Re: [REGRESSION] High, likely incorrect process cpu usage counters with kvm and 2.6.2[67]

On Sun, Aug 31, 2008 at 11:43 AM, Avi Kivity <avi@xxxxxxxxxxxx> wrote:
Running an idle Windows VM on Linux 2.6.26+ with kvm, one sees high values
for the kvm process in top (30%-70% cpu), where one would normally expect
0%-1%. Surprisingly, the per-cpu system counters show almost 100% idle,
leading me to believe this is an accounting error and that the process does
not actually consume this much cpu.

Busted process accounting - This looks the same as
http://bugzilla.kernel.org/show_bug.cgi?id=11209 .
Please verify. Peter's patch in latest git stops showing "incorrect
looking" CPU usage but at least the process times are still wrong,
horribly.
In fact the CPU usage thing in -rc5 is likely also incorrect but I
need to analyze that bit a little more.

From Today's Git -

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND

12961 parag 20 0 83000 8908 6628 R 0 0.1 5124415h npviewer.bin


I bisected this to a scheduler change, namely

commit 3e51f33fcc7f55e6df25d15b55ed10c8b4da84cd
Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Date: Sat May 3 18:29:28 2008 +0200

sched: add optional support for CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
this replaces the rq->clock stuff (and possibly cpu_clock()).
- architectures that have an 'imperfect' hardware clock can set
CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
- the 'jiffie' window might be superfulous when we update tick_gtod
before the __update_sched_clock() call in sched_clock_tick()
- cpu_clock() might be implemented as:
sched_clock_cpu(smp_processor_id())
if the accuracy proves good enough - how far can TSC drift in a
single jiffie when considering the filtering and idle hooks?
[ mingo@xxxxxxx: various fixes and cleanups ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>

That patch sounds like it had open questions?
Really giving this is a long standing bad regression, all the
offending patches should be reverted in absence of a fix, no?

Parag
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