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    Subject[patch] idle_task was wasting a lot of CPU
    I am going to send you some of my code from my tree.

    The first pending patch is a fix for a bug I discovered last week. The
    patch avoid the idle task to continue to run if an useful process is being
    wakenup. It make a _big_ difference. Compiling the kernel a `xload' show
    that the CPU is used at 100% all the time (5/10% kernel and the other is
    gcc). Without this patch xload is far from doing a straight line in the
    high part of the graph.

    I have not benchmarked the time differences in a kernel compilation and
    see the numbers, any volunteers btw?

    Here the patch against 2.1.130:

    Index: linux/kernel/sched.c
    diff -u linux/kernel/sched.c: linux/kernel/sched.c:
    --- linux/kernel/sched.c: Fri Nov 27 11:19:09 1998
    +++ linux/kernel/sched.c Fri Nov 27 11:41:41 1998
    @@ -130,8 +133,15 @@
    - if (p->policy != SCHED_OTHER || p->counter > current->counter + 3)
    - current->need_resched = 1;
    + /*
    + * If the current process is the idle one, we must reschedule ASAP.
    + * Checking for p->counter >= current->counter we have the idle task
    + * check implicit.
    + * -arca
    + */
    + if (/* idle_task == current || */ p->counter >= current->counter ||
    + p->policy != SCHED_OTHER)
    + current->need_resched = 1;


    Note I decreased the limit to stop the running process to run a
    schedule() to improve iteractive performance. I don' t know the impact in
    the scheduling() increase doing this change, but the system feel more
    responsive. If you want to go 100% safe apply the patch and uncomment the
    idle_task check that is implicit in my current code (because for the
    idle task ->counter is always 0) and return to check for counter+3...

    Andrea Arcangeli

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