Re: [PATCH v3 0/3] KVM: Yield to IPI target if necessary

From: Nadav Amit
Date: Mon Jun 10 2019 - 21:52:45 EST

> On Jun 10, 2019, at 6:45 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> On Tue, 11 Jun 2019 at 09:11, Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> wrote:
>> On Mon, Jun 10, 2019 at 04:34:20PM +0200, Radim KrÄmÃÅ wrote:
>>> 2019-05-30 09:05+0800, Wanpeng Li:
>>>> The idea is from Xen, when sending a call-function IPI-many to vCPUs,
>>>> yield if any of the IPI target vCPUs was preempted. 17% performance
>>>> increasement of ebizzy benchmark can be observed in an over-subscribe
>>>> environment. (w/ kvm-pv-tlb disabled, testing TLB flush call-function
>>>> IPI-many since call-function is not easy to be trigged by userspace
>>>> workload).
>>> Have you checked if we could gain performance by having the yield as an
>>> extension to our PV IPI call?
>>> It would allow us to skip the VM entry/exit overhead on the caller.
>>> (The benefit of that might be negligible and it also poses a
>>> complication when splitting the target mask into several PV IPI
>>> hypercalls.)
>> Tangetially related to splitting PV IPI hypercalls, are there any major
>> hurdles to supporting shorthand? Not having to generate the mask for
>> ->send_IPI_allbutself and ->kvm_send_ipi_all seems like an easy to way
>> shave cycles for affected flows.
> Not sure why shorthand is not used for native x2apic mode.

Why do you say so? native_send_call_func_ipi() checks if allbutself
shorthand should be used and does so (even though the check can be more
efficient - Iâm looking at that code right nowâ)