linux-kernel - Re: [v3 0/3] KVM: irqchip: synchronize srcu only if needed

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Date: Fri, 9 Feb 2024 01:00:07 -0800
From: Dongli Zhang <dongli.zhang@...cle.com>
To: Yi Wang <up2wing@...il.com>, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, foxywang@...cent.com
Cc: seanjc@...gle.com, pbonzini@...hat.com, mingo@...hat.com,
        tglx@...utronix.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        bp@...en8.de, wanpengli@...cent.com, oliver.upton@...ux.dev,
        anup@...infault.org, frankja@...ux.ibm.com, imbrenda@...ux.ibm.com,
        maz@...nel.org, atishp@...shpatra.org, borntraeger@...ux.ibm.com
Subject: Re: [v3 0/3] KVM: irqchip: synchronize srcu only if needed

Hi Yi,

On 1/24/24 03:34, Yi Wang wrote:
> From: Yi Wang <foxywang@...cent.com>
> 
> We found that it may cost more than 20 milliseconds very accidentally
> to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms
> already.

Would you mind explaining the reason that the *number of VMs* matters, as
KVM_CAP_SPLIT_IRQCHIP is a per-VM cap?

Or it meant it is more likely to have some VM workload impacted by the
synchronize_srcu_expedited() as in prior discussion?

https://lore.kernel.org/kvm/CAN35MuSkQf0XmBZ5ZXGhcpUCGD-kKoyTv9G7ya4QVD1xiqOxLg@mail.gmail.com/

Thank you very much!

Dongli Zhang

> 
> The reason is that when vmm(qemu/CloudHypervisor) invokes
> KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and
> might_sleep and kworker of srcu may cost some delay during this period.
> One way makes sence is setup empty irq routing when creating vm and
> so that x86/s390 don't need to setup empty/dummy irq routing.
> 
> Note: I have no s390 machine so the s390 patch has not been tested.
> 
> Changelog:
> ----------
> v3:
>   - squash setup empty routing function and use of that into one commit
>   - drop the comment in s390 part
> 
> v2:
>   - setup empty irq routing in kvm_create_vm
>   - don't setup irq routing in x86 KVM_CAP_SPLIT_IRQCHIP
>   - don't setup irq routing in s390 KVM_CREATE_IRQCHIP
> 
> v1: https://urldefense.com/v3/__https://lore.kernel.org/kvm/20240112091128.3868059-1-foxywang@tencent.com/__;!!ACWV5N9M2RV99hQ!LjwKfBaGVl3u1l9YQSskg_1RU6278h2-fYnYLsoihF9i43aq73eIDqolGzOmeRvO8UlPreQHLqXEL1bAuw$ 
> 
> Yi Wang (3):
>   KVM: setup empty irq routing when create vm
>   KVM: x86: don't setup empty irq routing when KVM_CAP_SPLIT_IRQCHIP
>   KVM: s390: don't setup dummy routing when KVM_CREATE_IRQCHIP
> 
>  arch/s390/kvm/kvm-s390.c |  9 +--------
>  arch/x86/kvm/irq.h       |  1 -
>  arch/x86/kvm/irq_comm.c  |  5 -----
>  arch/x86/kvm/x86.c       |  3 ---
>  include/linux/kvm_host.h |  1 +
>  virt/kvm/irqchip.c       | 19 +++++++++++++++++++
>  virt/kvm/kvm_main.c      |  4 ++++
>  7 files changed, 25 insertions(+), 17 deletions(-)
>