linux-kernel - Re: [PATCH 0/4] Alter steal-time reporting in the guest

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Date:	Thu, 07 Mar 2013 16:34:16 -0600
From:	Michael Wolf <mjw@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, riel@...hat.com, gleb@...hat.com,
	kvm@...r.kernel.org, peterz@...radead.org, glommer@...allels.com,
	mingo@...hat.com, anthony@...emonkey.ws
Subject: Re: [PATCH 0/4] Alter steal-time reporting in the guest

On Thu, 2013-03-07 at 18:25 -0300, Marcelo Tosatti wrote:
> On Thu, Mar 07, 2013 at 03:15:09PM -0600, Michael Wolf wrote:
> > > 
> > > Makes sense?
> > > 
> > > Not sure what the concrete way to report stolen time relative to hard
> > > capping is (probably easier inside the scheduler, where run_delay is
> > > calculated).
> > > 
> > > Reporting the hard capping to the guest is a good idea (which saves the
> > > user from having to measure it themselves), but better done separately
> > > via new field.
> > 
> > didnt respond to this in the previous response.  I'm not sure I'm
> > following you here.  I thought this is what I was doing by having a
> > consigned (expected steal) field add to the /proc/stat output.  Are you
> > looking for something else or a better naming convention?
> 
> Expected steal is not a good measure to use (because as mentioned in the
> previous email there is no expected steal over a fixed period of time).
> 
> It is fine to report 'maximum percentage of underlying physical CPU'
> (what percentage of the physical CPU time guest VM is allowed to make
> use of).
> 
> And then steal time is relative to maximum percentage of underlying
> physical CPU time allowed.
> 

So last August I had sent out an RFC set of patches to do this.  That
patchset was meant to handle the general overcommit case as well as the
capping case by having qemu pass a percentage to the host that would
then be passed onto the guest and used to adjust the steal time.
Here is the link to the discussion
http://lkml.indiana.edu/hypermail/linux/kernel/1208.3/01458.html

As you will see there Avi didn't like the idea of a percentage down in
the guest, among other reasons he was concerned about migration.  Also
in the email thread you will see that Anthony Liguori was opposed to the
idea of just changing the steal time, he wanted it split out.

What Glauber has suggested and I am working on implementing is taking
out the timer and adding a last read field in the host.  So in the host
I can determine the total time that has passed and compute a percentage
and apply that percentage to the steal time while the info is still on
the host.  Then pass the steal and consigned time to the guest.

Does that address your concerns?

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