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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+CyZcvuT80ixp9f0FNmjN+rTUtw8MshtBG0Uk4L1B1UjDw@mail.gmail.com>
Date:   Tue, 11 Jun 2019 09:57:36 -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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References: <1559178307-6835-1-git-send-email-wanpengli@tencent.com>
 <20190610143420.GA6594@flask> <20190611011100.GB24835@linux.intel.com>
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 <CANRm+CyZcvuT80ixp9f0FNmjN+rTUtw8MshtBG0Uk4L1B1UjDw@mail.gmail.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
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> 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.

Indeed - I was sure by the name that it does it correctly. That=E2=80=99s =
stupid.

I=E2=80=99ll add it to the patch-set I am working on (TLB shootdown =
improvements),
if you don=E2=80=99t mind.