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From: Wanpeng Li <kernellwp@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Jim Mattson" <jmattson@google.com>,
	"Joerg Roedel" <joro@8bytes.org>
Subject: Re: [PATCH 1/2] KVM: X86: Single target IPI fastpath
Date: Wed, 13 Nov 2019 14:05:59 +0800	[thread overview]
Message-ID: <CANRm+CzLvyWswEX1UDnESSLHO5xt2wPciL6b=TTr-ua7yKZSTA@mail.gmail.com> (raw)
In-Reply-To: <CANRm+CzK_h2E9XWFipkNpAALLCBcM2vrUkdBpumwmT9AP09hfA@mail.gmail.com>

On Tue, 12 Nov 2019 at 09:33, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> On Tue, 12 Nov 2019 at 05:59, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > On 09/11/19 08:05, Wanpeng Li wrote:
> > > From: Wanpeng Li <wanpengli@tencent.com>
> > >
> > > This patch tries to optimize x2apic physical destination mode, fixed delivery
> > > mode single target IPI by delivering IPI to receiver immediately after sender
> > > writes ICR vmexit to avoid various checks when possible.
> > >
> > > Testing on Xeon Skylake server:
> > >
> > > The virtual IPI latency from sender send to receiver receive reduces more than
> > > 330+ cpu cycles.
> > >
> > > Running hackbench(reschedule ipi) in the guest, the avg handle time of MSR_WRITE
> > > caused vmexit reduces more than 1000+ cpu cycles:
> > >
> > > Before patch:
> > >
> > >   VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time
> > > MSR_WRITE    5417390    90.01%    16.31%      0.69us    159.60us    1.08us
> > >
> > > After patch:
> > >
> > >   VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time
> > > MSR_WRITE    6726109    90.73%    62.18%      0.48us    191.27us    0.58us
> >
> > Do you have retpolines enabled?  The bulk of the speedup might come just
> > from the indirect jump.
>
> Adding 'mitigations=off' to the host grub parameter:
>
> Before patch:
>
>     VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time
> MSR_WRITE    2681713    92.98%    77.52%      0.38us     18.54us
> 0.73us ( +-   0.02% )
>
> After patch:
>
>     VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time
> MSR_WRITE    2953447    92.48%    62.47%      0.30us     59.09us
> 0.40us ( +-   0.02% )
>
> Actually, this is not the first attempt to add shortcut for MSR writes
> which performance sensitive, the other effort is tscdeadline timer
> from Isaku Yamahata, https://patchwork.kernel.org/cover/10541035/ ,
> ICR and TSCDEADLINE MSR writes cause the main MSR write vmexits in our
> product observation, multicast IPIs are not as common as unicast IPI
> like RESCHEDULE_VECTOR and CALL_FUNCTION_SINGLE_VECTOR etc. As far as
> I know, something similar to this patch has already been deployed in
> some cloud companies private kvm fork.

Hi Paolo,

Do you think I should continue for this?

    Wanpeng

  reply	other threads:[~2019-11-13  6:06 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-09  7:05 [PATCH 1/2] KVM: X86: Single target IPI fastpath Wanpeng Li
2019-11-09  7:05 ` [PATCH 2/2] KVM: LAPIC: micro-optimize fixed mode ipi delivery Wanpeng Li
2019-11-11 21:59   ` Paolo Bonzini
2019-11-12  1:34     ` Wanpeng Li
2019-11-10  1:59 ` [PATCH 1/2] KVM: X86: Single target IPI fastpath kbuild test robot
2019-11-10  1:59 ` [PATCH] KVM: X86: fix semicolon.cocci warnings kbuild test robot
2019-11-11 13:06 ` [PATCH 1/2] KVM: X86: Single target IPI fastpath Vitaly Kuznetsov
2019-11-12  1:18   ` Wanpeng Li
2019-11-11 21:59 ` Paolo Bonzini
2019-11-12  1:33   ` Wanpeng Li
2019-11-13  6:05     ` Wanpeng Li [this message]
2019-11-14  3:12     ` Wanpeng Li
2019-11-14 11:58 ` Paolo Bonzini
2019-11-14 15:22   ` Sean Christopherson
2019-11-14 15:44     ` Paolo Bonzini
2019-11-14 16:34       ` Sean Christopherson
2019-11-15  5:56         ` Wanpeng Li

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