[PATCH] hotplug: Optimize {get,put}_online_cpus()

[Peter Zijlstra]

Subject: hotplug: Optimize {get,put}_online_cpus()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Tue Sep 17 16:17:11 CEST 2013

The cpu hotplug lock is a purely reader biased read-write lock.

The current implementation uses global state, change it so the reader
side uses per-cpu state in the uncontended fast-path.

Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 include/linux/cpu.h |   33 ++++++++++++++-
 kernel/cpu.c        |  108 ++++++++++++++++++++++++++--------------------------
 2 files changed, 87 insertions(+), 54 deletions(-)

--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -16,6 +16,7 @@
 #include <linux/node.h>
 #include <linux/compiler.h>
 #include <linux/cpumask.h>
+#include <linux/percpu.h>
 
 struct device;
 
@@ -175,8 +176,36 @@ extern struct bus_type cpu_subsys;
 
 extern void cpu_hotplug_begin(void);
 extern void cpu_hotplug_done(void);
-extern void get_online_cpus(void);
-extern void put_online_cpus(void);
+
+extern struct task_struct *__cpuhp_writer;
+DECLARE_PER_CPU(unsigned int, __cpuhp_refcount);
+
+extern void __get_online_cpus(void);
+
+static inline void get_online_cpus(void)
+{
+	might_sleep();
+
+	this_cpu_inc(__cpuhp_refcount);
+	/*
+	 * Order the refcount inc against the writer read; pairs with the full
+	 * barrier in cpu_hotplug_begin().
+	 */
+	smp_mb();
+	if (unlikely(__cpuhp_writer))
+		__get_online_cpus();
+}
+
+extern void __put_online_cpus(void);
+
+static inline void put_online_cpus(void)
+{
+	barrier();
+	this_cpu_dec(__cpuhp_refcount);
+	if (unlikely(__cpuhp_writer))
+		__put_online_cpus();
+}
+
 extern void cpu_hotplug_disable(void);
 extern void cpu_hotplug_enable(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -49,88 +49,92 @@ static int cpu_hotplug_disabled;
 
 #ifdef CONFIG_HOTPLUG_CPU
 
-static struct {
-	struct task_struct *active_writer;
-	struct mutex lock; /* Synchronizes accesses to refcount, */
-	/*
-	 * Also blocks the new readers during
-	 * an ongoing cpu hotplug operation.
-	 */
-	int refcount;
-} cpu_hotplug = {
-	.active_writer = NULL,
-	.lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
-	.refcount = 0,
-};
+struct task_struct *__cpuhp_writer = NULL;
+EXPORT_SYMBOL_GPL(__cpuhp_writer);
+
+DEFINE_PER_CPU(unsigned int, __cpuhp_refcount);
+EXPORT_PER_CPU_SYMBOL_GPL(__cpuhp_refcount);
 
-void get_online_cpus(void)
+static DECLARE_WAIT_QUEUE_HEAD(cpuhp_wq);
+
+void __get_online_cpus(void)
 {
-	might_sleep();
-	if (cpu_hotplug.active_writer == current)
+	if (__cpuhp_writer == current)
 		return;
-	mutex_lock(&cpu_hotplug.lock);
-	cpu_hotplug.refcount++;
-	mutex_unlock(&cpu_hotplug.lock);
 
+again:
+	/*
+	 * Ensure a pending reading has a 0 refcount.
+	 *
+	 * Without this a new reader that comes in before cpu_hotplug_begin()
+	 * reads the refcount will deadlock.
+	 */
+	this_cpu_dec(__cpuhp_refcount);
+	wait_event(cpuhp_wq, !__cpuhp_writer);
+
+	this_cpu_inc(__cpuhp_refcount);
+	/*
+	 * See get_online_cpu().
+	 */
+	smp_mb();
+	if (unlikely(__cpuhp_writer))
+		goto again;
 }
-EXPORT_SYMBOL_GPL(get_online_cpus);
+EXPORT_SYMBOL_GPL(__get_online_cpus);
 
-void put_online_cpus(void)
+void __put_online_cpus(void)
 {
-	if (cpu_hotplug.active_writer == current)
-		return;
-	mutex_lock(&cpu_hotplug.lock);
+	unsigned int refcnt = 0;
+	int cpu;
 
-	if (WARN_ON(!cpu_hotplug.refcount))
-		cpu_hotplug.refcount++; /* try to fix things up */
+	if (__cpuhp_writer == current)
+		return;
 
-	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
-		wake_up_process(cpu_hotplug.active_writer);
-	mutex_unlock(&cpu_hotplug.lock);
+	for_each_possible_cpu(cpu)
+		refcnt += per_cpu(__cpuhp_refcount, cpu);
 
+	if (!refcnt)
+		wake_up_process(__cpuhp_writer);
 }
-EXPORT_SYMBOL_GPL(put_online_cpus);
+EXPORT_SYMBOL_GPL(__put_online_cpus);
 
 /*
  * This ensures that the hotplug operation can begin only when the
  * refcount goes to zero.
  *
- * Note that during a cpu-hotplug operation, the new readers, if any,
- * will be blocked by the cpu_hotplug.lock
- *
  * Since cpu_hotplug_begin() is always called after invoking
  * cpu_maps_update_begin(), we can be sure that only one writer is active.
- *
- * Note that theoretically, there is a possibility of a livelock:
- * - Refcount goes to zero, last reader wakes up the sleeping
- *   writer.
- * - Last reader unlocks the cpu_hotplug.lock.
- * - A new reader arrives at this moment, bumps up the refcount.
- * - The writer acquires the cpu_hotplug.lock finds the refcount
- *   non zero and goes to sleep again.
- *
- * However, this is very difficult to achieve in practice since
- * get_online_cpus() not an api which is called all that often.
- *
  */
 void cpu_hotplug_begin(void)
 {
-	cpu_hotplug.active_writer = current;
+	__cpuhp_writer = current;
 
 	for (;;) {
-		mutex_lock(&cpu_hotplug.lock);
-		if (likely(!cpu_hotplug.refcount))
+		unsigned int refcnt = 0;
+		int cpu;
+
+		/*
+		 * Order the setting of writer against the reading of refcount;
+		 * pairs with the full barrier in get_online_cpus().
+		 */
+
+		set_current_state(TASK_UNINTERRUPTIBLE);
+
+		for_each_possible_cpu(cpu)
+			refcnt += per_cpu(__cpuhp_refcount, cpu);
+
+		if (!refcnt)
 			break;
-		__set_current_state(TASK_UNINTERRUPTIBLE);
-		mutex_unlock(&cpu_hotplug.lock);
+
 		schedule();
 	}
+	__set_current_state(TASK_RUNNING);
 }
 
 void cpu_hotplug_done(void)
 {
-	cpu_hotplug.active_writer = NULL;
-	mutex_unlock(&cpu_hotplug.lock);
+	__cpuhp_writer = NULL;
+	wake_up_all(&cpuhp_wq);
 }
 
 /*
--
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