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    SubjectRe: [RFC 3/5] sched: Add CPU rate hard caps
    Con Kolivas wrote:
    > On Friday 26 May 2006 14:20, Peter Williams wrote:
    >> This patch implements hard CPU rate caps per task as a proportion of a
    >> single CPU's capacity expressed in parts per thousand.
    > A hard cap of 1/1000 could lead to interesting starvation scenarios where a
    > mutex or semaphore was held by a task that hardly ever got cpu. Same goes to
    > a lesser extent to a 0 soft cap.
    > Here is how I handle idleprio tasks in current -ck:
    > tags tasks that are holding a mutex
    > is the idleprio policy for staircase.
    > What it does is runs idleprio tasks as normal tasks when they hold a mutex or
    > are waking up after calling down() (ie holding a semaphore).

    I wasn't aware that you could detect those conditions. They could be
    very useful.

    > These two in
    > combination have shown resistance to any priority inversion problems in
    > widespread testing. An attempt was made to track semaphores held via a
    > down_interruptible() but unfortunately the lack of strict rules about who
    > could release the semaphore meant accounting was impossible of this scenario.
    > In practice, though there were no test cases that showed it to be an issue,
    > and the recent conversion en-masse of semaphores to mutexes in the kernel
    > means it has pretty much covered most possibilities.

    Peter Williams

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