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    SubjectRe: Realtime Preemption, 2.6.12, Beginners Guide?
    Date
    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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