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

[Wanpeng Li]

On Wed, 12 Jun 2019 at 09:37, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>
> > On Jun 11, 2019, at 6:18 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> >
> > On Wed, 12 Jun 2019 at 00:57, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
> >>> On Jun 11, 2019, at 3:02 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> >>>
> >>> On Tue, 11 Jun 2019 at 09:48, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
> >>>>> 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)
> >>>
> >>> Please continue to follow the apic/x2apic driver. Just apic_flat set
> >>> APIC_DEST_ALLBUT/APIC_DEST_ALLINC to ICR.
> >>
> >> Indeed - I was sure by the name that it does it correctly. Thatâs stupid.
> >>
> >> Iâll add it to the patch-set I am working on (TLB shootdown improvements),
> >> if you donât mind.
> >
> > Original for hotplug cpu safe.
> > https://lwn.net/Articles/138365/
> > https://lwn.net/Articles/138368/
> > Not sure shortcut native support is acceptable, I will play my
> > kvm_send_ipi_allbutself and kvm_send_ipi_all. :)
>
> Yes, I saw these threads before. But I think the test in
> native_send_call_func_ipi() should take care of it.

Good news, https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/log/?h=WIP.x86/ipi
Thomas who also is the hotplug state machine author introduces
shorthands support to native kernel now, I will add the support to
kvm_send_ipi_allbutself() and kvm_send_ipi_all() after his work
complete.

Regards,
Wanpeng Li