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

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Date:	Tue, 9 Oct 2007 11:49:53 -0700
From:	Mike Kravetz <kravetz@...ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 11:04:12PM -0400, Steven Rostedt wrote:
> On Mon, Oct 08, 2007 at 11:45:23AM -0700, Mike Kravetz wrote:
> > 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).

The more I try understand the IPI handling the more confused I get. :(
At fist I was concerned about an IPI happening in the middle of the
__schedule routine.  But, then it occurred to me that interrupts are
disabled when in this routine (when holding the runqueue lock).  So, IPIs
are not delivered during __schedule processing.  Right?

But, if this is case then I don't understand the following code in
schedule():

        local_irq_disable();

        do {
                __schedule();
        } while (unlikely(test_thread_flag(TIF_NEED_RESCHED) ||
                          test_thread_flag(TIF_NEED_RESCHED_DELAYED)));

        local_irq_enable();

How can the reschedule flags possibly be set AFTER running __schedule.
Especially when the call is explicitly surrounded by local_irq_disable/
local_irq_enable.

Can someone help me?
-- 
Mike
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