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

[Zhang, Qiang]

________________________________________
发件人: Uladzislau Rezki <urezki@xxxxxxxxx>
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@xxxxxxxxx>
> >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.

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