Re: [PATCH v3 0/5] s390/idle: CPU idle driver

[Alexander Gordeev <agordeev@linux.ibm.com>]

On Thu, Jun 18, 2026 at 02:00:14PM +0200, Mete Durlu wrote:
...
> This patch series introduces a CPU idle driver for s390 architecture
> that leverages the existing cpu idle infrastructure and idle governor
> to optimize idle state selection based on timer events and interrupt
> patterns.
> 
> - Implementation Overview
> -----------------------------------------------------------------------
> 
> The driver implements two idle states;
> 
> **Polling Idle (shallow state)**
> - Busy-loop implementation that prevents SIE-exit during short idle periods
> - Zero exit latency, optimized for latency-sensitive workloads
> - Maintains CPU with the guest, improving response times
> 
> **Enabled Wait (deep state)**
> - Traditional EW state that signals hypervisor availability
> - Higher exit latency but more efficient for longer idle periods
> 
> The idle governor dynamically selects between these states by monitoring
> next timer interrupts and system interrupt frequency, choosing the most
> appropriate idle state for each situation.
> 
> - Performance Benefits
> -----------------------------------------------------------------------
> 
> Benchmark results across LPAR, KVM, and z/VM guests demonstrate significant
> improvements for latency-sensitive and scheduler-intensive workloads:
> 
> ```
> Benchmark    | LPAR  | z/VM  | KVM   |
> -----------------------------------------
> cyclictest   | +45%  | +50%  | +30%  |
> hackbench    | +5%   | +12%  | -     |
> pgbench      | +2%   | +15%  | +2%   |
> uperf        | +50%* | +-5%  | +-5%  |
> ```
> 
> *Single-threaded uperf scenarios show 50-70% improvements on LPAR.
> Multi-threaded results show slight improvements overall.
> 
> Additionally, fio and ioping tests show latency distribution shifts toward
> lower values with reduced minimum and average latencies.
> 
> - Configuration
> -----------------------------------------------------------------------
> 
> Target residency and exit latency values are benchmark estimates. Due to
> s390's virtualized architecture, these values are non-deterministic.
> Idle state parameters are tuned per hypervisor type after benchmarks:
> 
> **LPAR:**
> - EW: 5us target residency, 5us exit latency
> 
> **KVM/z/VM:**
> - EW: 1us target residency, 1us exit latency
> 
> ---
> Mete Durlu (5):
>       s390/tick: Remove CIF_NOHZ_DELAY flag
>       tick: Remove arch_needs_cpu
>       s390: Enable TIF_POLLING_NRFLAG
>       s390/idle: Introduce cpuidle for s390
>       s390/configs: Enable cpuidle driver on s390
> 
>  MAINTAINERS                         |   8 +++
>  arch/s390/Kconfig                   |   5 ++
>  arch/s390/configs/debug_defconfig   |   2 +
>  arch/s390/configs/defconfig         |   2 +
>  arch/s390/include/asm/processor.h   |   4 --
>  arch/s390/include/asm/thread_info.h |   1 +
>  arch/s390/kernel/idle.c             |   1 -
>  arch/s390/kernel/irq.c              |   4 --
>  arch/s390/kernel/smp.c              |   1 -
>  drivers/cpuidle/Kconfig             |   5 ++
>  drivers/cpuidle/Kconfig.s390        |  11 ++++
>  drivers/cpuidle/Makefile            |   4 ++
>  drivers/cpuidle/cpuidle-s390.c      | 115 ++++++++++++++++++++++++++++++++++++
>  include/linux/tick.h                |   3 -
>  kernel/time/tick-sched.c            |   7 +--
>  15 files changed, 156 insertions(+), 17 deletions(-)
> ---
> base-commit: 2d3090a8aeb596a26935db0955d46c9a5db5c6ce
> change-id: 20260609-idledriver-217e2b2712a3
> 
> Best regards,
> -- 
> Mete Durlu <meted@linux.ibm.com>

Applied, thanks!