Re: [RFC]Two ideas to optimize updating irq routing table

[Paolo Bonzini <pbonzini@redhat.com>]

Il 26/03/2014 09:22, Gonglei (Arei) ha scritto:
> Yes, previously I was using synchronize_srcu, which is not good. When I
> changed it to synchronize_srcu_expedited, grace period delay is much better
> than synchronize_srcu. Though in our tests, we can still see some impact
> of KVM_SET_GSI_ROUTING ioctl.
>
> Our testing scenario is like this. In VM we run a script that sets smp_affinity
> for each IRQ every 0.5s (this leads QEMU to do KVM_SET_GSI_ROUTING ioctl).
> Outside the VM we ping that VM.

Does the affinity actually change every 0.5s?

> Without patches, ping time can jump from 0.3ms to 2ms-30ms. With synchronize_srcu
> patch, ping time is worse. With synchronize_srcu_expedited patch, ping time is
> overall good, though sometimes ping time jump to 1ms-3ms.
>
> With following raw patch, ping time is like call_rcu patch, that not influenced
> by setting IRQ affinity, keeps 0.3ms, and there is no vulnerability, frequent
> intermidiate KVM_SET_GSI_ROUTING settings are just skipped, and always the newest
> setting would take effect.

Interesting, but it only works for assigned-dev.c which is deprecated. 
If you used VFIO you'd see no improvement, and Christian's s390 usecase 
would also see no improvement.

Paolo