setting jiffies as the clocksource stops time

setting jiffies as the clocksource stops time

[Balaji Rao]

Hi,

When i set jiffies as the current_clocksource, date(1) tells me that wallclock time has stopped, and soon after that, the 
system becomes unresponsive. This is not seen with CONFIG_NO_HZ disabled.

This happens in 2.6.24-rc4,rc5, and even in rc8.

I wonder how can jiffies be used as a clocksource.. Its value depends on the tick and when we turn off ticks, we would stop 
incrementing jiffies and when we come come out of idle, we update the jiffies by reading the current_clocksource which now 
is 'jiffies', and hence jiffies wouldn't get updated. Could this be the explanation ?

I also encountered the same problem when i ran the kernel in qemu, and set pit as the current_clocksource. Why does this 
happen ?

So, i think should we unregister jiffies as a clocksource when we go into NO_HZ ?

Thomas, Ingo what are your thoughts on this ?

regards,
balaji rao

Re: setting jiffies as the clocksource stops time

[Daniel Walker]

On Thu, 2008-01-17 at 02:09 +0530, Balaji Rao wrote:
> Hi,
> 
> When i set jiffies as the current_clocksource, date(1) tells me that wallclock time has stopped, and soon after that, the 
> system becomes unresponsive. This is not seen with CONFIG_NO_HZ disabled.
> 
> This happens in 2.6.24-rc4,rc5, and even in rc8.
> 
> I wonder how can jiffies be used as a clocksource.. Its value depends on the tick and when we turn off ticks, we would stop 
> incrementing jiffies and when we come come out of idle, we update the jiffies by reading the current_clocksource which now 
> is 'jiffies', and hence jiffies wouldn't get updated. Could this be the explanation ?

Your right, It can't be used as a clocksource with nohz , and the system
will refuse to automatically switch to it ..

However, you can manually override the system, which is what your
doing .. That in turn forces the usage of an unstable clock which causes
your time and system not to function any more..

Daniel

Re: setting jiffies as the clocksource stops time

[Thomas Gleixner]

On Thu, 17 Jan 2008, Balaji Rao wrote:
> When i set jiffies as the current_clocksource, date(1) tells me that wallclock time has stopped, and soon after that, the 
> system becomes unresponsive. This is not seen with CONFIG_NO_HZ disabled.
> 
> This happens in 2.6.24-rc4,rc5, and even in rc8.

It happens since 2.6.21-rc1 on 32 bit.
 
> Thomas, Ingo what are your thoughts on this ?

http://lkml.org/lkml/2008/1/16/123

Thanks,
	tglx

Re: setting jiffies as the clocksource stops time

[Jan Engelhardt]

On Jan 16 2008 13:20, Daniel Walker wrote:
>On Thu, 2008-01-17 at 02:09 +0530, Balaji Rao wrote:
>> Hi,
>> 
>> When i set jiffies as the current_clocksource, date(1) tells me
>> that wallclock time has stopped, and soon after that, the system
>> becomes unresponsive. This is not seen with CONFIG_NO_HZ disabled.
>> 
>> I wonder how can jiffies be used as a clocksource.. Its value
>> depends on the tick and when we turn off ticks, we would stop
>> incrementing jiffies and when we come come out of idle, we update
>> the jiffies by reading the current_clocksource which now is
>> 'jiffies', and hence jiffies wouldn't get updated. Could this be
>> the explanation ?
>
>Your right, It can't be used as a clocksource with nohz , and the system
>will refuse to automatically switch to it ..

I think that manually changing to jiffies by echoing into sysfs
should also be prohibited.

Re: setting jiffies as the clocksource stops time

[Daniel Walker]

On Thu, 2008-01-17 at 23:34 +0100, Jan Engelhardt wrote:
> On Jan 16 2008 13:20, Daniel Walker wrote:
> >On Thu, 2008-01-17 at 02:09 +0530, Balaji Rao wrote:
> >> Hi,
> >> 
> >> When i set jiffies as the current_clocksource, date(1) tells me
> >> that wallclock time has stopped, and soon after that, the system
> >> becomes unresponsive. This is not seen with CONFIG_NO_HZ disabled.
> >> 
> >> I wonder how can jiffies be used as a clocksource.. Its value
> >> depends on the tick and when we turn off ticks, we would stop
> >> incrementing jiffies and when we come come out of idle, we update
> >> the jiffies by reading the current_clocksource which now is
> >> 'jiffies', and hence jiffies wouldn't get updated. Could this be
> >> the explanation ?
> >
> >Your right, It can't be used as a clocksource with nohz , and the system
> >will refuse to automatically switch to it ..
> 
> I think that manually changing to jiffies by echoing into sysfs
> should also be prohibited.

Yeah, I wasn't suggesting we shouldn't prevent it .. However, there are
tons of way to kill your system by manual changing things ..

Daniel

Re: setting jiffies as the clocksource stops time

[Balaji Rao]

On Friday 18 January 2008 04:04:33 am Jan Engelhardt wrote:
> 
> On Jan 16 2008 13:20, Daniel Walker wrote:
> >On Thu, 2008-01-17 at 02:09 +0530, Balaji Rao wrote:
> >> Hi,
> >> 
> >> When i set jiffies as the current_clocksource, date(1) tells me
> >> that wallclock time has stopped, and soon after that, the system
> >> becomes unresponsive. This is not seen with CONFIG_NO_HZ disabled.
> >> 
> >> I wonder how can jiffies be used as a clocksource.. Its value
> >> depends on the tick and when we turn off ticks, we would stop
> >> incrementing jiffies and when we come come out of idle, we update
> >> the jiffies by reading the current_clocksource which now is
> >> 'jiffies', and hence jiffies wouldn't get updated. Could this be
> >> the explanation ?
> >
> >Your right, It can't be used as a clocksource with nohz , and the system
> >will refuse to automatically switch to it ..
> 
> I think that manually changing to jiffies by echoing into sysfs
> should also be prohibited.
> 
Yea, right. But why not unregister jiffies as a clocksource itself when we get into NO_HZ ? I think it's much cleaner 
provided it has no other consequences.

Re: setting jiffies as the clocksource stops time

[Thomas Gleixner]

On Fri, 18 Jan 2008, Balaji Rao wrote:

> On Friday 18 January 2008 04:04:33 am Jan Engelhardt wrote:
> > 
> > On Jan 16 2008 13:20, Daniel Walker wrote:
> > >On Thu, 2008-01-17 at 02:09 +0530, Balaji Rao wrote:
> > >> Hi,
> > >> 
> > >> When i set jiffies as the current_clocksource, date(1) tells me
> > >> that wallclock time has stopped, and soon after that, the system
> > >> becomes unresponsive. This is not seen with CONFIG_NO_HZ disabled.
> > >> 
> > >> I wonder how can jiffies be used as a clocksource.. Its value
> > >> depends on the tick and when we turn off ticks, we would stop
> > >> incrementing jiffies and when we come come out of idle, we update
> > >> the jiffies by reading the current_clocksource which now is
> > >> 'jiffies', and hence jiffies wouldn't get updated. Could this be
> > >> the explanation ?
> > >
> > >Your right, It can't be used as a clocksource with nohz , and the system
> > >will refuse to automatically switch to it ..
> > 
> > I think that manually changing to jiffies by echoing into sysfs
> > should also be prohibited.
> > 
> Yea, right. But why not unregister jiffies as a clocksource itself when we get into NO_HZ ? I think it's much cleaner 
> provided it has no other consequences.

As I said before. I have a patch lined up for the same issue vs. PIT
clocksource and I'm adding that for jiffies as well. It's just not an
urgent issue, which needs to go into .24

Thanks

	tglx