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

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Date:   Mon, 10 Jun 2019 18:48:15 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Wanpeng Li <kernellwp@...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 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…)