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Re: [Xen-devel] A clocksource question

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] A clocksource question
From: Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>
Date: Fri, 12 Mar 2010 22:01:45 +0100
Cc: Tobias Geiger <tobias.geiger@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On 03/12/2010 11:02 PM, Jeremy Fitzhardinge wrote:
> On 03/12/2010 01:56 PM, Tobias Geiger wrote:
>> kbpc2:~# cat
>> /sys/devices/system/clocksource/clocksource0/current_clocksource
>> xen
>> kbpc2:~# cat
>> /sys/devices/system/clocksource/clocksource0/available_clocksource
>> xen
>> kbpc2:~# echo "jiffies"
>>> /sys/devices/system/clocksource/clocksource0/current_clocksource
>> kbpc2:~# dmesg  | tail -1
>> [ 7898.642404] Override clocksource jiffies is not HRT compatible.
>> Cannot switch
>> while in HRT/NOHZ mode
>> kbpc2:~#
>> Seems like it's not possible to switch to jiffies - however:
>> kbpc2:~# zcat /proc/config.gz | grep NO_HZ
>> # CONFIG_NO_HZ is not set
> Interesting.  BTW, does NO_HZ change the behaviour of this?  In
> principle NO_HZ should be preferable, and particularly in a virtual
> machine since it should limit the number of timer interrupts.
> Joanna, are you using NO_HZ?
Yes, I have:

BTW, FWIW, I'm attaching my kernel config. It's almost the same as the
previously mentioned "Young's Dec23" kernel (the only difference being
some more recent pvops git patches applied and pcifront being enabled as
a module).

While trying to switch from xen source to jiffies source, I got the same
message as Tobias:

jiffies clocksource is not HRT compatible. Cannot switch while in

I can definitely say that this previously mentioned warning from hrtimer
about interrupt being too slow is the marking of the start of all the
troubles. E.g. when I run my Dom0 for some time without starting any
other VMs, the system behaves good, and no warning in dmesg. Only after
I start a VM, and probably depending on how heavy load it produces, the
warning appears in Dom0 dmesg and the problem becomes "feelable". And,
depending on what new "delta" is being chosen, the system is either
still usable -- e.g. with a delta like in this case:

hrtimer: interrupt too slow, forcing clock min delta to 82595226 ns

which is around 80ms (still the very subtle hiccups can be observed when
you press and hold a key for some time), or totally unusable, if one is
unlucky, and the "safe" delta got chosen to be something around 0.5s
(which interestingly happens most often in my case).

The "clocksource tsc unstable" messages trouble me too BTW. They seem to
appear before the hrtimer "interrupt too slow" messages.


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