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

[Uladzislau Rezki <urezki@gmail.com>]

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