ACK: [SRU][N/Q][PATCH 0/1] KVM: s390: improve interrupt cpu for wakeup

[Tim Whisonant]

On Mon, Nov 24, 2025 at 10:07:13PM +0100, Massimiliano Pellizzer wrote:
> BugLink: https://bugs.launchpad.net/bugs/2132317
> 
> [ Impact ]
> 
> Commit 352ccf890a3e (KVM: s390: improve interrupt cpu for wakeup)
> addresses a performance optimization in the KVM subsystem
> for the s390 architecture, specifically concerning the selection
> of target virtual CPUs (vCPUs) for floating interrupt delivery.
> The modification changes the heuristic algorithm used to determine
> which vCPU should be awakened when a floating interrupt arrives,
> shifting from an idle-CPU-preference strategy to a
> last-sleep-CPU-preference strategy.
> This algorithmic change yields significant performance improvements
> by reducing inter-processor interrupt overhead
> and improving cache locality.
> 
> [ Fix ]
> 
> Backport 352ccf890a3e (KVM: s390: improve interrupt cpu for wakeup)
> from mainline.
> 
> [ Test Plan ]
> 
> To test the patch it is possible to run I/O-intensive workloads
> (fio, uperf) with varying vCPU configurations, on a s390x machine,
> to ensure interrupt delivery remains correct without errors,
> timeouts, or kernel warnings. Moreover, during I/O-intensive workloads
> a performance improvement should be visible.
> 
> [ Regression Potential]
> 
> The patch modifies interrupt delivery heuristics within s390 KVM
> but does not change core interrupt handling logic
> or data structures significantly.
> The primary risk is suboptimal vCPU selection in workload patterns
> that differ from the tested scenarios (fio, uperf), potentially
> causing performance regressions rather than functional failures.
> 
> Christian Borntraeger (1):
>   KVM: s390: improve interrupt cpu for wakeup
> 
>  arch/s390/include/asm/kvm_host.h |  2 +-
>  arch/s390/kvm/interrupt.c        | 20 +++++++++-----------
>  2 files changed, 10 insertions(+), 12 deletions(-)
> 
> -- 
> 2.51.0
> 
 
Acked-by: Tim Whisonant <tim.whisonant at canonical.com>