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

[Peter Zijlstra]

On Tue, Sep 24, 2013 at 07:42:36AM -0700, Paul E. McKenney wrote:
> > +#define cpuhp_writer_wake()						\
> > +	wake_up_process(cpuhp_writer_task)
> > +
> > +#define cpuhp_writer_wait(cond)						\
> > +do {									\
> > +	for (;;) {							\
> > +		set_current_state(TASK_UNINTERRUPTIBLE);		\
> > +		if (cond)						\
> > +			break;						\
> > +		schedule();						\
> > +	}								\
> > +	__set_current_state(TASK_RUNNING);				\
> > +} while (0)
> 
> Why not wait_event()?  Presumably the above is a bit lighter weight,
> but is that even something that can be measured?

I didn't want to mix readers and writers on cpuhp_wq, and I suppose I
could create a second waitqueue; that might also be a better solution
for the NULL thing below.

> > +	atomic_inc(&cpuhp_waitcount);
> > +
> > +	/*
> > +	 * We either call schedule() in the wait, or we'll fall through
> > +	 * and reschedule on the preempt_enable() in get_online_cpus().
> > +	 */
> > +	preempt_enable_no_resched();
> > +	wait_event(cpuhp_wq, !__cpuhp_writer);
> 
> Finally!  A good use for preempt_enable_no_resched().  ;-)

Hehe, there were a few others, but tglx removed most with the
schedule_preempt_disabled() primitive.

In fact, I considered a wait_event_preempt_disabled() but was too lazy.
That whole wait_event macro fest looks like it could use an iteration or
two of collapse anyhow.

> > +	preempt_disable();
> > +
> > +	/*
> > +	 * It would be possible for cpu_hotplug_done() to complete before
> > +	 * the atomic_inc() above; in which case there is no writer waiting
> > +	 * and doing a wakeup would be BAD (tm).
> > +	 *
> > +	 * If however we still observe cpuhp_writer_task here we know
> > +	 * cpu_hotplug_done() is currently stuck waiting for cpuhp_waitcount.
> > +	 */
> > +	if (atomic_dec_and_test(&cpuhp_waitcount) && cpuhp_writer_task)
> 
> OK, I'll bite...  What sequence of events results in the
> atomic_dec_and_test() returning true but there being no
> cpuhp_writer_task?
> 
> Ah, I see it...

<snip>

Indeed, and

> But what prevents the following sequence of events?

<snip>

> o	Task B's call to cpuhp_writer_wake() sees a NULL pointer.

Quite so.. nothing. See there was a reason I kept being confused about
it.

> >  void cpu_hotplug_begin(void)
> >  {
> > +	unsigned int count = 0;
> > +	int cpu;
> > +
> > +	lockdep_assert_held(&cpu_add_remove_lock);
> > 
> > +	__cpuhp_writer = 1;
> > +	cpuhp_writer_task = current;
> 
> At this point, the value of cpuhp_slowcount can go negative.  Can't see
> that this causes a problem, given the atomic_add() below.

Agreed.

> > +
> > +	/* After this everybody will observe writer and take the slow path. */
> > +	synchronize_sched();
> > +
> > +	/* Collapse the per-cpu refcount into slowcount */
> > +	for_each_possible_cpu(cpu) {
> > +		count += per_cpu(__cpuhp_refcount, cpu);
> > +		per_cpu(__cpuhp_refcount, cpu) = 0;
> >  	}
> 
> The above is safe because the readers are no longer changing their
> __cpuhp_refcount values.

Yes, I'll expand the comment.

So how about something like this?

---
--- 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;
 
@@ -173,10 +174,50 @@ extern struct bus_type cpu_subsys;
 #ifdef CONFIG_HOTPLUG_CPU
 /* Stop CPUs going up and down. */
 
+extern void cpu_hotplug_init_task(struct task_struct *p);
+
 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();
+
+	/* Support reader-in-reader recursion */
+	if (current->cpuhp_ref++) {
+		barrier();
+		return;
+	}
+
+	preempt_disable();
+	if (likely(!__cpuhp_writer))
+		__this_cpu_inc(__cpuhp_refcount);
+	else
+		__get_online_cpus();
+	preempt_enable();
+}
+
+extern void __put_online_cpus(void);
+
+static inline void put_online_cpus(void)
+{
+	barrier();
+	if (--current->cpuhp_ref)
+		return;
+
+	preempt_disable();
+	if (likely(!__cpuhp_writer))
+		__this_cpu_dec(__cpuhp_refcount);
+	else
+		__put_online_cpus();
+	preempt_enable();
+}
+
 extern void cpu_hotplug_disable(void);
 extern void cpu_hotplug_enable(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
@@ -200,6 +241,8 @@ static inline void cpu_hotplug_driver_un
 
 #else		/* CONFIG_HOTPLUG_CPU */
 
+static inline void cpu_hotplug_init_task(struct task_struct *p) {}
+
 static inline void cpu_hotplug_begin(void) {}
 static inline void cpu_hotplug_done(void) {}
 #define get_online_cpus()	do { } while (0)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1454,6 +1454,9 @@ struct task_struct {
 	unsigned int	sequential_io;
 	unsigned int	sequential_io_avg;
 #endif
+#ifdef CONFIG_HOTPLUG_CPU
+	int		cpuhp_ref;
+#endif
 };
 
 /* Future-safe accessor for struct task_struct's cpus_allowed. */
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -49,88 +49,100 @@ 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;
+EXPORT_SYMBOL_GPL(__cpuhp_writer);
 
-void get_online_cpus(void)
-{
-	might_sleep();
-	if (cpu_hotplug.active_writer == current)
-		return;
-	mutex_lock(&cpu_hotplug.lock);
-	cpu_hotplug.refcount++;
-	mutex_unlock(&cpu_hotplug.lock);
+DEFINE_PER_CPU(unsigned int, __cpuhp_refcount);
+EXPORT_PER_CPU_SYMBOL_GPL(__cpuhp_refcount);
+
+static atomic_t cpuhp_waitcount;
+static atomic_t cpuhp_slowcount;
+static DECLARE_WAIT_QUEUE_HEAD(cpuhp_readers);
+static DECLARE_WAIT_QUEUE_HEAD(cpuhp_writer);
 
+void cpu_hotplug_init_task(struct task_struct *p)
+{
+	p->cpuhp_ref = 0;
 }
-EXPORT_SYMBOL_GPL(get_online_cpus);
 
-void put_online_cpus(void)
+void __get_online_cpus(void)
 {
-	if (cpu_hotplug.active_writer == current)
+	/* Support reader-in-writer recursion */
+	if (__cpuhp_writer == current)
 		return;
-	mutex_lock(&cpu_hotplug.lock);
 
-	if (WARN_ON(!cpu_hotplug.refcount))
-		cpu_hotplug.refcount++; /* try to fix things up */
+	atomic_inc(&cpuhp_waitcount);
 
-	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
-		wake_up_process(cpu_hotplug.active_writer);
-	mutex_unlock(&cpu_hotplug.lock);
+	/*
+	 * We either call schedule() in the wait, or we'll fall through
+	 * and reschedule on the preempt_enable() in get_online_cpus().
+	 */
+	preempt_enable_no_resched();
+	wait_event(cpuhp_readers, !__cpuhp_writer);
+	preempt_disable();
+
+	if (atomic_dec_and_test(&cpuhp_waitcount))
+		wake_up_all(&cpuhp_writer);
+}
+EXPORT_SYMBOL_GPL(__get_online_cpus);
+
+void __put_online_cpus(void)
+{
+	if (__cpuhp_writer == current)
+		return;
 
+	if (atomic_dec_and_test(&cpuhp_slowcount))
+		wake_up_all(&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;
+	unsigned int count = 0;
+	int cpu;
+
+	lockdep_assert_held(&cpu_add_remove_lock);
 
-	for (;;) {
-		mutex_lock(&cpu_hotplug.lock);
-		if (likely(!cpu_hotplug.refcount))
-			break;
-		__set_current_state(TASK_UNINTERRUPTIBLE);
-		mutex_unlock(&cpu_hotplug.lock);
-		schedule();
+	__cpuhp_writer = current;
+
+	/* 
+	 * After this everybody will observe writer and take the slow path.
+	 */
+	synchronize_sched();
+
+	/* 
+	 * Collapse the per-cpu refcount into slowcount. This is safe because
+	 * readers are now taking the slow path (per the above) which doesn't
+	 * touch __cpuhp_refcount.
+	 */
+	for_each_possible_cpu(cpu) {
+		count += per_cpu(__cpuhp_refcount, cpu);
+		per_cpu(__cpuhp_refcount, cpu) = 0;
 	}
+	atomic_add(count, &cpuhp_slowcount);
+
+	/* Wait for all readers to go away */
+	wait_event(cpuhp_writer, !atomic_read(&cpuhp_slowcount));
 }
 
 void cpu_hotplug_done(void)
 {
-	cpu_hotplug.active_writer = NULL;
-	mutex_unlock(&cpu_hotplug.lock);
+	/* Signal the writer is done */
+	cpuhp_writer = NULL;
+	wake_up_all(&cpuhp_readers);
+
+	/* 
+	 * Wait for any pending readers to be running. This ensures readers
+	 * after writer and avoids writers starving readers.
+	 */
+	wait_event(cpuhp_writer, !atomic_read(&cpuhp_waitcount));
 }
 
 /*
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1736,6 +1736,8 @@ static void __sched_fork(unsigned long c
 	INIT_LIST_HEAD(&p->numa_entry);
 	p->numa_group = NULL;
 #endif /* CONFIG_NUMA_BALANCING */
+
+	cpu_hotplug_init_task(p);
 }
 
 #ifdef CONFIG_NUMA_BALANCING
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