Re: [RFC v2 4/7] change kernel accounting to include steal time

[Rik van Riel <riel@redhat.com>]

On 08/30/2010 06:56 PM, Jeremy Fitzhardinge wrote:
>   On 08/30/2010 12:45 PM, Rik van Riel wrote:
>> On 08/30/2010 03:20 PM, Peter Zijlstra wrote:
>>> On Mon, 2010-08-30 at 15:17 -0400, Rik van Riel wrote:
>>>>
>>>> When time is accounted as steal time, it is NOT accounted as
>>>> to the current process user/system/..., which in turn should
>>>> help it in the scheduler.
>>>>
>>>> Am I overlooking something?
>>>
>>> Yeah, the scheduler doesn't care about the user/system time accounting
>>> at all... :-)
>>
>> Uh oh. This would seem like something we'll want to fix
>> in an architecture independent way, so s390, etc. also
>> benefit from it.
>>
>> I can see this being a real problem when the host and guest
>> OS have the same time slice - which is quite possible since
>> they may both be the same version of Linux.
>>
>> Guest 1, alternating between processes A and B, may end up
>> with process A getting a lot of actual CPU time, and process
>> B being scheduled in when the VCPU itself is not running...
>
> Yep.  I'd been trying to do that with sched_clock games, but that never
> worked out.
>
> I think it basically comes down to adding "sched_clock_unstolen()" which
> the scheduler can use to measure time a process spends running, and
> sched_clock() for measuring sleep times.  In the normal case,
> sched_clock_unstolen() would be the same as sched_clock().

That requires the host to export (any time the guest is scheduled
in), the amount of CPU time the VCPU thread has used, and the time
the VCPU was scheduled in.

Since the VCPU must be running when it is examining these variables,
it can calculate the additional time (since it was last scheduled)
to account to the task, and remember the currently calculated time
in its own per-vcpu variable, so next time it can get a delta again.

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