linux-kernel - Re: [PATCH v5 0/4] KVM: X86: Paravirt remote TLB flush

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Date:   Wed, 15 Nov 2017 16:05:02 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v5 0/4] KVM: X86: Paravirt remote TLB flush

On Mon, Nov 13, 2017 at 02:01:16AM -0800, Wanpeng Li wrote:
> Remote flushing api's does a busy wait which is fine in bare-metal
> scenario. But with-in the guest, the vcpus might have been pre-empted
> or blocked. In this scenario, the initator vcpu would end up
> busy-waiting for a long amount of time.
> 
> This patch set implements para-virt flush tlbs making sure that it
> does not wait for vcpus that are sleeping. And all the sleeping vcpus
> flush the tlb on guest enter. Idea was discussed here:
> https://lkml.org/lkml/2012/2/20/157
> 
> The best result is achieved when we're overcommiting the host by running 
> multiple vCPUs on each pCPU. In this case PV tlb flush avoids touching 
> vCPUs which are not scheduled and avoid the wait on the main CPU.
> 
> In addition, thanks for commit 9e52fc2b50d ("x86/mm: Enable RCU based 
> page table freeing (CONFIG_HAVE_RCU_TABLE_FREE=y)")
> 
> Test on a Haswell i7 desktop 4 cores (2HT), so 8 pCPUs, running ebizzy 
> in one linux guest.

8 pCPUS?
> 
> ebizzy -M 
>               vanilla    optimized     boost
>  8 vCPUs       10152       10083       -0.68% 
> 16 vCPUs        1224        4866       297.5% 
> 24 vCPUs        1109        3871       249%
> 32 vCPUs        1025        3375       229.3% 

so this is all just one guest? What happens if you have say a 64pCPU
machine with eight of these guests? That is more of a realistic
workload in todays cloud situations.

> 
> Note: The patchset is rebased against "locking/qspinlock/x86: Avoid
>    test-and-set when PV_DEDICATED is set" v3
> 
> v4 -> v5:
>  * flushmask instead of cpumask
> 
> v3 -> v4:
>  * use READ_ONCE()
>  * use try_cmpxchg instead of cmpxchg
>  * add {} to if
>  * no FLUSH flags to preserve during set_preempted
>  * "KVM: X86" prefix to patch subject
> 
> v2 -> v3: 
>  * percpu cpumask
> 
> v1 -> v2:
>  * a new CPUID feature bit
>  * fix cmpxchg check
>  * use kvm_vcpu_flush_tlb() to get the statistics right
>  * just OR the KVM_VCPU_PREEMPTED in kvm_steal_time_set_preempted
>  * add a new bool argument to kvm_x86_ops->tlb_flush
>  * __cpumask_clear_cpu() instead of cpumask_clear_cpu()
>  * not put cpumask_t on stack
>  * rebase the patchset against "locking/qspinlock/x86: Avoid
>    test-and-set when PV_DEDICATED is set" v3
> 
> Wanpeng Li (4):
>   KVM: X86: Add vCPU running/preempted state
>   KVM: X86: Add paravirt remote TLB flush
>   KVM: X86: introduce invalidate_gpa argument to tlb flush
>   KVM: X86: Add flush_on_enter before guest enter
> 
>  Documentation/virtual/kvm/cpuid.txt  |  4 ++++
>  arch/x86/include/asm/kvm_host.h      |  2 +-
>  arch/x86/include/uapi/asm/kvm_para.h |  6 +++++
>  arch/x86/kernel/kvm.c                | 46 ++++++++++++++++++++++++++++++++++--
>  arch/x86/kvm/cpuid.c                 |  3 ++-
>  arch/x86/kvm/svm.c                   | 14 +++++------
>  arch/x86/kvm/vmx.c                   | 21 ++++++++--------
>  arch/x86/kvm/x86.c                   | 25 +++++++++++++-------
>  8 files changed, 88 insertions(+), 30 deletions(-)
> 
> -- 
> 2.7.4
>