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

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Message-ID: <CANRm+CyZcvuT80ixp9f0FNmjN+rTUtw8MshtBG0Uk4L1B1UjDw@mail.gmail.com>
Date:   Tue, 11 Jun 2019 18:02:25 +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 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.

Regards,
Wanpeng Li