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Subject: Re: [PATCH v3 0/3] KVM: Yield to IPI target if necessary
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CANRm+Cx6Z=jxLaXqwhBDpVTsKH8mgoo4iC=U8GbAAJz-5gk5ZA@mail.gmail.com>
Date:   Tue, 11 Jun 2019 18:37:36 -0700
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        =?utf-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>
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To:     Wanpeng Li <kernellwp@gmail.com>
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> On Jun 11, 2019, at 6:18 PM, Wanpeng Li <kernellwp@gmail.com> wrote:
>=20
> On Wed, 12 Jun 2019 at 00:57, Nadav Amit <nadav.amit@gmail.com> wrote:
>>> On Jun 11, 2019, at 3:02 AM, Wanpeng Li <kernellwp@gmail.com> wrote:
>>>=20
>>> On Tue, 11 Jun 2019 at 09:48, Nadav Amit <nadav.amit@gmail.com> =
wrote:
>>>>> On Jun 10, 2019, at 6:45 PM, Wanpeng Li <kernellwp@gmail.com> =
wrote:
>>>>>=20
>>>>> On Tue, 11 Jun 2019 at 09:11, Sean Christopherson
>>>>> <sean.j.christopherson@intel.com> wrote:
>>>>>> On Mon, Jun 10, 2019 at 04:34:20PM +0200, Radim Kr=C4=8Dm=C3=A1=C5=99=
 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).
>>>>>>>=20
>>>>>>> Have you checked if we could gain performance by having the =
yield as an
>>>>>>> extension to our PV IPI call?
>>>>>>>=20
>>>>>>> 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.)
>>>>>>=20
>>>>>> 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.
>>>>>=20
>>>>> Not sure why shorthand is not used for native x2apic mode.
>>>>=20
>>>> 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=E2=80=99m looking at that code right now=E2=80=A6)
>>>=20
>>> Please continue to follow the apic/x2apic driver. Just apic_flat set
>>> APIC_DEST_ALLBUT/APIC_DEST_ALLINC to ICR.
>>=20
>> Indeed - I was sure by the name that it does it correctly. That=E2=80=99=
s stupid.
>>=20
>> I=E2=80=99ll add it to the patch-set I am working on (TLB shootdown =
improvements),
>> if you don=E2=80=99t mind.
>=20
> 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.

I=E2=80=99ll recheck.=