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    SubjectRe: [patch, 2.6.11-rc2] sched: RLIMIT_RT_CPU_RATIO feature
    Ingo Molnar wrote:
    > pretty much the only criticism of the RT-CPU patch was that the global
    > sysctl is too rigid and that it doesnt allow privileged tasks to ignore
    > the limit. I've uploaded a new RT-CPU-limit patch that solves this
    > problem:
    >
    > http://redhat.com/~mingo/rt-limit-patches/
    >
    > i've removed the global sysctl and implemented a new rlimit,
    > RT_CPU_RATIO: the maximum amount of CPU time RT tasks may use, in
    > percent. For testing purposes it defaults to 80%.
    >
    > the RT-limit being an rlimit makes it much more configurable: root tasks
    > can have unlimited CPU time limit, while users could have a more
    > conservative setting of say 30%. This also makes it per-process and
    > runtime configurable as well. The scheduler will instantly act upon any
    > new RT_CPU_RATIO rlimit.
    >
    > (this approach is fundamentally different from the previous patch that
    > made the "maximum RT-priority available to an unprivileged task" value
    > an rlimit - with priorities being an rlimit we still havent made RT
    > priorities safe against deadlocks.)
    >
    > multiple tasks can have different rlimits as well, and the scheduler
    > interprets it the following way: it maintains a per-CPU "RT CPU use"
    > load-average value and compares it against the per-task rlimit. If e.g.
    > the task says "i'm in the 60% range" and the current average is 70%,
    > then the scheduler delays this RT task - if the next task has an 80%
    > rlimit then it will be allowed to run. This logic is straightforward and
    > can be used as a further control mechanism against runaway highprio RT
    > tasks.
    >
    > other properties of the RT_CPU_RATIO rlimit:
    >
    > - if there's idle time in the system then RT tasks will be
    > allowed to use more than the limit.
    >
    > - if an RT task goes above the limit all the time then there
    > is no guarantee that exactly the limit will be allowed for
    > it. (i.e. you should set the limit to somewhat above the real
    > needs of the RT task in question.)
    >
    > - a zero RLIMIT_RT_CPU_RATIO value means unlimited CPU time to that
    > RT task. If the task is not an RT task then it may not change to RT
    > priority. (i.e. a zero value makes it fully compatible with previous
    > RT scheduling semantics.)
    >
    > - a nonzero rt_cpu_limit value also has the effect of allowing
    > the use of RT priorities to nonprivileged users.
    >
    > - on SMP the limit is measured and enforced per-CPU.
    >
    > - runtime overhead is minimal, especially if the limit is set to 0.
    >
    > - the CPU-use measurement code has a 'memory' of roughly 300 msecs.
    > I.e. if an RT task runs 100 msecs nonstop then it will increase
    > its CPU use by about 30%. This should be fast enough for users for
    > the limit to be human-inperceptible, but slow enough to allow
    > occasional longer timeslices to RT tasks.

    As I understand this (and I may be wrong), the intention is that if a
    task has its RT_CPU_RATIO rlimit set to a value greater than zero then
    setting its scheduling policy to SCHED_RR or SCHED_FIFO is allowed.
    This causes me to ask the following questions:

    1. Why is current->signal->rlim[RLIMIT_RT_CPU_RATIO].rlim_cur being used
    in setscheduler() instead of p->signal->rlim[RLIMIT_RT_CPU_RATIO].rlim_cur?

    2. What stops a task that had a non zero RT_CPU_RATIO rlimit and changed
    its policy to SCHED_RR or SCHED_FIFO from then setting RT_CPU_RATIO
    rlimit back to zero and escaping the controls? As far as I can see
    (and, once again, I may be wrong) the mechanism for setting rlimits only
    requires CAP_SYS_RESOURCE privileges in order to increase the value.

    Peter
    --
    Peter Williams pwil3058@bigpond.net.au

    "Learning, n. The kind of ignorance distinguishing the studious."
    -- Ambrose Bierce
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