Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

[Zachary Amsden <zach@vmware.com>]

Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
>
>   
>> no, that's not the case: next_timer_interrupt() is the NO_IDLE_HZ 
>> method of doing things - while in the NO_HZ case you are supposed to 
>> use clockevent devices to program timer hardware.
>>     

We don't have a clockevent device.  But we need NO_IDLE_HZ support, 
which NO_HZ has now subsumed.

> a proper CE device also has the added bonus of making high-res timers 
> guests work automatically. It should be simple: just pass it through to 
> your hypervisor, a hyper-CE-device, like a hyper-clocksource device has 
> essentially no guest-side complexity.
>   

It is not so simple.  In theory it works great.  In reality, the i386 
implementation is completely hardwired to work the way hardware works, 
and breaking the clockevent code out of the deep ties to the APIC is 
extremely non-trivial.  We tried, and could not accomplish it for 2.6.21 
because the hrtimers integration was complex, and introduced many bugs 
for us.  We worked around this by keeping NO_IDLE_HZ support, which now 
you deprecated.  So now we are using NO_HZ without a hyper-CE device, 
and it is working fine.  We understand the benefits of moving to the CE 
model - but it cannot be done overnight.

Xen has the same requirements for integrating their timer code.

Zach