linux-kernel - Re: -rt more realtime scheduling issues

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Date:	Mon, 8 Oct 2007 23:04:12 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Kravetz <kravetz@...ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: -rt more realtime scheduling issues

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.

-- Steve

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