linux-kernel - Re: [PATCH] sched/rt: Unthrottle the highest RT task of the rq if there are no another available tasks to be picked

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Date:	Tue, 12 Feb 2013 13:15:14 +0100
From:	Mike Galbraith <bitbucket@...ine.de>
To:	Stanislav Meduna <stano@...una.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH] sched/rt: Unthrottle the highest RT task of the rq if
 there are no another available tasks to be picked

On Tue, 2013-02-12 at 09:12 +0100, Stanislav Meduna wrote: 
> On 12.02.2013 08:06, Mike Galbraith wrote:
> 
> >> In this case pick_next_task takes idle tasks and idle wastes cpu
> >> time.
> 
> > That's not a waste of CPU time, that's utilization enforcement the thing
> > it is designed to do.
> 
> Well this is a philosophical question and the opinions will IMHO
> vary strongly. If the throttling kicks in, the system already
> is in the out-of-spec state.

Exactly, please don't feed the wild eyed psychopaths ;-)

> Is the goal now just to allow
> e.g. the ssh login to be able to kill the task and still try
> to do the best if otherwise (possibly masking the problem for
> months), or is it to enforce the utilization?

Both.  It has two modes of enforcement, sane mode is I WILL constrain
this thing you turned loose should it acts up, and not so sane mode,
where borrowing a cup of CPU from the neighbors is ok.  Workqueues.

> For example we have a PLC software where the end-user develops
> an application that will be executed in our realtime task.
> The application usually has a longer initialization part where
> the excess utilization can happen and should be tolerated
> and the running part where it is a bug if it happens. Here
> I would prefer the throttling to alert the user, but not
> to actually throttle if there is no non-RT task actually
> wanting to run. In other cases I would maybe prefer even
> killing the task, alerting the user to the fact.

That's not in the throttles job description.  It's not a monitor and
report system, it's a constraint system for very dangerous beasts.

> I have a related question: is the information that the throttling
> happened available somewhere except the log (where it gets only
> written once)? If not, would a patch exporting the count
> of throttlings via /sys be accepted?

I'm not the maintainer, so can't say.  Seems to me a trace point would
be better though.

-Mike

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