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