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

[Avi Kivity <avi@redhat.com>]

  On 08/30/2010 03:42 PM, Glauber Costa wrote:
> On Sun, Aug 29, 2010 at 12:59:36PM +0300, Avi Kivity wrote:
>>   On 08/26/2010 12:43 AM, Glauber Costa wrote:
>>> This patch proposes a common steal time implementation. When no
>>> steal time is accounted, we just add a branch to the current
>>> accounting code, that shouldn't add much overhead.
>>>
>>> When we do want to register steal time, we proceed as following:
>>> - if we would account user or system time in this tick, and there is
>>>    out-of-cpu time registered, we skip it altogether, and account steal
>>>    time only.
>>> - if we would account user or system time in this tick, and we got the
>>>    cpu for the whole slice, we proceed normaly.
>>> - if we are idle in this tick, we flush out-of-cpu time to give it the
>>>    chance to update whatever last-measure internal variable it may have.
>>>
>>> This approach is simple, but proved to work well for my test scenarios.
>>> in a UP guest on UP host, with a cpu-hog in both guest and host shows
>>> ~ 50 % steal time. steal time is also accounted proportionally, if
>>> nice values are given to the host cpu-hog.
>>>
>>> A cpu-hog in the host with no load in the guest, produces 0 % steal time,
>>> with 100 % idle, as one would expect.
>>>
>> The scheduler people and lkml need to be copied on this patch.
>>
>> Since s390 does steal time (I think?), can this code be shared?
> AFAIK, s390 enables CONFIG_VIRT_CPU_ACCOUNTING, so all timings
> comes from the hypervisor, and statistical sampling is not involved.

Ok.  I see ppc does something similar as well (taking care of 
user/kernel transitions itself).

> We could do that, if our hardware had any method to say precisely
> how much time we spent in each state, which I don't think we do.

We don't, though I'm sure everyone is wondering why we can't have cheap 
accurate global clocks on x86.

> So in a summary, s390 is in a totally different ifdef side.

Yes.

> Who should we copy at the scheduler side?

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   Ingo Molnar <mingo@elte.hu>
   Peter Zijlstra <peterz@infradead.org>

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