KVM and rtc missing interupts 2.6.21-rc3

KVM and rtc missing interupts 2.6.21-rc3

[David Brown]

When ever I try and start a guest OS with kvm I get a lot of these rtc
missing interupt messages from the kernel

[  468.510878] rtc: lost some interrupts at 1024Hz.
[  468.530876] rtc: lost some interrupts at 1024Hz.
[  468.550868] rtc: lost some interrupts at 1024Hz.
[  468.570864] rtc: lost some interrupts at 1024Hz.
[  468.590856] rtc: lost some interrupts at 1024Hz.
[  468.610850] rtc: lost some interrupts at 1024Hz.
[  468.630844] rtc: lost some interrupts at 1024Hz.
[  468.650839] rtc: lost some interrupts at 1024Hz.
[  468.670832] rtc: lost some interrupts at 1024Hz.
[  468.690826] rtc: lost some interrupts at 1024Hz.
[  468.710819] rtc: lost some interrupts at 1024Hz.
[  468.730813] rtc: lost some interrupts at 1024Hz.
[  468.750807] rtc: lost some interrupts at 1024Hz.
[  468.770801] rtc: lost some interrupts at 1024Hz.
[  468.790795] rtc: lost some interrupts at 1024Hz.
[  468.810790] rtc: lost some interrupts at 1024Hz.
[  468.830783] rtc: lost some interrupts at 1024Hz.
[  468.850778] rtc: lost some interrupts at 1024Hz.
[  468.870774] rtc: lost some interrupts at 1024Hz.
[  468.890772] rtc: lost some interrupts at 1024Hz.
[  468.910760] rtc: lost some interrupts at 1024Hz.
[  468.930754] rtc: lost some interrupts at 1024Hz.

I don't have the dynamic ticker set or some of the other high
resolution timer things set

# CONFIG_TICK_ONESHOT is not set
# CONFIG_NO_HZ is not set
# CONFIG_HIGH_RES_TIMERS is not set

I do have the rtc group selected as modules though

CONFIG_RTC=m
CONFIG_GEN_RTC=m
CONFIG_GEN_RTC_X=y
...
#
# Real Time Clock
#
CONFIG_RTC_LIB=m
CONFIG_RTC_CLASS=m

#
# RTC interfaces
#
CONFIG_RTC_INTF_SYSFS=m
CONFIG_RTC_INTF_PROC=m
CONFIG_RTC_INTF_DEV=m
CONFIG_RTC_INTF_DEV_UIE_EMUL=y

#
# RTC drivers
#
CONFIG_RTC_DRV_CMOS=m
CONFIG_RTC_DRV_X1205=m
CONFIG_RTC_DRV_DS1307=m
CONFIG_RTC_DRV_DS1553=m
CONFIG_RTC_DRV_ISL1208=m
... and the rest of the drivers as modules

I have the latest kvm-16 and here's how I'm starting the guest OS.

KVER="2.6.9-42.EL"
INITRD_BASE="initrd"
INITRD_APPEND=".img"
APPEND="ro root=LABEL=/ 3 console=ttyS0"
DEV=hda

kvm -kernel "/boot/vmlinuz-${KVER}" \
    -initrd "/boot/${INITRD_BASE}-${KVER}${INITRD_APPEND}" \
    -append "${APPEND}" \
    -nographic -m 256 \
    -net nic,vlan0 \
    -net tap,vlan=0,ifname=tap1,script=no \
    /dev/hda


Anymore information can be easily provided.

Thanks,
- David Brown

Re: KVM and rtc missing interupts 2.6.21-rc3

[Roland Dreier]

 > When ever I try and start a guest OS with kvm I get a lot of these rtc
 > missing interupt messages from the kernel
 > 
 > [  468.510878] rtc: lost some interrupts at 1024Hz.

I started to debug this a little while ago but I never got too far.
However it doesn't seem connected with kvm -- it is something to do
with tun/tap or bridging holding a lock too long I think.

Try running your guest with -no-kvm (and even with the kvm module not
loaded, just to be sure).  In my case I still saw the messages.
However, removing the "-net tap" line from my command line did get rid
of the messages.

 - R.

Re: KVM and rtc missing interupts 2.6.21-rc3

[David Brown]

> I started to debug this a little while ago but I never got too far.
> However it doesn't seem connected with kvm -- it is something to do
> with tun/tap or bridging holding a lock too long I think.
>
> Try running your guest with -no-kvm (and even with the kvm module not
> loaded, just to be sure).  In my case I still saw the messages.
> However, removing the "-net tap" line from my command line did get rid
> of the messages.

Hmmm I'm getting the same thing on 2.6.20.1 as well.

- David Brown

Re: KVM and rtc missing interupts 2.6.21-rc3

[Lee Revell]

On 3/8/07, Roland Dreier <rdreier@cisco.com> wrote:
>  > When ever I try and start a guest OS with kvm I get a lot of these rtc
>  > missing interupt messages from the kernel
>  >
>  > [  468.510878] rtc: lost some interrupts at 1024Hz.
>
> I started to debug this a little while ago but I never got too far.
> However it doesn't seem connected with kvm -- it is something to do
> with tun/tap or bridging holding a lock too long I think.

Holding a lock too long should be easy to debug with the -rt patch -
set the preemption related options to match mainline, and enable the
latency tracer.

Lee

Re: KVM and rtc missing interupts 2.6.21-rc3

[Roland Dreier]

 > > Try running your guest with -no-kvm (and even with the kvm module not
 > > loaded, just to be sure).  In my case I still saw the messages.
 > > However, removing the "-net tap" line from my command line did get rid
 > > of the messages.
 > 
 > Hmmm I'm getting the same thing on 2.6.20.1 as well.

With or without kvm?

 - R.

Re: KVM and rtc missing interupts 2.6.21-rc3

[Roland Dreier]

 > Holding a lock too long should be easy to debug with the -rt patch -
 > set the preemption related options to match mainline, and enable the
 > latency tracer.

Good idea, I'll give it a shot when I get a chance.