回复: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
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  • 回复: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
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Zhang, Qiang Jan. 23, 2021, 9:31 a.m. UTC
>________________________________________
>发件人: Uladzislau Rezki (Sony) <urezki@gmail.com>
>发送时间: 2021年1月21日 0:21
>收件人: LKML; RCU; Paul E . McKenney; Michael Ellerman
>抄送: Andrew Morton; Daniel Axtens; Frederic Weisbecker; Neeraj >Upadhyay; Joel Fernandes; Peter Zijlstra; Michal Hocko; Thomas >Gleixner; Theodore Y . Ts'o; Sebastian Andrzej Siewior; Uladzislau >Rezki; Oleksiy Avramchenko
>主题: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
>
>Since the page is obtained in a fully preemptible context, dropping
>the lock can lead to migration onto another CPU. As a result a prev.
>bnode of that CPU may be underutilised, because a decision has been
>made for a CPU that was run out of free slots to store a pointer.
>
>migrate_disable/enable() are now independent of RT, use it in order
>to prevent any migration during a page request for a specific CPU it
>is requested for.


Hello Rezki

The critical migrate_disable/enable() area is not allowed to block, under RT and non RT.  
There is such a description in preempt.h 


* Notes on the implementation.
 *
 * The implementation is particularly tricky since existing code patterns
 * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
 * This means that it cannot use cpus_read_lock() to serialize against hotplug,
 * nor can it easily migrate itself into a pending affinity mask change on
 * migrate_enable().


How about the following changes:



Thanks
Qiang



>
>Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
>---
> kernel/rcu/tree.c | 2 ++
>1 file changed, 2 insertions(+)
>
>diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>index 454809514c91..cad36074366d 100644
>--- a/kernel/rcu/tree.c
>+++ b/kernel/rcu/tree.c
>@@ -3489,10 +3489,12 @@ add_ptr_to_bulk_krc_lock(struct >kfree_rcu_cpu **krcp,
>                        (*krcp)->bkvhead[idx]->nr_records == >KVFREE_BULK_MAX_ENTR) {
>                bnode = get_cached_bnode(*krcp);
>                if (!bnode && can_alloc) {
>+                       migrate_disable();
>                        krc_this_cpu_unlock(*krcp, *flags);
>                        bnode = (struct kvfree_rcu_bulk_data *)
>                                __get_free_page(GFP_KERNEL | >__GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN);
>                       *krcp = krc_this_cpu_lock(flags);
>+                       migrate_enable();
>                }
>
>                if (!bnode)
>--
>2.20.1

Comments

Uladzislau Rezki Jan. 24, 2021, 9:57 p.m. UTC | #1
Hello, Zhang.

> >________________________________________
> >发件人: Uladzislau Rezki (Sony) <urezki@gmail.com>
> >发送时间: 2021年1月21日 0:21
> >收件人: LKML; RCU; Paul E . McKenney; Michael Ellerman
> >抄送: Andrew Morton; Daniel Axtens; Frederic Weisbecker; Neeraj >Upadhyay; Joel Fernandes; Peter Zijlstra; Michal Hocko; Thomas >Gleixner; Theodore Y . Ts'o; Sebastian Andrzej Siewior; Uladzislau >Rezki; Oleksiy Avramchenko
> >主题: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
> >
> >Since the page is obtained in a fully preemptible context, dropping
> >the lock can lead to migration onto another CPU. As a result a prev.
> >bnode of that CPU may be underutilised, because a decision has been
> >made for a CPU that was run out of free slots to store a pointer.
> >
> >migrate_disable/enable() are now independent of RT, use it in order
> >to prevent any migration during a page request for a specific CPU it
> >is requested for.
> 
> 
> Hello Rezki
> 
> The critical migrate_disable/enable() area is not allowed to block, under RT and non RT.  
> There is such a description in preempt.h 
> 
> 
> * Notes on the implementation.
>  *
>  * The implementation is particularly tricky since existing code patterns
>  * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
>  * This means that it cannot use cpus_read_lock() to serialize against hotplug,
>  * nor can it easily migrate itself into a pending affinity mask change on
>  * migrate_enable().
> 
How i interpret it is migrate_enable()/migrate_disable() are not allowed to
use any blocking primitives, such as rwsem/mutexes/etc. in order to mark a
current context as non-migratable.

void migrate_disable(void)
{
 struct task_struct *p = current;

 if (p->migration_disabled) {
  p->migration_disabled++;
  return;
 }

 preempt_disable();
 this_rq()->nr_pinned++;
 p->migration_disabled = 1;
 preempt_enable();
}

It does nothing that prevents you from doing schedule() or even wait for any
event(mutex slow path behaviour), when the process is removed from the run-queue.
I mean after the migrate_disable() is invoked. Or i miss something?

>
> How about the following changes:
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index e7a226abff0d..2aa19537ac7c 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3488,12 +3488,10 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
>                         (*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
>                 bnode = get_cached_bnode(*krcp);
>                 if (!bnode && can_alloc) {
> -                       migrate_disable();
>                         krc_this_cpu_unlock(*krcp, *flags);
>                         bnode = (struct kvfree_rcu_bulk_data *)
>                                 __get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN);
> -                       *krcp = krc_this_cpu_lock(flags);
> -                       migrate_enable();
> +                       raw_spin_lock_irqsave(&(*krcp)->lock, *flags);
>
Hm.. Taking the former lock can lead to a pointer leaking, i mean a CPU associated
with "krcp" might go offline during a page request process, so a queuing occurs on
off-lined CPU. Apat of that, acquiring a former lock still does not solve:

- CPU1 in process of page allocation;
- CPU1 gets migrated to CPU2;
- another task running on CPU1 also allocate a page;
- both bnodes are added to krcp associated with CPU1.

I agree that such scenario probably will never happen or i would say, can be
considered as a corner case. We can drop the:

[PATCH 3/3] kvfree_rcu: use migrate_disable/enable()

and live with: an allocated bnode can be queued to another CPU, so its prev.
"bnode" can be underutilized. What is also can be considered as a corner case.
According to my tests, it is hard to achieve:

Running kvfree_rcu() simultaneously in a tight loop, 1 000 000 allocations/freeing:

- 64 CPUs and 64 threads showed 1 migration;
- 64 CPUs and 128 threads showed 0 migrations;
- 64 CPUs and 32 threads showed 0 migration. 

Thoughts?

Thank you for your comments!

--
Vlad Rezki
Uladzislau Rezki Jan. 25, 2021, 1:49 p.m. UTC | #2
> 
> ________________________________________
> 发件人: Uladzislau Rezki <urezki@gmail.com>
> 发送时间: 2021年1月25日 5:57
> 收件人: Zhang, Qiang
> 抄送: Uladzislau Rezki (Sony); LKML; RCU; Paul E . McKenney; Michael Ellerman; Andrew Morton; Daniel Axtens; Frederic Weisbecker; Neeraj Upadhyay; Joel Fernandes; Peter Zijlstra; Michal Hocko; Thomas Gleixner; Theodore Y . Ts'o; Sebastian Andrzej Siewior; Oleksiy Avramchenko
> 主题: Re: 回复: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
> 
> >Hello, Zhang.
> 
> > >________________________________________
> > >发件人: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > >发送时间: 2021年1月21日 0:21
> > >收件人: LKML; RCU; Paul E . McKenney; Michael Ellerman
> > >抄送: Andrew Morton; Daniel Axtens; Frederic Weisbecker; Neeraj >Upadhyay; Joel Fernandes; Peter Zijlstra; Michal Hocko; Thomas >Gleixner; Theodore Y . Ts'o; Sebastian Andrzej Siewior; Uladzislau >Rezki; Oleksiy Avramchenko
> > >主题: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
> > >
> > >Since the page is obtained in a fully preemptible context, dropping
> > >the lock can lead to migration onto another CPU. As a result a prev.
> > >bnode of that CPU may be underutilised, because a decision has been
> > >made for a CPU that was run out of free slots to store a pointer.
> > >
> > >migrate_disable/enable() are now independent of RT, use it in order
> > >to prevent any migration during a page request for a specific CPU it
> > >is requested for.
> >
> >
> > Hello Rezki
> >
> > The critical migrate_disable/enable() area is not allowed to block, under RT and non RT.
> > There is such a description in preempt.h
> >
> >
> > * Notes on the implementation.
> >  *
> >  * The implementation is particularly tricky since existing code patterns
> >  * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
> >  * This means that it cannot use cpus_read_lock() to serialize against hotplug,
> >  * nor can it easily migrate itself into a pending affinity mask change on
> >  * migrate_enable().
> >
> >How i interpret it is migrate_enable()/migrate_disable() are not allowed to
> >use any blocking primitives, such as rwsem/mutexes/etc. in order to mark a
> >current context as non-migratable.
> >
> >void migrate_disable(void)
> >{
> > struct task_struct *p = current;
> >
> > if (p->migration_disabled) {
> >  p->migration_disabled++;
> >  return;
> > }
> 
> > preempt_disable();
> > this_rq()->nr_pinned++;
> > p->migration_disabled = 1;
> > preempt_enable();
> >}
> >
> >It does nothing that prevents you from doing schedule() or even wait for any
> >event(mutex slow path behaviour), when the process is removed from the run-queue.
> >I mean after the migrate_disable() is invoked. Or i miss something?
> 
> Hello Rezki
> 
> Sorry, there's something wrong with the previous description.
> There are the following scenarios
> 
> Due to migrate_disable will increase  this_rq()->nr_pinned , after that
> if get_free_page be blocked, and this time, CPU going offline,
> the sched_cpu_wait_empty() be called in per-cpu "cpuhp/%d" task,
> and be blocked.
> 
But after the migrate_disable() is invoked a CPU can not be brought down.
If there are pinned tasks a "hotplug path" will be blocked on balance_hotplug_wait()
call.

> blocked:
> sched_cpu_wait_empty()
> {
>       struct rq *rq = this_rq();
>        rcuwait_wait_event(&rq->hotplug_wait,
>                            rq->nr_running == 1 && !rq_has_pinned_tasks(rq),
>                            TASK_UNINTERRUPTIBLE);
> }
>
Exactly.

> wakeup:
> balance_push()
> {
>         if (is_per_cpu_kthread(push_task) || is_migration_disabled(push_task)) {
>               
>                 if (!rq->nr_running && !rq_has_pinned_tasks(rq) &&
>                     rcuwait_active(&rq->hotplug_wait)) {
>                         raw_spin_unlock(&rq->lock);
>                         rcuwait_wake_up(&rq->hotplug_wait);
>                         raw_spin_lock(&rq->lock);
>                 }
>                 return;
>         }
> }
> 
> One of the conditions for this function to wake up is "rq->nr_pinned  == 0"
> that is to say between migrate_disable/enable, if blocked will defect CPU going
> offline longer blocking time.
> 
Indeed, the hotplug time is affected. For example in case of waiting for
a mutex to be released, an owner will wakeup waiters. But this is expectable.

>
> I'm not sure that's a problem,and I didn't find it in the kernel code  between 
>  migrate_disable/enable possible sleep calls.
> 
For example z3fold.c:

/* Add to the appropriate unbuddied list */
static inline void add_to_unbuddied(struct z3fold_pool *pool,
				struct z3fold_header *zhdr)
{
	if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0 ||
			zhdr->middle_chunks == 0) {
		struct list_head *unbuddied;
		int freechunks = num_free_chunks(zhdr);

		migrate_disable();
		unbuddied = this_cpu_ptr(pool->unbuddied);
		spin_lock(&pool->lock);
		list_add(&zhdr->buddy, &unbuddied[freechunks]);
		spin_unlock(&pool->lock);
		zhdr->cpu = smp_processor_id();
		migrate_enable();
	}
}

for PREEMPT_RT kernel a spinlock is converted to rt-mutex, thus it can sleep.

--
Vlad Rezki
Uladzislau Rezki Jan. 26, 2021, 1:43 p.m. UTC | #3
On Tue, Jan 26, 2021 at 09:33:40AM +0000, Zhang, Qiang wrote:
> 
> 
> ________________________________________
> 发件人: Uladzislau Rezki <urezki@gmail.com>
> 发送时间: 2021年1月25日 21:49
> 收件人: Zhang, Qiang
> 抄送: Uladzislau Rezki; LKML; RCU; Paul E . McKenney; Michael Ellerman; Andrew Morton; Daniel Axtens; Frederic Weisbecker; Neeraj Upadhyay; Joel Fernandes; Peter Zijlstra; Michal Hocko; Thomas Gleixner; Theodore Y . Ts'o; Sebastian Andrzej Siewior; Oleksiy Avramchenko
> 主题: Re: 回复: 回复: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
> 
> > >Hello, Zhang.
> >
> > > >________________________________________
> > > >发件人: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > >发送时间: 2021年1月21日 0:21
> > > >收件人: LKML; RCU; Paul E . McKenney; Michael Ellerman
> > > >抄送: Andrew Morton; Daniel Axtens; Frederic Weisbecker; Neeraj >Upadhyay; Joel Fernandes; Peter Zijlstra; Michal Hocko; Thomas >Gleixner; Theodore Y . Ts'o; Sebastian Andrzej Siewior; Uladzislau >Rezki; Oleksiy Avramchenko
> > > >主题: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
> > > >
> > > >Since the page is obtained in a fully preemptible context, dropping
> > > >the lock can lead to migration onto another CPU. As a result a prev.
> > > >bnode of that CPU may be underutilised, because a decision has been
> > > >made for a CPU that was run out of free slots to store a pointer.
> > > >
> > > >migrate_disable/enable() are now independent of RT, use it in order
> > > >to prevent any migration during a page request for a specific CPU it
> > > >is requested for.
> > >
> > >
> > > Hello Rezki
> > >
> > > The critical migrate_disable/enable() area is not allowed to block, under RT and non RT.
> > > There is such a description in preempt.h
> > >
> > >
> > > * Notes on the implementation.
> > >  *
> > >  * The implementation is particularly tricky since existing code patterns
> > >  * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
> > >  * This means that it cannot use cpus_read_lock() to serialize against hotplug,
> > >  * nor can it easily migrate itself into a pending affinity mask change on
> > >  * migrate_enable().
> > >
> > >How i interpret it is migrate_enable()/migrate_disable() are not allowed to
> > >use any blocking primitives, such as rwsem/mutexes/etc. in order to mark a
> > >current context as non-migratable.
> > >
> > >void migrate_disable(void)
> > >{
> > > struct task_struct *p = current;
> > >
> > > if (p->migration_disabled) {
> > >  p->migration_disabled++;
> > >  return;
> > > }
> >
> > > preempt_disable();
> > > this_rq()->nr_pinned++;
> > > p->migration_disabled = 1;
> > > preempt_enable();
> > >}
> > >
> > >It does nothing that prevents you from doing schedule() or even wait for any
> > >event(mutex slow path behaviour), when the process is removed from the run-queue.
> > >I mean after the migrate_disable() is invoked. Or i miss something?
> >
> > Hello Rezki
> >
> > Sorry, there's something wrong with the previous description.
> > There are the following scenarios
> >
> > Due to migrate_disable will increase  this_rq()->nr_pinned , after that
> > if get_free_page be blocked, and this time, CPU going offline,
> > the sched_cpu_wait_empty() be called in per-cpu "cpuhp/%d" task,
> > and be blocked.
> >
> >But after the migrate_disable() is invoked a CPU can not be brought down.
> >If there are pinned tasks a "hotplug path" will be blocked on balance_hotplug_wait()
> >call.
> 
> > blocked:
> > sched_cpu_wait_empty()
> > {
> >       struct rq *rq = this_rq();
> >        rcuwait_wait_event(&rq->hotplug_wait,
> >                            rq->nr_running == 1 && !rq_has_pinned_tasks(rq),
> >                            TASK_UNINTERRUPTIBLE);
> > }
> >
> >Exactly.
> 
> > wakeup:
> > balance_push()
> > {
> >         if (is_per_cpu_kthread(push_task) || is_migration_disabled(push_task)) {
> >
> >                 if (!rq->nr_running && !rq_has_pinned_tasks(rq) &&
> >                     rcuwait_active(&rq->hotplug_wait)) {
> >                         raw_spin_unlock(&rq->lock);
> >                         rcuwait_wake_up(&rq->hotplug_wait);
> >                         raw_spin_lock(&rq->lock);
> >                 }
> >                 return;
> >         }
> > }
> >
> > One of the conditions for this function to wake up is "rq->nr_pinned  == 0"
> > that is to say between migrate_disable/enable, if blocked will defect CPU going
> > offline longer blocking time.
> >
> >Indeed, the hotplug time is affected. For example in case of waiting for
> >a mutex to be released, an owner will wakeup waiters. But this is expectable.
> 
> >
> > I'm not sure that's a problem,and I didn't find it in the kernel code  between
> >  migrate_disable/enable possible sleep calls.
> >
> >For example z3fold.c:
> 
> >/* Add to the appropriate unbuddied list */
> >static inline void add_to_unbuddied(struct z3fold_pool *pool,
> >                                struct z3fold_header *zhdr)
> >{
> >       if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0 ||
> >                        zhdr->middle_chunks == 0) {
> >                struct list_head *unbuddied;
> >              int freechunks = num_free_chunks(zhdr);
> >
> >                migrate_disable();
> >                unbuddied = this_cpu_ptr(pool->unbuddied);
> >                spin_lock(&pool->lock);
> >                list_add(&zhdr->buddy, &unbuddied[freechunks]);
> >                spin_unlock(&pool->lock);
> >                zhdr->cpu = smp_processor_id();
> >                migrate_enable();
> >        }
> >}
> 
> >for PREEMPT_RT kernel a spinlock is converted to rt-mutex, thus it can sleep.
> 
>  I forgot that. Thank you for your explanation.
> 
>  
No problem. I also has recently learned about spinlock and rt-mutexes :)

--
Vlad Rezki

Patch
diff mbox series

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e7a226abff0d..2aa19537ac7c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3488,12 +3488,10 @@  add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
                        (*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
                bnode = get_cached_bnode(*krcp);
                if (!bnode && can_alloc) {
-                       migrate_disable();
                        krc_this_cpu_unlock(*krcp, *flags);
                        bnode = (struct kvfree_rcu_bulk_data *)
                                __get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN);
-                       *krcp = krc_this_cpu_lock(flags);
-                       migrate_enable();
+                       raw_spin_lock_irqsave(&(*krcp)->lock, *flags);
                }
 
                if (!bnode)