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SubjectRe: [kvm-devel] [BUG] Oops with KVM-27
Il Mon, Jun 11, 2007 at 10:44:45AM +0300, Avi Kivity ha scritto: 
> Luca wrote:
> >>
> >>I've managed to reproduce this on kvm-21 (it takes many boots for this
> >>to happen, but it does eventually).
> >
> >Hum, any clue on the cause?
>
> From what I've seen, it's the new Linux clocksource code.
>
> >Should I test older versions?
>
> They're unlikely to be better. Instead, it would be best to see what
> the guest is doing.

RCU is not working. Network initialization hangs because it happens to
be the first RCU user.
The guest is stuck waiting for RCU syncronization:

[ 4.992207] [<c04321c5>] synchronize_rcu+0x4e/0x80
[ 4.994379] [<c0431db5>] wakeme_after_rcu+0x0/0x8
[ 4.996521] [<c0599ad1>] synchronize_net+0x64/0x8c
[ 4.998678] [<c05d70e0>] inet_register_protosw+0xef/0x151
[ 5.000984] [<c072d79e>] inet_init+0x1cd/0x498

wait_for_completion() in synchronize_rcu() calls schedule() and the
completion is never signaled (wakeme_after_rcu is never called).
The completion AFAICS would be signaled via rcu_process_callbacks(),
which is called in tasklet context.
Scheduler and completion are working fine since they're used in other
part of the kernel without problems.

To recap:

i686 F7 kernel: always works.

i586 F7 kernel: sometime hangs due to RCU problems. When it does work
it's because the LAPIC is disabled on boot:

Using local APIC timer interrupts.
calibrating APIC timer ...
... lapic delta = 25745109
... PM timer delta = 0
..... delta 25745109
..... mult: 1105912110
..... calibration result: 4119217
..... CPU clock speed is 8794.0417 MHz.
..... host bus clock speed is 4119.0217 MHz.
... verify APIC timer
... jiffies delta = 103
APIC timer disabled due to verification failure.

When it doesn't work LAPIC passes the test:

[ 1.304717] Using local APIC timer interrupts.
[ 1.304719] calibrating APIC timer ...
[ 1.718823] ... lapic delta = 25251444
[ 1.720582] ... PM timer delta = 0
[ 1.722219] ..... delta 25251444
[ 1.723827] ..... mult: 1084706136
[ 1.725470] ..... calibration result: 4040231
[ 1.727374] ..... CPU clock speed is 8625.0780 MHz.
[ 1.729396] ..... host bus clock speed is 4040.0231 MHz.
[ 1.731540] ... verify APIC timer
[ 2.158342] ... jiffies delta = 102
[ 2.160035] ... jiffies result ok

i586 F7 kernel, with 'nolapic': always works.

Luca
--
Sbagliare è umano, ma per incasinare davvero le cose serve un computer.
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