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

[Massimiliano Pellizzer]

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(-)

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2.51.0