Date: Fri, 28 Jan 2005 09:40:49 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "Jack O'Quin" <joq@io.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>,
       Paul Davis <paul@linuxaudiosystems.com>,
       Con Kolivas <kernel@kolivas.org>, linux <linux-kernel@vger.kernel.org>,
       rlrevell@joe-job.com, CK Kernel <ck@vds.kolivas.org>,
       utz <utz@s2y4n2c.de>, Andrew Morton <akpm@osdl.org>, alexn@dsv.su.se,
       Rui Nuno Capela <rncbc@rncbc.org>, Chris Wright <chrisw@osdl.org>,
       Arjan van de Ven <arjanv@redhat.com>
Subject: Re: [patch, 2.6.11-rc2] sched: RLIMIT_RT_CPU_RATIO feature
Message-ID: <20050128084049.GA5004@elte.hu>
References: <87hdl940ph.fsf@sulphur.joq.us> <20050124085902.GA8059@elte.hu> <20050124125814.GA31471@elte.hu> <20050125135613.GA18650@elte.hu> <87sm4opxto.fsf@sulphur.joq.us> <20050126070404.GA27280@elte.hu> <87fz0neshg.fsf@sulphur.joq.us> <1106782165.5158.15.camel@npiggin-nld.site> <20050128080802.GA2860@elte.hu> <871xc62bot.fsf@sulphur.joq.us>
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* Jack O'Quin <joq@io.com> wrote:

> > i'm wondering, couldnt Jackd solve this whole issue completely in
> > user-space, via a simple setuid-root wrapper app that does nothing else
> > but validates whether the user is in the 'jackd' group and then keeps a
> > pipe open to to the real jackd process which it forks off, deprivileges
> > and exec()s? Then unprivileged jackd could request RT-priority changes
> > via that pipe in a straightforward way. Jack normally gets installed as
> > root/admin anyway, so it's not like this couldnt be done.
> 
> Perhaps.
> 
> Until recently, that didn't work because of the longstanding rlimits
> bug in mlockall().  For scheduling only, it might be possible.
> 
> Of course, this violates your requirement that the user not be able to
> lock up the CPU for DoS.  The jackd watchdog is not perfect.

there is a legitimate fear that if it's made "too easy" to acquire some
sort of SCHED_FIFO priority, that an "arm's race" would begin between
desktop apps, each trying to set themselves to SCHED_FIFO (or SCHED_ISO)
and advising users to 'raise the limit if they see delays' - just to get
snappier than the rest.

thus after a couple of years we'd end up with lots of desktop apps
running as SCHED_FIFO, and latency would go down the drain again.

(yeah, this feels like going back to the drawing board.)

	Ingo
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