Re: Warning from swake_up_all in 4.14.15-rt13 non-RT

[Peter Zijlstra]

On Mon, Mar 12, 2018 at 11:51:13AM +0100, Sebastian Andrzej Siewior wrote:
> On 2018-03-09 23:26:43 [+0100], Peter Zijlstra wrote:
> > On Fri, Mar 09, 2018 at 09:25:50PM +0100, Sebastian Andrzej Siewior wrote:
> > > Is it just about the irqsave() usage or something else? I doubt it is
> > > the list walk. It is still unbound if not called from irq-off region.
> > 
> > The current list walk is preemptible. You put the entire iteration (of
> > unbound length) inside a single critical section which destroy RT.
> 
> I considered that list walk as cheap. We don't do any wake ups with the
> list walk - just mark the task for a later wake up. But if it is not I
> could add an upper limit of 20 iterations or so.

So the problem is that as soon as this is exposed to userspace you've
lost.

If a user can stack like 10000 tasks on the completion before triggering
it, you've got yourself a giant !preempt section. Yes the wake_q stuff
is cheaper, but unbound is still unbound.

wake_all must not be used from !preemptible (IRQ or otherwise) sections.
And I'm not seeing how waking just the top 20 helps.

> > Why isn't this a problem on RT?
> So we remain in the preempt_disable() section due to RCU-sched so we
> have this, yes. But the "disabled interrupts" part is due to
> spin_lock_irqsave() which is a non-issue on RT. So if we managed to get
> rid of the rcu-sched then the swait can go and we can stick with the
> wake_up_all() on RT, too.

OK, so for RT we simply loose the IRQ-disable thing, but its still a
!preemptible section.