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cpuhotplug: make get_online_cpus() scalability by using percpu counter

PATCH In review Lai Jiangshan 3 participants 12 messages Apr 5, 2010 – Apr 13, 2010

Subsystems (4)
MM core Scheduler exec and binfmt CPU hotplug
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Lai Jiangshan

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#0 lore raw

PATCH 2/2 cpuhotplug: make get_online_cpus() scalability by using percpu counter

Current get_online_cpus() acquires a mutex lock and then release it. It is not scale and it hurts cache. This patch rewrite it.

1) get_online_cpus() must be allowed to be called recursively, so I added get_online_cpus_nest for every task for new code.

This patch just allows get_online_cpus() to be called recursively, but when it is not nested, get_online_cpus() will wait until cpuhotplug finished, so the potential starvation is avoided.

And, the livelock of cpu_hotplug_begin() is avoided, so the comment is removed.

2) This new code use PER_CPU counters, and this counters protected by RCU. These counters acts like the reference counters of a modules. (Actually, all these code is stolen from module.c: try_refcount_get() is stolen from try_module_get(), put_online_cpus() from module_put()...)

After this patch applied, get_online_cpus() is very light and scale when cpuhotplug is not running. It just disables preemption and increase the cpu counter and then enables preemption.

3) Since we have try_refcount_get(), I add a new API try_get_online_cpus().

Signed-off-by Lai Jiangshan <[email protected]>
---
 include/linux/cpu.h   |    2
 include/linux/sched.h |    3 +
 kernel/cpu.c          |  131 ++++++++++++++++++++++++++++++++------------------
 kernel/fork.c         |    3 +
 4 files changed, 94 insertions(+), 45 deletions(-)
include/linux/cpu.h +2 -0
index e287863..a32809c 100644
@@ -112,6 +112,7 @@ extern struct sysdev_class cpu_sysdev_class;
 
 extern void get_online_cpus(void);
 extern void put_online_cpus(void);
+extern int try_get_online_cpus(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)

@@ -134,6 +135,7 @@ static inline void cpu_hotplug_driver_unlock(void)
 
 #define get_online_cpus()	do { } while (0)
 #define put_online_cpus()	do { } while (0)
+#define try_get_online_cpus()	(1)
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
include/linux/sched.h +3 -0
index c46b6e5..0422ea3 100644
@@ -1501,6 +1501,9 @@ struct task_struct {
 		unsigned long memsw_bytes; /* uncharged mem+swap usage */
 	} memcg_batch;
 #endif
+#ifdef CONFIG_HOTPLUG_CPU
+	int get_online_cpus_nest;
+#endif
 };
 
 /* Future-safe accessor for struct task_struct's cpus_allowed. */
kernel/cpu.c +86 -45

174 lines not on this page · raw message

kernel/fork.c +3 -0
index d67f1db..b162014 100644
@@ -1109,6 +1109,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	p->memcg_batch.memcg = NULL;
 #endif
 	p->stack_start = stack_start;
+#ifdef CONFIG_HOTPLUG_CPU
+	p->get_online_cpus_nest = 0;
+#endif
 
 	/* Perform scheduler related setup. Assign this task to a CPU. */
 	sched_fork(p, clone_flags);
#1 lore raw
Quoted 3 lines · Lai Jiangshan

1) get_online_cpus() must be allowed to be called recursively, so I added get_online_cpus_nest for every task for new code.

Well, iirc one of the goals of

	cpu-hotplug: replace lock_cpu_hotplug() with get_online_cpus()
	86ef5c9a8edd78e6bf92879f32329d89b2d55b5a

was avoiding the new members in task_struct. I leave this up to you and Gautham.

Lai, I didn't read this patch carefully yet (and I can't apply it to Linus's tree). But at first glance,

kernel/cpu.c static int cpu_hotplug_disabled; · line 119

Quoted 8 lines

void put_online_cpus(void)
{
...

kernel/cpu.c +5 -0
+	if (!--current->get_online_cpus_nest) {
+		preempt_disable();
+		__get_cpu_var(refcount)--;
+		if (cpu_hotplug_task)
+			wake_up_process(cpu_hotplug_task);

This looks unsafe. In theory nothing protects cpu_hotplug_task from exiting if refcount_sum() becomes zero, this means wake_up_process() can hit the freed/reused/unmapped task_struct. Probably cpu_hotplug_done() needs another synhronize_sched() before return.

OTOH, I do not understand why the result of __get_cpu_var(refcount) must be visible to refcount_sum() if we race with cpu_hotplug_begin(), so it seems to me cpu_hotplug_begin() also needs synchronize_sched() before refcount_sum().

Oleg.

#2 lore raw
Quoted 11 lines · Oleg Nesterov
Quoted 3 lines · Lai Jiangshan

1) get_online_cpus() must be allowed to be called recursively, so I added get_online_cpus_nest for every task for new code.

Well, iirc one of the goals of

	cpu-hotplug: replace lock_cpu_hotplug() with get_online_cpus()
	86ef5c9a8edd78e6bf92879f32329d89b2d55b5a
Quoted 6 lines

was avoiding the new members in task_struct. I leave this up to you and Gautham.

Lai, I didn't read this patch carefully yet (and I can't apply it to Linus's tree). But at first glance,

because I tried to apply it without 1/2 ;)

Quoted 5 lines
Quoted 5 lines

void put_online_cpus(void)
{
...

kernel/cpu.c +2 -0
+	if (!--current->get_online_cpus_nest) {
+		preempt_disable();
Quoted 8 lines

kernel/cpu.c static int cpu_hotplug_disabled; · line 119

Quoted 3 lines
kernel/cpu.c +3 -0
+		__get_cpu_var(refcount)--;
+		if (cpu_hotplug_task)
+			wake_up_process(cpu_hotplug_task);

This looks unsafe. In theory nothing protects cpu_hotplug_task from exiting if refcount_sum() becomes zero, this means wake_up_process() can hit the freed/reused/unmapped task_struct. Probably cpu_hotplug_done() needs another synhronize_sched() before return.

Yes, I think this is true, at least in theory.

Quoted 4 lines

OTOH, I do not understand why the result of __get_cpu_var(refcount) must be visible to refcount_sum() if we race with cpu_hotplug_begin(), so it seems to me cpu_hotplug_begin() also needs synchronize_sched() before refcount_sum().

No, I misread the unapplied patch, sorry for noise.

Oleg.

author
#3 lore raw
Quoted 11 lines · Oleg Nesterov
Quoted 10 lines · Oleg Nesterov
Quoted 2 lines · Lai Jiangshan

1) get_online_cpus() must be allowed to be called recursively, so I added get_online_cpus_nest for every task for new code.

Well, iirc one of the goals of

	cpu-hotplug: replace lock_cpu_hotplug() with get_online_cpus()
	86ef5c9a8edd78e6bf92879f32329d89b2d55b5a

was avoiding the new members in task_struct. I leave this up to you and Gautham.

Old get_online_cpus() is read-preference, I think the goal of this ability is allow get_online_cpus()/put_online_cpus() to be called nested.

But read-preference RWL may cause write side starvation, so I abandon this ability, and use per-task counter for allowing get_online_cpus()/put_online_cpus() to be called nested, I think this deal is absolutely worth.

Quoted 20 lines
Quoted 4 lines

Lai, I didn't read this patch carefully yet (and I can't apply it to Linus's tree). But at first glance,

because I tried to apply it without 1/2 ;)

Quoted 4 lines
Quoted 4 lines

void put_online_cpus(void)
{
...

kernel/cpu.c +1 -0
+	if (!--current->get_online_cpus_nest) {
Quoted 8 lines

kernel/cpu.c static int cpu_hotplug_disabled; · line 119

Quoted 4 lines
kernel/cpu.c +4 -0
+		preempt_disable();
+		__get_cpu_var(refcount)--;
+		if (cpu_hotplug_task)
+			wake_up_process(cpu_hotplug_task);

This looks unsafe. In theory nothing protects cpu_hotplug_task from exiting if refcount_sum() becomes zero, this means wake_up_process() can hit the freed/reused/unmapped task_struct. Probably cpu_hotplug_done() needs another synhronize_sched() before return.

Quoted 1 line

Yes, I think this is true, at least in theory.

preempt_disable() prevent cpu_hotplug_task from exiting.

Thanks, Lai

#4 lore raw
Quoted 8 lines · Lai Jiangshan
Quoted 6 lines · Oleg Nesterov
Quoted 5 lines · Oleg Nesterov
Quoted 2 lines · Lai Jiangshan

1) get_online_cpus() must be allowed to be called recursively, so I added get_online_cpus_nest for every task for new code.

Well, iirc one of the goals of

Quoted 8 lines
Quoted 5 lines
Quoted 5 lines
	cpu-hotplug: replace lock_cpu_hotplug() with get_online_cpus()
	86ef5c9a8edd78e6bf92879f32329d89b2d55b5a

was avoiding the new members in task_struct. I leave this up to you and Gautham.

Old get_online_cpus() is read-preference, I think the goal of this ability is allow get_online_cpus()/put_online_cpus() to be called nested.

Sure, I understand why you added task_struct->get_online_cpus_nest.

Quoted 2 lines

and use per-task counter for allowing get_online_cpus()/put_online_cpus() to be called nested, I think this deal is absolutely worth.

As I said, I am not going to argue. I can't justify this tradeoff.

Quoted 13 lines
Quoted 13 lines
Quoted 12 lines

kernel/cpu.c static int cpu_hotplug_disabled; · line 119

Quoted 8 lines

void put_online_cpus(void)
{
...

kernel/cpu.c +5 -0
+	if (!--current->get_online_cpus_nest) {
+		preempt_disable();
+		__get_cpu_var(refcount)--;
+		if (cpu_hotplug_task)
+			wake_up_process(cpu_hotplug_task);

This looks unsafe. In theory nothing protects cpu_hotplug_task from exiting if refcount_sum() becomes zero, this means wake_up_process() can hit the freed/reused/unmapped task_struct. Probably cpu_hotplug_done() needs another synhronize_sched() before return.

Quoted 3 lines
Quoted 1 line

Yes, I think this is true, at least in theory.

preempt_disable() prevent cpu_hotplug_task from exiting.

If the cpu_down() is the caller of cpu_hotplug_begin/done, then yes.

But unless I missed something, nothing protects from cpu_up() which takes this lock too.

Just in case... I am not saying this is really possible in practice.

Oleg.

#5 lore raw
Quoted 6 lines · Oleg Nesterov
Quoted 3 lines · Lai Jiangshan

Old get_online_cpus() is read-preference, I think the goal of this ability is allow get_online_cpus()/put_online_cpus() to be called nested.

Quoted 6 lines

Sure, I understand why you added task_struct->get_online_cpus_nest.

Quoted 2 lines

and use per-task counter for allowing get_online_cpus()/put_online_cpus() to be called nested, I think this deal is absolutely worth.

As I said, I am not going to argue. I can't justify this tradeoff.

But, I must admit, I'd like to avoid adding the new member to task_struct.

What do you think about the code below?

I didn't even try to compile it, just to explain what I mean.

In short: we have the per-cpu fast counters, plus the slow counter which is only used when cpu_hotplug_begin() is in progress.

Oleg.

static DEFINE_PER_CPU(long, cpuhp_fast_ctr);
static struct task_struct *cpuhp_writer;
static DEFINE_MUTEX(cpuhp_slow_lock)
static long cpuhp_slow_ctr;

static bool update_fast_ctr(int inc)

{
	bool success = true;

	preempt_disable();
	if (likely(!cpuhp_writer))
		__get_cpu_var(cpuhp_fast_ctr) += inc;
	else if (cpuhp_writer != current)
		success = false;
	preempt_enable();

	return success;
}

void get_online_cpus(void)

{
	if (likely(update_fast_ctr(+1));
		return;

	mutex_lock(&cpuhp_slow_lock);
	cpuhp_slow_ctr++;
	mutex_unlock(&cpuhp_slow_lock);
}

void put_online_cpus(void)

{
	if (likely(update_fast_ctr(-1));
		return;

	mutex_lock(&cpuhp_slow_lock);
	if (!--cpuhp_slow_ctr && cpuhp_writer)
		wake_up_process(cpuhp_writer);
	mutex_unlock(&cpuhp_slow_lock);
}

static void clear_fast_ctr(void)

{
	long total = 0;
	int cpu;

	for_each_possible_cpu(cpu) {
		total += per_cpu(cpuhp_fast_ctr, cpu);
		per_cpu(cpuhp_fast_ctr, cpu) = 0;
	}

	return total;
}

static void cpu_hotplug_begin(void)

{
	cpuhp_writer = current;
	synchronize_sched();

	/* Nobody except us can use can use cpuhp_fast_ctr */

	mutex_lock(&cpuhp_slow_lock);
	cpuhp_slow_ctr += clear_fast_ctr();

	while (cpuhp_slow_ctr) {
		__set_current_state(TASK_UNINTERRUPTIBLE);
		mutex_unlock(&&cpuhp_slow_lock);
		schedule();
		mutex_lock(&cpuhp_slow_lock);
	}
}

static void cpu_hotplug_done(void)

{
	cpuhp_writer = NULL;
	mutex_unlock(&cpuhp_slow_lock);
}
author
#6 lore raw
Quoted 12 lines · Oleg Nesterov
Quoted 8 lines · Oleg Nesterov
Quoted 2 lines · Lai Jiangshan

Old get_online_cpus() is read-preference, I think the goal of this ability is allow get_online_cpus()/put_online_cpus() to be called nested.

Sure, I understand why you added task_struct->get_online_cpus_nest.

Quoted 2 lines

and use per-task counter for allowing get_online_cpus()/put_online_cpus() to be called nested, I think this deal is absolutely worth.

As I said, I am not going to argue. I can't justify this tradeoff.

But, I must admit, I'd like to avoid adding the new member to task_struct.

Quoted 9 lines

What do you think about the code below?

I didn't even try to compile it, just to explain what I mean.

In short: we have the per-cpu fast counters, plus the slow counter which is only used when cpu_hotplug_begin() is in progress.

Oleg.

get_online_cpus() in your code is still read-preference. I wish we quit this ability of get_online_cpus().

Lai.

maintainer
#7 lore raw
Quoted 17 lines · Lai Jiangshan
Quoted 16 lines · Oleg Nesterov
Quoted 8 lines · Oleg Nesterov
Quoted 2 lines · Lai Jiangshan

Old get_online_cpus() is read-preference, I think the goal of this ability is allow get_online_cpus()/put_online_cpus() to be called nested.

Sure, I understand why you added task_struct->get_online_cpus_nest.

Quoted 2 lines

and use per-task counter for allowing get_online_cpus()/put_online_cpus() to be called nested, I think this deal is absolutely worth.

As I said, I am not going to argue. I can't justify this tradeoff.

But, I must admit, I'd like to avoid adding the new member to task_struct.

What do you think about the code below?

I didn't even try to compile it, just to explain what I mean.

Quoted 8 lines
Quoted 5 lines

In short: we have the per-cpu fast counters, plus the slow counter which is only used when cpu_hotplug_begin() is in progress.

Oleg.

get_online_cpus() in your code is still read-preference. I wish we quit this ability of get_online_cpus().

Why?

author
#8 lore raw
Quoted 20 lines · Peter Zijlstra
Quoted 19 lines · Lai Jiangshan
Quoted 18 lines · Oleg Nesterov
Quoted 8 lines · Oleg Nesterov
Quoted 2 lines · Lai Jiangshan

Old get_online_cpus() is read-preference, I think the goal of this ability is allow get_online_cpus()/put_online_cpus() to be called nested.

Sure, I understand why you added task_struct->get_online_cpus_nest.

Quoted 2 lines

and use per-task counter for allowing get_online_cpus()/put_online_cpus() to be called nested, I think this deal is absolutely worth.

As I said, I am not going to argue. I can't justify this tradeoff.

But, I must admit, I'd like to avoid adding the new member to task_struct.

What do you think about the code below?

I didn't even try to compile it, just to explain what I mean.

In short: we have the per-cpu fast counters, plus the slow counter which is only used when cpu_hotplug_begin() is in progress.

Quoted 6 lines
Quoted 4 lines
Quoted 2 lines

Oleg.

get_online_cpus() in your code is still read-preference. I wish we quit this ability of get_online_cpus().

Why?

Because read-preference RWL will cause write site starvation.

A user run the following code will cause cpuhotplug starvation. (100 processes run sched_setaffinity().)

Lai

#define _GNU_SOURCE
#include <sched.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>

#define NCPU 4
#define NPROCESS 100

cpu_set_t set;
pid_t target;

void stress_test(void)

{
	int cpu;

	srand((int)target);
	for (;;) {
		cpu = rand() % NCPU;
		CPU_SET(cpu, &set);
		sched_setaffinity(target, sizeof(set), &set);
		CPU_CLR(cpu, &set);
	}
}

int main(int argc, char *argv[])

{
	pid_t ret;
	int i;

	target = getpid();
	for (i = 1; i < NPROCESS; i++) {
		ret = fork();
		if (ret < 0)
			break;
		else if (ret)
			target = ret;
		else
			stress_test();
	}

	stress_test();
}
maintainer
#9 lore raw
Quoted 1 line · Lai Jiangshan

Because read-preference RWL will cause write site starvation.

Sure, but why is that a problem? Hotplug should be a rare event, who cares if it takes a while to come through.

author
#10 lore raw
Quoted 7 lines · Peter Zijlstra
Quoted 1 line · Lai Jiangshan

Because read-preference RWL will cause write site starvation.

Sure, but why is that a problem? Hotplug should be a rare event,

who cares if it takes a while to come through.

It is totally starvation, hotplug can not complete. I think it is a kind of DOS attack.

#11 lore raw
In reply to Lai Jiangshan
Quoted 5 lines · Lai Jiangshan
Quoted 3 lines · Oleg Nesterov

But, I must admit, I'd like to avoid adding the new member to task_struct.

Quoted 8 lines
Quoted 6 lines

What do you think about the code below?

I didn't even try to compile it, just to explain what I mean.

In short: we have the per-cpu fast counters, plus the slow counter which is only used when cpu_hotplug_begin() is in progress.

get_online_cpus() in your code is still read-preference.

Yes,

Quoted 1 line

I wish we quit this ability of get_online_cpus().

Heh. Since I never read the changelogs, I didn't even notice this was one of the goals of your patch. I thought this is just the side-effect.

Yes, if we want to block the new readers, I don't see how it is possible to avoid the counter in task_struct.

I can't judge whether this new member worth the trouble. Once again, I am not arguing, just I don't know. And I think your patch is correct (apart from pure theoretical race with cpu_hotplug_done afaics).

Oleg.