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

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Date:	Tue, 09 Oct 2007 10:16:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mike Kravetz <kravetz@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: -rt more realtime scheduling issues

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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