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

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Message-ID: <20161107154401.16f0214e@gandalf.local.home>
Date:   Mon, 7 Nov 2016 15:44:01 -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 21:33:02 +0100
Daniel Bristot de Oliveira <daniel@...stot.me> wrote:

> On 11/07/2016 09:16 PM, Steven Rostedt wrote:
> > I'm confused? Are you saying that RR tasks don't get throttled in the
> > current code? That sounds like a bug to me.  
> 
> If the RT_RUNTIME_SHARING is enabled, the CPU in which the RR tasks are
> running (and pinned) will borrow RT runtime from another CPU, allowing
> the RR tasks to run forever. For example:
> 
> [root@...on debug]# cat /proc/sched_debug | grep rt_runtime
>   .rt_runtime                    : 950.000000
>   .rt_runtime                    : 950.000000
>   .rt_runtime                    : 950.000000
>   .rt_runtime                    : 950.000000
> [root@...on debug]# echo RT_RUNTIME_SHARE > sched_features
> [root@...on debug]# taskset -c 2 chrt -r 5 /home/bristot/f &
> [1] 23908
> [root@...on debug]# taskset -c 2 chrt -r 5 /home/bristot/f &
> [2] 23915
> [root@...on debug]# cat /proc/sched_debug | grep rt_runtime
>   .rt_runtime                    : 900.000000
>   .rt_runtime                    : 950.000000
>   .rt_runtime                    : 1000.000000
>   .rt_runtime                    : 950.000000
> 
> You see? the rt_runtime of the CPU 2 was borrowed time from CPU 0.
> 
> It is not a BUG but a feature (no jokes haha). With RT_RUNTIME_SHARE,
> the rt_runtime is such a global runtime. It works fine for tasks that
> can migrate... but that is not the case for per-cpu kworkers.

This still looks like a bug, or not the expected result. Perhaps we
shouldn't share when tasks are pinned. It doesn't make sense. It's like
pinning two deadline tasks to the same CPU and giving them 100% of that
CPU and saying that it's really just 1/nr_cpus of usage, which would
have the same effect.

OK, it appears this is specific to RT_RUNTIME_SHARE which is what
causes this strange behavior, and even more rational to make this a
default option and perhaps even turn RT_RUNTIME_SHARE off by default.

-- Steve