Re: [PATCH] x86: add cpuidle_kvm driver to allow guest side halt polling

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, May 20, 2019 at 03:46:50PM +0200, Christian Borntraeger wrote:
> 
> 
> On 20.05.19 14:07, Christian Borntraeger wrote:
> > 
> > 
> > On 20.05.19 13:51, Paolo Bonzini wrote:
> >> On 17/05/19 19:48, Marcelo Tosatti wrote:
> >>>
> >>> The cpuidle_kvm driver allows the guest vcpus to poll for a specified
> >>> amount of time before halting. This provides the following benefits
> >>> to host side polling:
> >>>
> >>> 	1) The POLL flag is set while polling is performed, which allows
> >>> 	   a remote vCPU to avoid sending an IPI (and the associated
> >>>  	   cost of handling the IPI) when performing a wakeup.
> >>>
> >>> 	2) The HLT VM-exit cost can be avoided.
> >>>
> >>> The downside of guest side polling is that polling is performed
> >>> even with other runnable tasks in the host.
> >>>
> >>> Results comparing halt_poll_ns and server/client application
> >>> where a small packet is ping-ponged:
> >>>
> >>> host                                        --> 31.33	
> >>> halt_poll_ns=300000 / no guest busy spin    --> 33.40	(93.8%)
> >>> halt_poll_ns=0 / guest_halt_poll_ns=300000  --> 32.73	(95.7%)
> >>>
> >>> For the SAP HANA benchmarks (where idle_spin is a parameter 
> >>> of the previous version of the patch, results should be the
> >>> same):
> >>>
> >>> hpns == halt_poll_ns
> >>>
> >>>                           idle_spin=0/   idle_spin=800/	   idle_spin=0/
> >>> 			  hpns=200000    hpns=0            hpns=800000
> >>> DeleteC06T03 (100 thread) 1.76           1.71 (-3%)        1.78	  (+1%)
> >>> InsertC16T02 (100 thread) 2.14     	 2.07 (-3%)        2.18   (+1.8%)
> >>> DeleteC00T01 (1 thread)   1.34 		 1.28 (-4.5%)	   1.29   (-3.7%)
> >>> UpdateC00T03 (1 thread)	  4.72		 4.18 (-12%)	   4.53   (-5%)
> >>
> >> Hi Marcelo,
> >>
> >> some quick observations:
> >>
> >> 1) This is actually not KVM-specific, so the name and placement of the
> >> docs should be adjusted.
> >>
> >> 2) Regarding KVM-specific code, however, we could add an MSR so that KVM
> >> disables halt_poll_ns for this VM when this is active in the guest?
> > 
> > The whole code looks pretty much architecture independent. I have also seen cases
> > on s390 where this kind of code would make sense. Can we try to make this
> > usable for other archs as well?
> 
> I did a quick hack (not yet for  the list as it contains some uglyness).
> and the code seems to run ok on s390. 
> So any chance to move this into drivers/cpuidle/ so that !x86 can also enable that
> when appropriate?

Done that, but you have to provide default_idle function 
and then later change:

+config HALTPOLL_CPUIDLE
+       tristate "Halt poll cpuidle driver"
+       depends on X86
+       default y
+       help

> I actually agree with Paolo that we should disable host halt polling as soon as
> the guest does it. Maybe we should have some arch specific callback (that can be
> an MSR).

Yep.