linux-kernel - Re: [PATCH] sched/rt: RT_RUNTIME

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Date:   Mon, 7 Nov 2016 14:16:02 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Bristot de Oliveira <daniel@...stot.me>
Cc:     Christoph Lameter <cl@...ux.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/rt: RT_RUNTIME_GREED sched feature

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