Re: [git pull] scheduler fixes

Ingo Molnar
2009-01-18T08:38:30Z

* Avi Kivity <avi@redhat.com> wrote:

> Ingo Molnar wrote:
>> * Mike Galbraith <efault@gmx.de> wrote:
>>
>>
>>> On Sat, 2009-01-17 at 04:43 -0800, Andrew Morton wrote:
>>>
>>>> http://bugzilla.kernel.org/show_bug.cgi?id=12465 just popped up - another
>>>> scheduler regression. It has been bisected.
>>>>
>>> Seems pretty clear. I'd suggest reverting it.
>>>
>>
>> We can revert it (and will revert it if no solution is found), but i'd
>> also like to understand why it happens, because that kind of regression
>> from this change is unexpected - we might be hiding some bug that could
>> pop up under less debuggable circumstances, so we need to understand it
>> while we have a chance.
>>
>> Below is the commit in question. Avi, any ideas what makes KVM special
>> here? Perhaps its use of "preempt notifiers" is causing a problem
>> somehow?
>>
>
> preempt notifiers use should cause additional context switch costs of a
> few thousand cycles and possible an IPI (if a vcpu was migrated). So
> I'd suspect scheduling latency here.
>
> Is it possible to trace this (the time between a wake up and actual
> scheduling of a task)?

Can you reproduce those latencies? We didnt get similar reports from
elsewhere so there seems to be a KVM angle.

Ingo