Re: [PATCH 1/1] kthread: break dependency between worker->lock and task_struct->pi_lock

[Suren Baghdasaryan]

On Tue, Apr 28, 2020 at 9:31 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Apr 27, 2020 at 11:43:58AM -0700, Suren Baghdasaryan wrote:
> > A number of kthread-related functions indirectly take task_struct->pi_lock
> > while holding worker->lock in the call chain like this:
> >     spin_lock(&worker->lock)
> >     kthread_insert_work
> >     wake_up_process
> >     try_to_wake_up
> >     raw_spin_lock_irqsave(&p->pi_lock, flags)
> >
> > This lock dependency exists whenever kthread_insert_work is called either
> > directly or indirectly via __kthread_queue_delayed_work in the following
> > functions:
> >     kthread_queue_work
> >     kthread_delayed_work_timer_fn
> >     kthread_queue_delayed_work
> >     kthread_flush_work
> >     kthread_mod_delayed_work
> >
> > This creates possibilities for circular dependencies like the one reported
> > at: https://lkml.org/lkml/2020/4/24/954
>
> Please, do not use lkml.org links.

Thanks for the review! Would
https://lore.kernel.org/lkml/CAJuCfpG4NkhpQvZjgXZ_3gm6Hf1QgN_eUOQ8iX9Cv1k9whLwSQ@xxxxxxxxxxxxxx
be better or should I just add the body of that report here? Or do not
mention it at all?

>
> Also, ideally, we'd pull that kthread_queue_delayed_work() out from
> under rq->lock.

I understand but I don't see an easy way to do that. We need to start
PSI polling whenever a monitored PSI state changes:
https://elixir.bootlin.com/linux/v5.6.7/source/kernel/sched/psi.c#L783.
This is happening under rq->lock because PSI accounting is done from
inside enqueue_task/dequeue_task - the call chain is:

enqueue_task > psi_enqueue > psi_task_change > psi_group_change >
psi_schedule_poll_work > psi_task_change

IIUC enqueue_task/dequeue_task are called with rq->lock taken, so
moving kthread_queue_delayed_work out is not trivial.

>
> In fact, looking at it, WTH is the delayed branch of
> kthread_queue_delayed_work() under that lock? That whole
> delayed_work_list thing smells like bong-hits.

I have the poll_scheduled atomic specifically to ensure that
kthread_queue_delayed_work does not block as commented here:
https://elixir.bootlin.com/linux/v5.7-rc3/source/kernel/sched/psi.c#L551.
I understand this is not ideal. If there is a better way to schedule
that kworker while ensuring it does not block I would be happy to
rework this. Any suggestions?

>
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