linux-kernel - Re: RT scheduler config, suggestions and questions

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Date:	Wed, 06 Feb 2008 07:36:55 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Max Krasnyanskiy <maxk@...lcomm.com>
Cc:	Steven Rostedt <srostedt@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: RT scheduler config, suggestions and questions


On Tue, 2008-02-05 at 15:37 -0800, Max Krasnyanskiy wrote:
> Folks,
> 
> I just realized that in latest Linus' tree following sysctls are under SCHED_DEBUG:
> 	sched_rt_period
> 	sched_rt_ratio
> 
> I do not believe that is correct. I know that we do not want to expose scheduler knobs
> in general but theses are not the heuristic kind of knobs. There is no way the scheduler 
> can magically figure out what the correct setting should be here.

Yeah, since fixed.

> Also shouldn't those new RT features that recently went be configurable and _disabled_ 
> by default ? For example "RT watchdog" and "RT throttling" actually seem very questionable. 
> SCHED_FIFO is clearly defined as
> "
>   A SCHED_FIFO process runs until either it is blocked by an I/O request, it is preempted 
>   by a higher priority process, or it calls sched_yield(2).
> "

The watchdog is disabled by default, the bandwidth is .95s every 1s,
which is mainly a safe-guard against run-away real-time tasks. As long
as real-time usage stays within those limits nothing happens. If you
don't like it set sched_rt_runtime [*] to -1.

[*] provided in the interface changes posted a few days ago.

> Both the watchdog and the throttling are clearly braking that rule. I think it's good to have 
> those features but not enabled by default and certainly not with sysctls that disable them 
> hidden under debugging.
> How about this:
> - We introduce Kconfig options for them ?

I don't see why this would be needed.

> - Expose all rt sysctls outside of #ifdef DEBUG

Already did this somewhere along the line.

> btw I can see "watchdog" being very useful to catch hard-RT tasks that exceed the deadline.
> But's it gotta be per thread.

It is.

> Single setting per user is not enough. Unless a use has a single RT task.

?

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