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    SubjectRe: [PATCH]sched: Isochronous class v2 for unprivileged soft rt scheduling
    utz lehmann wrote:
    > I had experimented with throttling runaway RT tasks. I use a similar
    > accounting. I saw a difference between counting with or without the
    > calling from fork. If i remember correctly the timeout expired too fast
    > if the non-RT load was "while /bin/true; do :; done".
    > With "while true; do :; done" ("true" is bash buildin) it worked good.
    > But maybe it's not important in the real world.

    It won't be relevant if we move to a sched_clock() based timesource
    anyway, which looks to be the next major development for this.

    > If i understand sched_clock correctly it only has higher resolution if
    > you can use tsc. In the non tsc case it's jiffies based. (On x86).
    > I think you can easily fool a timer tick/jiffies based accounting and do
    > a local DoS.

    The same timer is used for accounting of SCHED_NORMAL tasks, so if you
    can work around that you can DoS the system with even the correct
    combination of SCHED_NORMAL tasks. When Ingo implemented sched_clock all
    the architectures slowly came on board. If I recall correctly the lowest
    resolution one still had microsecond accuracy which is more than enough
    given the time a context switch takes.

    > Making SCHED_ISO privileged if you don't have a high resolution
    > sched_clock is ugly.
    > I really like the idea of a unprivileged SCHED_ISO but it has to be safe
    > for a multi user system. And the kernel default should be safe for multi
    > user.

    Agreed; this is exactly what this work is about.

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