linux-kernel - Re: [PATCH] cpu idle ticks show twice in xen pvm guest

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Date:	Sat, 17 Dec 2011 16:46:26 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Ferenc Wagner <wferi@...f.hu>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Zhenzhong Duan <zhenzhong.duan@...cle.com>,
	linux-x86_64@...r.kernel.org, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com
Subject: Re: [PATCH] cpu idle ticks show twice in xen pvm guest

On Fri, Dec 09, 2011 at 04:47:39PM +0100, Ferenc Wagner wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
> 
> >> On Wed, Oct 05, 2011 at 10:11:58PM -0700, Zhenzhong Duan wrote:
> >>
> >>> Run below test on xen pvm.
> >>> # x=$(cat /proc/stat | grep cpu0 | awk '{print $5}') && sleep 60  \
> >>> && y=$(cat /proc/stat | grep cpu0 | awk '{print $5}') \
> >>> && echo -e  "X:$x\nY:$y\nIDLE:" $(echo "scale=3; ($y-$x)/6000*100" | bc)
> >>>
> >>> @ X:58562301
> >>> @ Y:58574282
> >>> @ IDLE: 199.600
> >>>
> >>> Normal idle percent should be around 100%.
> >>> xen_timer_interrupt called account_idle_ticks to account hypervisor stolen idle ticks 
> >>> but these ticks will be accounted again when idle ticks restarted.
> >>>
> >>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
> >>> Signed-off-by: Joe Jin <joe.jin@...cle.com>
> >
> > Does this affect the accounting of stolen ticks?  If it does, that's not
> > necessarily a showstopper for this patch, but we'll need to do some more
> > thinking about it.  Certainly, accurate accounting for idleness is
> > important.
> 
> Please see also http://thread.gmane.org/gmane.linux.kernel/734441, where
> I found that the counter doubling isn't always present under 2.6.26.
> However, after going to 2.6.32 (Debian lenny-backports kernel, 4th of
> April on the graph below) that instability seems to disappear.  Please
> note that the following graph shows halved idle and iowait percentages.
> 

What happenend in Feb?

> 
> (I haven't collected steal values, so the numbers don't sum up to 100%.)
> I'd be grateful if this discrepancy could be cleared up eventually!
> It's heartening to see some progress after more than three years. :)
> 
> Actually, as Munin doesn't half the idle and iowait values, but
> truncates the (then overflowing) graph at 100%, I was rather surprised
> to see iowait completely disappear after the kernel upgrade, and
> concluded that it was somehow converted into buggy-looping in blkfront.
> Now I see this isn't the case, but the steadily increasing system CPU
> usage between reboots is still a mystery.  I'll start a separate thread
> for that, just wanted to provide some motivation for this topic.

Did you add more memory in the system?
> --
> Thanks,
> Feri.

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