linux-kernel - Re: [PATCH 6/6] sched: disabled rt-bandwidth by default

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Date:	Tue, 19 Aug 2008 21:17:51 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Stefani Seibold <stefani@...bold.net>,
	Dario Faggioli <raistlin@...ux.it>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 6/6] sched: disabled rt-bandwidth by default

On Tuesday 19 August 2008 21:05, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > Disable bandwidth control by default.
> >
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > ---
> >  kernel/sched.c |   17 +++++++----------
> >  1 file changed, 7 insertions(+), 10 deletions(-)
> >
> > Index: linux-2.6/kernel/sched.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/sched.c
> > +++ linux-2.6/kernel/sched.c
> > @@ -824,9 +824,9 @@ static __read_mostly int scheduler_runni
> >
> >  /*
> >   * part of the period that we allow rt tasks to run in us.
> > - * default: 0.95s
> > + * default: inf
> >   */
> > -int sysctl_sched_rt_runtime = 950000;
> > +int sysctl_sched_rt_runtime = -1;
>
> The fixes look good to me, but this enabling of infinite RT task lockups
> is not an improvement.
>
> The thing is, i got far more bugreports about locked up RT tasks where
> the lockup was unintentional, than real bugreports about anyone
> _intending_ for the whole box to come to a grinding halt because a
> high-prio RT tasks is monopolizing the CPU.

Why are all these people running poorly written apps then?

We don't cater to poorly at the expense of the properly written
code.


> In fact there's only been this artificial test so far.

No, someone reported that it broke their app.


> So could you please just increase the chunking to 10 seconds or so, from
> the current 1 second? Anyone locking up the system for more than 10
> seconds via an RT task has to deal with many other issues already.
>
> I.e. keep the system borderline debuggable (up to 10 seconds delays are
> _not_ nice so people will notice) - but it's still a marked improvement
> from completly locked up desktops.
>
> And those who really need longer than 10 second periods can set it
> higher, or even (if they want to live dangerously or run POSIX
> conformance tests) make it infinite (set it to -1) - and will have to
> deal with other things like the softlockup watchdog as well.
>
> Ok?

Nack. Let's retain our API specifications and backwards compatibilty
by default. Advertise the sysrq switch and the setting of the sysctl
to throttle, but don't break this by default please.
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