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

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Message-ID: <47238aa4-cc41-96ac-4d6e-c182a340db85@bristot.me>
Date:   Mon, 7 Nov 2016 19:49:03 +0100
From:   Daniel Bristot de Oliveira <daniel@...stot.me>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Christoph Lameter <cl@...ux.com>
Cc:     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 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.

This problem was fixed on -rt using a -rt specific lock mechanism, but
the problem still exist in the non-rt kernel. Btw, this is just an
example of side effects of the starvation of non-rt-tasks. We can have
more examples...

-- Daniel