Re: [kvm-devel] [BUG] Oops with KVM-27

[Luca Tettamanti <kronos.it@gmail.com>]

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.