cpus_allowed_ptr(current, cpumask);
+ free_cpumask_var(cpumask);
+ preempt_disable();
+ mycpu = smp_processor_id();
+ if (mycpu == cpu) {
+ printk(KERN_ERR "Yuck! Still on unplug CPU\n!");
+ preempt_enable();
+ return -EBUSY;
+ }
+ preempt_enable();
+
cpu_hotplug_begin();
+ ret = cpu_unplug_begin(cpu);
+ if (ret) {
+ printk("cpu_unplug_begin(%d) failed\n", cpu);
+ goto out_cancel;
+ }
cpuhp_tasks_frozen = tasks_frozen;
@@ -845,6 +961,8 @@ static int __ref _cpu_down(unsigned int
hasdied = prev_state != st->state && st->state == CPUHP_OFFLINE;
out:
+ cpu_unplug_done(cpu);
+out_cancel:
cpu_hotplug_done();
/* This post dead nonsense must die */
if (!ret && hasdied)
|
