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    SubjectRe: -rt more realtime scheduling issues
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    On Mon, 2007-10-08 at 23:04 -0400, Steven Rostedt wrote:
    > On Mon, Oct 08, 2007 at 11:45:23AM -0700, Mike Kravetz wrote:
    > > On Fri, Oct 05, 2007 at 07:15:48PM -0700, Mike Kravetz wrote:
    > > > After applying the fix to try_to_wake_up() I was still seeing some large
    > > > latencies for realtime tasks.
    > >
    > > I've been looking for places in the code where reschedule IPIs should
    > > be sent in the case of 'overload' to redistribute RealTime tasks based
    > > on priority. However, an even more basic question to ask might be: Are
    > > the use of reschedule IPIs reliable enough for this purpose. In the
    > > code, there is the following comment:
    > >
    > > /*
    > > * this function sends a 'reschedule' IPI to another CPU.
    > > * it goes straight through and wastes no time serializing
    > > * anything. Worst case is that we lose a reschedule ...
    > > */
    > >
    > > After a quick read of the code, it does appear that reschedule's can
    > > be lost if the the IPI is sent at just the right time in schedule
    > > processing. Can someone confirm this is actually the case?
    > >
    > > The issue I see is that the 'rt_overload' mechanism depends on reschedule
    > > IPIs for RealTime scheduling semantics. If this is not a reliable
    > > mechanism then this can lead to breakdowns in RealTime scheduling semantics.
    > >
    > > Are these accurate statements? I'll start working on a reliable delivery
    > > mechanism for RealTime scheduling. But, I just want to make sure that
    > > is really necessary.
    >
    > For i386 I don't think so. Seems that the interrupt handler will set the
    > current task to "need_resched" and on exit of the interrupt handler, the
    > schedule should take place. I don't see the race (that doesn't mean
    > there is one).
    >
    > For x86_64 though, I don't think that we schedule. All the reschedule
    > vector does is return with a comment:
    >
    > /*
    > * Reschedule call back. Nothing to do,
    > * all the work is done automatically when
    > * we return from the interrupt.
    > */
    > asmlinkage void smp_reschedule_interrupt(void)
    > {
    > ack_APIC_irq();
    > }
    >
    > I'm thinking that this was the case for i386 a while back, and we fixed
    > it for RT.
    >
    > /me does a quick search...
    >
    > http://lkml.org/lkml/2005/5/13/174
    >
    > Yep! This is a bug in x86_64. I'll fix this up tomorrow and send out a
    > patch.

    Hmm, my understanding is that the IPI caller needs to set
    TIF_NEED_RESCHED before issuing the IPI.

    So I'm inclined to not like this 'fix'.

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