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

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Date:   Tue, 8 Nov 2016 12:17:10 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        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 Tue, 8 Nov 2016 17:51:33 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> You really should already know this.

I know what we want to do, but there's some momentous problems that
need to be solved first. Until then, we may be forced to continue with
hacks.

> 
> As stands the current rt cgroup code (and all this throttling code) is a
> giant mess (as in, its not actually correct from a RT pov). We should
> not make it worse by adding random hacks to it.
> 
> The right way to to about doing this is by replacing it with something
> better; like the proposed DL server for FIFO tasks -- which is entirely
> non-trivial as well, see existing discussion on that.

Right. The biggest issue that I see is how to assign affinities to
FIFO tasks and use a DL server to keep them from starving other tasks?

> 
> I'm not entirely sure what this patch was supposed to fix, but it could
> be running CFS tasks with higher priority than RT for a window, instead

I'm a bit confused with the above sentence. Do you mean that this patch
causes CFS tasks to run for a period with a higher priority than RT?
Well, currently we have the both CFS tasks and the "idle" task run
higher than RT, but this patch changes that to be just CFS tasks.

> of throttling RT tasks. This seems fairly ill specified, but something
> like that could easily done with an explicit or slack time DL server for
> CFS tasks.

If we can have a DL scheduler that can handle arbitrary affinities,
then all could be solved with that.

-- Steve