From: Con Kolivas <kernel@kolivas.org>
To: Roberto Orenstein <rstein@brturbo.com>,
       pat erley <paterley@drunkencodepoets.com>
Subject: Re: patch-O1int-0306281420 for 2.5.73 interactivity
Date: Sun, 29 Jun 2003 11:32:49 +1000
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On Sun, 29 Jun 2003 08:45, Roberto Orenstein wrote:
> On Sat, 2003-06-28 at 17:26, pat erley wrote:
> > I made a small error when I sent Con a piece of magic I wrote up to help
> > the sleep period.
> >
> > what it says right now:
> >
> > /kernel/sched.c around line 325
> >
> >
> > sleep_period = (sleep_period *
> > 	17 * sleep_period / ((17 * sleep_period / (5 * tau) + 2) * 5 * tau));
> > ----------------------------------------------------------^
> >
> > it should be:
> >
> > sleep_period = (sleep_period *
> > 	17 * sleep_period / ((17 * sleep_period / (5 * tau + 2)) * 5 * tau));
> > --------------------------------------------------------------^
> >
> > stupid parenthesis.
> >
> > a little background.  what this essentially is is a taylor approximation
> > of the function ln(66x+1) normalized.  ln(66x+1) happens to do a great
> > job oas a weighting function on the range of 0 to 1, and because the
> > input only happens to range from 0 to 1, only 2 terms were needed to do a
> > 'good enough' job.
> >
> > Pat
>
> I did your correction and I got a kernel panic(attempting to kill init)
> on boot. It didn't flushed to disk, so it isn't attached, but it panics
> at effective_prio+0xcc/0xe0.
> With objdump I could see it traps a division by 0:
> cc:       f7 fb                   idiv   %ebx
>
> I remember cleary %ebx being 0 on the panic report.
> And I tracked down and the code is on this else in effective_prio:
> ----------- sched.c 341-----------------
> else {
> 	sleep_period = (sleep_period *
> 		17 * sleep_period / ((17 * sleep_period / (5 * tau + 2)) * 5 * tau));
> 	if (!sleep_period)
> 		return p->static_prio;
> 	}
> --------------------------------------
>
> I don't have the time now to track this further today, but what happens
> if sleep_period is too small and tau is too big?
>  Could this (17 * sleep_period / (5 * tau + 2) give 0 and so a division
> by 0?

This will give a divide by zero.

I will put together a new patch soon with this correction and appropriate 
logic.

Con

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