________________________________________
??????: Uladzislau Rezki <[email protected]>
????ʱ??: 2021??1??25?? 5:57
?ռ???: Zhang, Qiang
????: 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
????: Re: ?ظ?: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
>Hello, Zhang.
> >________________________________________
> >??????: Uladzislau Rezki (Sony) <[email protected]>
> >????ʱ??: 2021??1??21?? 0:21
> >?ռ???: 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; Uladzislau >Rezki; Oleksiy Avramchenko
> >????: [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.
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);
}
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.
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.
>
> 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:
I agree with you here
>- 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!
Maybe migrate_disable/enable() can be removed
Thanks
Qiang
>--
>Vlad Rezki
>
> ________________________________________
> 发件人: Uladzislau Rezki <[email protected]>
> 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) <[email protected]>
> > >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 <[email protected]>
????ʱ??: 2021??1??25?? 21:49
?ռ???: Zhang, Qiang
????: 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
????: Re: ?ظ?: ?ظ?: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()
> >Hello, Zhang.
>
> > >________________________________________
> > >??????: Uladzislau Rezki (Sony) <[email protected]>
> > >????ʱ??: 2021??1??21?? 0:21
> > >?ռ???: 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; Uladzislau >Rezki; Oleksiy Avramchenko
> > >????: [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.
>--
>Vlad Rezki