linux-kernel - Re: Inquiry: Should we remove "isolcpus= kernel boot option? (may have realtime uses)

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Date:	Thu, 5 Jun 2008 16:57:03 +0200
From:	Michael Trimarchi <trimarchimichael@...oo.it>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	fabio@...dalf.sssup.it, Max Krasnyansky <maxk@...lcomm.com>,
	Mark Hounschell <dmarkh@....rr.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Oeser <ioe-lkml@...eria.de>, Paul Jackson <pj@....com>,
	linux-kernel@...r.kernel.org, Con Kolivas <kernel@...ivas.org>,
	"Derek L. Fults" <dfults@....com>, devik <devik@....cz>,
	Dimitri Sivanich <sivanich@....com>,
	Dinakar Guniguntala <dino@...ibm.com>,
	Emmanuel Pacaud <emmanuel.pacaud@...v-poitiers.fr>,
	Frederik Deweerdt <deweerdt@...e.fr>,
	Ingo Molnar <mingo@...e.hu>,
	Matthew Dobson <colpatch@...ibm.com>, rostedt@...dmis.org,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Paul Menage <menage@...gle.com>,
	"Randy.Dunlap" <rddunlap@...l.org>, suresh.b.siddha@...el.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Dario <faggioli@...dalf.sssup.it>
Subject: Re: Inquiry: Should we remove "isolcpus= kernel boot option? (may have realtime uses)

> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Thu, Jun 05, 2008 02:07:40PM +0200
>
> On Thu, 2008-06-05 at 11:16 +0000, Michael Trimarchi wrote:
...
> > By now I'm facing some problems.  I still have not clear what
> > parameters a task forked from a sched_edf task should get, as it would
> > involve some form of admission control, 
> 
> I'd start with something like:
> 
> u64 sched_param::edf_period  [ns]
> u64 sched_param::edf_runtime [ns]
> 
> so that deadline = time_of_schedule + edf_period, and his allowance
> within that period is edf_runtime.
> 

This is what I'm doing right now (apart from using timespec structs
instead of u64 values to align the sched_param struct specified by
POSIX on systems with SCHED_SPORADIC support).

I'll clean up the code and post it here in the next few days.


> fork would inherit the parent's settings, and we'd need to do admission
> control on all tasks entering SCHED_EDF, either through setscheduler()
> or fork(). We could fail with -ENOTIME or somesuch error.
> 
> > and how to deal with tasks that run longer than their nominal
> > execution time (i.e., should we use some server mechanism to limit the
> > amount of cpu they're using, or handle that in some other way?)
> 
> Yeah - we already account the amount of runtime, we can send them
> SIGXCPU and stop running them. Look at the rt_bandwidth code upstream -
> it basically stops rt task groups from running once their quota is
> depleted - waking them up once it gets refreshed due to the period
> expiring.
> 
> For single tasks its easier, just account their time and dequeue them
> once it exceeds the quota, and enqueue them on a refresh timer thingy to
> start them again once the period rolls over.
> 

Ok, using the same mechanism even for SCHED_EDF tasks seems the
right way to go.


> The only tricky bit here is PI :-) it would need to keep running despite
> being over quota.
> 

There is some work in this area, and there are some protocols
handling that, but that simple solution will be a good starting
point.
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