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    SubjectRe: upstream regression (IO-APIC?)
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    On Sun, 2008-11-02 at 12:24 -0800, Bartlomiej Zolnierkiewicz wrote:
    > 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"

    That's not right, the magic number 4 thing would not be correct.
    On one of my systems for eg, i get this in dmesg

    Detected 2010.400 MHz processor.
    ...
    Calibrating delay using timer specific routine.. 4022.47 BogoMIPS
    (lpj=2011235)

    This is with an earlier kernel, the HZ value is 1000. And the lpj value
    that we get from the calculation of (tsc_khz * 1000)/HZ is correct in
    this case. And on all the systems that i have checked this assumption
    holds true.

    One of the things that i suspect is that you are not using delay_tsc in
    this case, i.e. tsc is not used for delay which is causing that panic

    can you please try the patch below on your system ?

    [test-patch]

    Index: linux-2.6/arch/x86/kernel/tsc.c
    ===================================================================
    --- linux-2.6.orig/arch/x86/kernel/tsc.c 2008-10-15 10:51:14.000000000 -0700
    +++ linux-2.6/arch/x86/kernel/tsc.c 2008-11-03 09:43:01.000000000 -0800
    @@ -847,10 +847,6 @@
    cpu_khz = calibrate_cpu();
    #endif

    - lpj = ((u64)tsc_khz * 1000);
    - do_div(lpj, HZ);
    - lpj_fine = lpj;
    -
    printk("Detected %lu.%03lu MHz processor.\n",
    (unsigned long)cpu_khz / 1000,
    (unsigned long)cpu_khz % 1000);
    @@ -871,6 +867,10 @@
    tsc_disabled = 0;

    use_tsc_delay();
    + lpj = ((u64)tsc_khz * 1000);
    + do_div(lpj, HZ);
    + lpj_fine = lpj;
    +
    /* Check and install the TSC clocksource */
    dmi_check_system(bad_tsc_dmi_table);
    check_system_tsc_reliable();

    > but the major question is whether can we reliably depend on the tsc_khz
    > for lpj?

    If the patch above doesn't help, I think the answer to your question is
    - not on some particular hardware, but we would know.
    Btw, what h/w are you running this on ?

    Thanks,
    Alok
    >
    > Thanks,
    > Bart



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