Re: Realtime Preemption, 2.6.12, Beginners Guide?

[Alistair John Strachan <s0348365@sms.ed.ac.uk>]

On Wednesday 13 Jul 2005 16:30, Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
> > it worked upon the first try, and indeed my testbox crashed within 10
> > seconds:
> >
> >  BUG: Unable to handle kernel NULL pointer dereference
> >  BUG: Unable to handle kernel NULL pointer dereference at virtual address
> > 00000006
>
> a couple of crashes later i got an important clue:
>
>  BUG: bad soft irq-flag value 00000f64, openvpn/3386!
>   [<c0104052>] dump_stack+0x1f/0x21 (20)
>   [<c013b883>] check_soft_flags+0x73/0xc9 (24)
>   [<00000f78>] 0xf78 (1066836133)
>
> it turns out that a small portion of the softirq processing path was
> still using the soft IRQ-flag, instead of the raw IRQ-flag! Given enough
> irq and softirq workload, we were interrupted in a piece of code where
> the data structure was inconsistent. (tinfo.task was already changed,
> but %esp not yet) Since interrupts were enabled during the crash
> printout, it would crash again and again as it got more interrupts. The
> backtrace printout crashed too due to the inconsistency. That's why you
> got those repeat ============= lines.
>
> the patch below should fix this bug and i've uploaded the -51-30 patch
> with this fix included. Could you check whether 4K stacks are now stable
> for you under PREEMPT_RT?
>
> so your intuitition about this being related to 4K stacks was completely
> right.
>

Ingo,

This fixes the issue. It's one more 'crossed t' on the rt-preempt patches.

I'll let you know if I discover anything else; your work has already allowed 
us to bring the responsiveness of our instrument to 300us which is low enough 
for the real-time PCR industry. Thanks a lot!

This debugging process has been extremely eye opening, thanks for the detailed 
descriptions of what's gone wrong at every stage. I just wish I was competent 
enough to fix these things myself.

-- 
Cheers,
Alistair.

personal:   alistair()devzero!co!uk
university: s0348365()sms!ed!ac!uk
student:    CS/CSim Undergraduate
contact:    1F2 55 South Clerk Street,
            Edinburgh. EH8 9PP.
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