Re: [PATCH] KVM: s390: improve interrupt cpu for wakeup

["Christoph Schlameuss" <schlameuss@linux.ibm.com>]

With nit fixed.

Reviewed-by: Christoph Schlameuß <schlameuss@de.ibm.com>

On Thu Sep 4, 2025 at 1:39 PM CEST, Christian Borntraeger wrote:
> Turns out that picking an idle CPU for floating interrupts has some
> negative side effects. The guest will keep the IO workload on its CPU
> and rather use an IPI from the interrupt CPU instead of moving workload.
> For example a guest with 2 vCPUss and 1 fio process might run that fio on
----------------------------------^
nit: vCPUs

> vcpu1. If after diag500 both vCPUs are idle then vcpu0 is woken up. The
> guest will then do an IPI from vcpu0 to vcpu1.
>
> So lets change the heuristics and prefer the last CPU that went to
> sleep. This one is likely still in halt polling and can be woken up
> quickly.
>
> This patch shows significant improvements in terms of bandwidth or
> cpu consumption for fio and uperf workloads and seems to be a net
> win.
>
> Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> ---
>  arch/s390/include/asm/kvm_host.h |  2 +-
>  arch/s390/kvm/interrupt.c        | 20 +++++++++-----------
>  2 files changed, 10 insertions(+), 12 deletions(-)