linux-kernel - Re: [PATCH] kthread: run kthreadd with max priority SCHED

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Date:	Sat, 22 Dec 2007 02:11:35 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jon Masters <jcm@...hat.com>
Cc:	Michal Schmidt <mschmidt@...hat.com>, linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
Subject: Re: [PATCH] kthread: run kthreadd with max priority SCHED_FIFO

On Sat, 22 Dec 2007 04:52:50 -0500 Jon Masters <jcm@...hat.com> wrote:

> > The general approach we've taken to this is "don't do that".  Yes, we could
> > boost lots of kernel threads in the way which this patch does but this
> > actually takes control *away* from userspace.  Userspace no longer has the
> > ability to guarantee itself minimum possible latency without getting
> > preempted by kernel threads.
> > 
> > And yes, giving userspace this minimum-latency capability does imply that
> > userspace has a responsibility to not 100% starve kernel threads.  It's a
> > reasonable compromise, I think?
> 
> So, user tasks running with SCHED_FIFO should be able to lock a system?

yup.  root can damage the system in all sorts of ways.

> I guess I see both sides of this argument - yes, it's userspace at
> fault, but in other cases when userspace is at fault, we take action
> (OOM, segfault, others). Isn't this situation just another case where
> the kernel needs to avoid the evils of userland going awry?

Well...  the problem is that if we add a safety net to catch run-away
SCHED_FIFO processes, we've permanently degraded the service which we
provide to well-behaved programs.

Should there be a watchdog which checks for a process which has run
realtime for a certain period and which then takes some action?  Such as
descheduling it for a while, generating warnings, demoting its policy,
killing it etc?
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