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Date: Wed, 14 Oct 2015 09:20:41 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Christoph Lameter <cl@linux.com>
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Cc: ksummit-discuss@lists.linuxfoundation.org
Subject: Re: [Ksummit-discuss] [TECH TOPIC] Mainlining PREEMPT_RT
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On Wed, 14 Oct 2015 06:45:31 -0500 (CDT)
Christoph Lameter <cl@linux.com> wrote:

> On Tue, 13 Oct 2015, Steven Rostedt wrote:
> 
> >  preempt_disable and local_irq_disable annotations
> 
> Hrm... Would there be a way to do Realtime without PREEMPT / PREEMPT_RT
> and such things? Doing this performance degradation is pretty significant
> and as far as I know makes this unacceptable for many uses. CONFIG_PREEMPT
> is already a problem.

The only RT that this would be good for is for CPU isolation and running
tasks that don't ever interact with the kernel. This is a completely
different use case than what PREEMPT_RT is about, although, NO_HZ_FULL
would be beneficial to both scenarios.

> 
> Or change CONFIG_PREEMPT first to be less intrusive? Have defined points
> where preemption can occur like CONFIG_PREEMPT_VOLUNTARY? Could we have
> realtime based on that? If we can keep the time between voluntary
> preemption short enough then this should do this as well. Plus we can
> likely reduce kernel complexity because code can rely on  not being
> rescheduled unless an explicit call was done.

Again, what you want has nothing to do with PREEMPT_RT, and not what
I'm proposing here for a tech topic.

-- Steve