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    SubjectRe: sched /HT processor

    >From: Steven Rostedt <>
    >To: Jesper Juhl <>
    >CC: Arun Srinivas <>, LKML
    >Subject: Re: sched /HT processor
    >Date: Sun, 03 Apr 2005 11:31:03 -0400
    >On Sun, 2005-04-03 at 13:17 +0200, Jesper Juhl wrote:
    > >
    > > A reschedule can happen once every ms, but also upon returning to
    > > userspace and when returning from an interrupt handler, and also when
    > > something in the kernel explicitly calls schedule() or sleeps (which in
    > > turn results in a call to schedule()). And each CPU runs schedule()
    > > independently.
    > > At least that's my understanding of it - if I'm wrong I hope someone on
    > > the list will correct me.
    >You're correct, but I'll add some more details here. The actual
    >schedule happens when needed. A schedule may not take place at every
    >ms, if the task running is not done with its time slice and no events
    >happened where another task should preempt it. If an RT task is running
    >in a FIFO policy, then it will continue to run until it calls schedule
    >itself or another process of higher priority preempts it.
    >Now if you don't have PREEMPT turned on, than the schedule won't take
    >place at all while a task is in the kernel, unless the task explicitly
    >calls schedule.
    >What happens on a timer interrupt where a task is done with its time
    >slice or another event where a schedule should take place, is just the
    >need_resched flag is set for the task. On return from the interrupt the
    >flag is checked, and if set a schedule is called.
    >This is still a pretty basic description of what really happens, and if
    >you want to learn more, just start searching the kernel code for
    >schedule and need_resched. Don't forget to look in the asm code (ie
    >entry.S, and dependent on your arch other *.S files).
    >-- Steve

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