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    SubjectRe: [PATCH] [request for inclusion] Realtime LSM
    utz lehmann wrote:
    > On Thu, 2005-01-13 at 16:25 -0500, Lee Revell wrote:
    >>On Thu, 2005-01-13 at 22:07 +0100, Arjan van de Ven wrote:
    >>>On Thu, Jan 13, 2005 at 03:04:26PM -0600, Jack O'Quin wrote:
    >>>>(Probably, this simplistic analysis misses some other, more subtle,
    >>>I think you can do nasty things to the locks held by those threads too
    >>>>RT threads should not do FS writes of their own. But, a badly broken
    >>>>or malicious one could, I suppose. So, that might provide a mechanism
    >>>>for losing more data than usual. Is that what you had in mind?
    >>>basically yes.
    >>>note that "FS writes" can come from various things, including library calls
    >>>made and such. But I think you got my point; even though it might seem a bit
    >>>theoretical it sure is unpleasant.
    >>I added Con to the cc: because this thread is starting to converge with
    >>an email discussion we've been having.
    >>The basic issue is that the current semantics of SCHED_FIFO seem make
    >>the deadlock/data corruption due to runaway RT thread issue difficult.
    >>The obvious solution is a new scheduling class equivalent to SCHED_FIFO
    >>but with a mechanism for the kernel to demote the offending thread to
    >>SCHED_OTHER in an emergency. The problem can be solved in userspace
    >>with a SCHED_FIFO watchdog thread that runs at a higher RT priority than
    >>all other RT processes.
    >>This all seems to imply that introducing an rlimit for MAX_RT_PRIO is an
    >>excellent solution. The RT watchdog thread could run as root, and the
    >>rlimit would be used to ensure than even nonroot users in the RT group
    >>could never preempt the watchdog thread.
    > Just an idea. What about throttling runaway RT tasks?
    > If the system spend more than 98% in RT tasks for 5s consider this as a
    > _fatal error_. Print an error message and throttle RT tasks by inserting
    > ticks where only SCHED_OTHER tasks allowed. For a limit of 98% this
    > means one SCHED_OTHER only tick all 50 ticks.
    > The limit and timeout should be configurable and of course it can be
    > disabled.
    > I know this is against RT task preempt all SCHED_OTHER but this is only
    > for a fatal system state to be able to recover sanely. A locked up
    > machine is is the worse alternative.

    There is a patch in -mm currently designed to use a sysrq key
    combination which converts all real time tasks to sched normal to save
    you if you desire in a lockup situation. We do want to preserve RT
    scheduling behaviour at all times without caveats for privileged users.

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