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

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Message-ID: <CANRm+CyKx4uMkAe7maTg8nwBvkA7SCi2CnSwrghDs1nPuWNg5A@mail.gmail.com>
Date:   Fri, 28 Jun 2019 17:18:06 +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 Fri, 28 Jun 2019 at 17:12, Wanpeng Li <kernellwp@...il.com> wrote:
>
> On Wed, 12 Jun 2019 at 09:37, Nadav Amit <nadav.amit@...il.com> wrote:
> >
> > > On Jun 11, 2019, at 6:18 PM, Wanpeng Li <kernellwp@...il.com> wrote:
> > >
> > > 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. :)
> >
> > 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.

Hmm, should fallback to native shorthands when support.

Regards,
Wanpeng Li