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

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Date:   Wed, 12 Jun 2019 09:18:51 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 0/3] KVM: Yield to IPI target if necessary

On Wed, 12 Jun 2019 at 00:57, Nadav Amit <nadav.amit@...il.com> wrote:
>
> > On Jun 11, 2019, at 3:02 AM, Wanpeng Li <kernellwp@...il.com> wrote:
> >
> > On Tue, 11 Jun 2019 at 09:48, Nadav Amit <nadav.amit@...il.com> wrote:
> >>> On Jun 10, 2019, at 6:45 PM, Wanpeng Li <kernellwp@...il.com> wrote:
> >>>
> >>> On Tue, 11 Jun 2019 at 09:11, Sean Christopherson
> >>> <sean.j.christopherson@...el.com> wrote:
> >>>> On Mon, Jun 10, 2019 at 04:34:20PM +0200, Radim Krčmář 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 - I’m looking at that code right now…)
> >
> > 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. That’s stupid.
>
> I’ll add it to the patch-set I am working on (TLB shootdown improvements),
> if you don’t 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. :)

Regards,
Wanpeng Li