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.
