Re: [PATCH] sched/rt: RT_RUNTIME_GREED sched feature

From: Steven Rostedt
Date: Mon Nov 07 2016 - 14:17:12 EST

On Mon, 7 Nov 2016 19:49:03 +0100
Daniel Bristot de Oliveira <daniel@xxxxxxxxxx> wrote:

> On 11/07/2016 07:32 PM, Steven Rostedt wrote:
> >> Excellent this would improve the situation with deadlocks as a result of
> >> > cgroup_locks not being released due to lack of workqueue processing.
> > ?? What deadlocks do you see? I mean, can you show the situation that
> > throttling RT tasks will cause deadlock?
> >
> > Sorry, but I'm just not seeing it.
> It is not a deadlock in the theoretical sense of the word, but it is
> more a side effect of the starvation - that looks like a deadlock.
> There is a case where the removal of a cgroup dir calls
> lru_add_drain_all(), that might schedule a kworker in the CPU that is
> running the spinning-rt task. The kworker will starve - because they are
> SCHED_OTHER by design, the lru_add_drain_all() will wait forever while
> holding the cgroup lock and this will cause a lot of problems on other
> tasks.

I understand the issue with not throttling an RT task, but this patch
is about not not throttling! That is, what scenario is there that will
cause a "deadlock" or deadlock like to happen when we *do* throttle,
where not throttling will work better, as this patch would have?

-- Steve