linux-kernel - Re: [PATCH] proc: Add RLIMIT_RTTIME to /proc/<pid>/limits

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Date:	Fri, 11 Apr 2008 11:01:25 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Michael Kerrisk <mtk.manpages@...glemail.com>
Cc:	Michael Kerrisk <mtk.manpages@...il.com>,
	Eugene Teo <eugeneteo@...nel.sg>, linux-kernel@...r.kernel.org,
	Neil Horman <nhorman@...driver.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] proc: Add RLIMIT_RTTIME to /proc/<pid>/limits

On Fri, 2008-04-11 at 10:56 +0200, Michael Kerrisk wrote:
> On Thu, Feb 28, 2008 at 5:21 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >
> > On Thu, 2008-02-28 at 16:12 +0100, Michael Kerrisk wrote:
> > > Peter,
> > >
> > > Could you please provide some text describing RLIMIT_RTTIMEfor the
> > > getrlimit.2 man page.
> >
> > The rlimit sets a timeout in [us] for SCHED_RR and SCHED_FIFO tasks.
> > This time is measured between sleeps, so a schedule in RR or a
> > preemption in either is not a sleep - the task needs to be dequeued and
> > enqueued for the timer to reset.
> >
> > Upon reaching the cur limit we start giving SIGXCPU every second, upon
> > reaching the hard limit we give SIGKILL - matching RLIMIT_CPU.
> >
> > Time is measured in tick granularity (for now).
> 
> So I have another question: why is the granularity of this rlimit
> microseconds?  On the one hand, specifying limits down at the
> microsecond level seems (to my naive eye) unlikely to be useful.  (But
> perhaps I have missed a thread where this was explained.)  On the
> other hand, it means that on 32-bit the largest time limit we can set
> is ~4000 seconds, and I wonder if there are scenarios where it might
> be useful to have larger limits than that.
> 
> Why not, for example, have a granularity of milliseconds?

The us scale seemed the best fit in that it allows sub-ms granularity
while still allowing for quite long periods too. I'd preferred ns scale
as that is what we use throughout the scheduler where possible - but
that seemed too restrictive at the high end.

No real hard arguments either way.

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