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    SubjectRe: RT patch acceptance

    * Andi Kleen <ak@muc.de> wrote:

    > > how would you do that, if even a first step (PREEMPT_VOLUNTARY) was
    > > opposed by some as possibly hurting throughput? I'm really curious, what
    > > would you do to improve PREEMPT_NONE's latencies?
    >
    > Mostly in the classical way. Add cond_resched where needed. Break up a
    > few locks. Perhaps convert a few spinlocks that block preemption to
    > long and which are not taken that often to a new kind of
    > sleep/spinlock (TBD). Then add more reschedule point again.

    been there, done that. A couple of years ago i started out with a
    somewhat similar opinion to yours, which could be summed up as: "this
    cannot be that hard, just break up the code, damnit". Wrote tools to see
    where the latencies come from, and started sticking cond_resched()s in.
    A few years down the road and after multiple restarts (lowlatency patch,
    the first preempt prototype patch, -VP patchset, etc.) i ended up with
    the -RT patch and with two new preemption models (PREEMPT_VOLUNTARY and
    PREEMPT_RT) in addition to PREEMPT_NONE and PREEMPT. (With the extra
    twist that when i started then the kernel was only 2 million lines big,
    now it's 6+ million lines of code.)

    Ingo
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