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    SubjectRe: upstream regression (IO-APIC?)
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    On Sunday 02 November 2008, Bartlomiej Zolnierkiewicz wrote:
    > On Thursday 30 October 2008, Robert Hancock wrote:
    > > Bartlomiej Zolnierkiewicz wrote:
    > > > The current Linus tree as of commit e946217e4fdaa67681bbabfa8e6b18641921f750
    > > > is broken for me. I get either the following panic (see log from qemu below)
    > > > or lost IRQs on ATA init... Is this a known issue?
    > > >
    > > > PS The tree that I used before and was supposedly good (sorry, I'm too tired
    > > > to verify it now) had commit 57f8f7b60db6f1ed2c6918ab9230c4623a9dbe37 at head.
    >
    > Unfortunately 57f8f7b60db6f1ed2c6918ab9230c4623a9dbe37 (v2.6.28-rc1)
    > is also bad. Bisecting it further was a real pain (i.e. I hit broken
    > build with x86 irqbalance changes, broken build with netfilter nat
    > changes and jbd journal problem). In the end it turned out that 2.6.27
    > is bad too! However with 2.6.27 the panic occurs only once per several
    > attempts and if there is no panic kernel boots normally (no lost IRQs).
    >
    > [...]
    >
    > I finally managed to narrow it down to change making x86 use tsc_khz
    > for loops_per_jiffy -- commit 3da757daf86e498872855f0b5e101f763ba79499
    > ("x86: use cpu_khz for loops_per_jiffy calculation"). This approach
    > seems too simplistic (as I see now Arjan & Pavel expressed concerns
    > about it back when the patch was posted initially [1][2]). Also it
    > would probably be preferred to re-use existing preset_lpj variable
    > (just like KVM does it for similar purpose [3]) instead of adding a
    > lpj_tsc one and increasing complexity.

    It turned out that I can boot a kernel with different config with
    HZ == 250 just fine and switching to HZ == 1000 makes it fail.


    Looking into it some more:

    HZ == 250 kernel (good):

    Calibrating delay loop (skipped), value calculated using timer frequency.. 2986.79 BogoMIPS (lpj=5973580)

    HZ == 1000 kernel (bad):

    Calibrating delay loop (skipped), using tsc calculated value.. 2990.35 BogoMIPS (lpj=1495176)

    HZ == 1000 kernel with hackyfix (good):

    Calibrating delay using timer specific routine.. 3016.68 BogoMIPS (lpj=6033376)


    Argggh... lpj is used for udelay() & friends so this bug is quite
    dangerous (since udelay() & friends are used for hardware delays)...

    [ The commit works for HZ == 250 because it does tsc_khz * 1000 / HZ,
    tsc_khz * 4 => lpj assumption holds true and there is no frequency
    scaling at boot. ]

    The quick fix would be to replace 1000 / HZ by the magic number "4"
    but the major question is whether can we reliably depend on the tsc_khz
    for lpj?

    Thanks,
    Bart


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